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<TITLE>The Whole Python FAQ</TITLE>

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<H1>The Whole Python FAQ</H1>

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Last changed on Wed Feb 12 21:31:08 2003 CET

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<P>(Entries marked with ** were changed within the last 24 hours;

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entries marked with * were changed within the last 7 days.)

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<P>

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<P>

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<HR>

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<H2>1. General information and availability</H2>

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<UL>

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<LI><A HREF="#1.1">1.1. What is Python?</A>

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<LI><A HREF="#1.2">1.2. Why is it called Python?</A>

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<LI><A HREF="#1.3">1.3. How do I obtain a copy of the Python source?</A>

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<LI><A HREF="#1.4">1.4. How do I get documentation on Python?</A>

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<LI><A HREF="#1.5">1.5. Are there other ftp sites that mirror the Python distribution?</A>

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<LI><A HREF="#1.6">1.6. Is there a newsgroup or mailing list devoted to Python?</A>

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<LI><A HREF="#1.7">1.7. Is there a WWW page devoted to Python?</A>

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<LI><A HREF="#1.8">1.8. Is the Python documentation available on the WWW?</A>

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<LI><A HREF="#1.9">1.9. Are there any books on Python?</A>

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<LI><A HREF="#1.10">1.10. Are there any published articles about Python that I can reference?</A>

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<LI><A HREF="#1.11">1.11. Are there short introductory papers or talks on Python?</A>

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<LI><A HREF="#1.12">1.12. How does the Python version numbering scheme work?</A>

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<LI><A HREF="#1.13">1.13. How do I get a beta test version of Python?</A>

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<LI><A HREF="#1.14">1.14. Are there copyright restrictions on the use of Python?</A>

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<LI><A HREF="#1.15">1.15. Why was Python created in the first place?</A>

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<LI><A HREF="#1.16">1.16. Do I have to like "Monty Python's Flying Circus"?</A>

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<LI><A HREF="#1.17">1.17. What is Python good for?</A>

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<LI><A HREF="#1.18">1.18. Can I use the FAQ Wizard software to maintain my own FAQ?</A>

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<LI><A HREF="#1.19">1.19. Which editor has good support for editing Python source code?</A>

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<LI><A HREF="#1.20">1.20. I've never programmed before. Is there a Python tutorial?</A>

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<LI><A HREF="#1.21">1.21. Where in the world is www.python.org located?</A>

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</UL>

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<P>

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<HR>

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<H2>2. Python in the real world</H2>

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<UL>

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<LI><A HREF="#2.1">2.1. How many people are using Python?</A>

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<LI><A HREF="#2.2">2.2. Have any significant projects been done in Python?</A>

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<LI><A HREF="#2.3">2.3. Are there any commercial projects going on using Python?</A>

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<LI><A HREF="#2.4">2.4. How stable is Python?</A>

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<LI><A HREF="#2.5">2.5. What new developments are expected for Python in the future?</A>

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<LI><A HREF="#2.6">2.6. Is it reasonable to propose incompatible changes to Python?</A>

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<LI><A HREF="#2.7">2.7. What is the future of Python?</A>

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<LI><A HREF="#2.8">2.8. What was the PSA, anyway?</A>

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<LI><A HREF="#2.9">2.9. Deleted</A>

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<LI><A HREF="#2.10">2.10. Deleted</A>

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<LI><A HREF="#2.11">2.11. Is Python Y2K (Year 2000) Compliant?</A>

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<LI><A HREF="#2.12">2.12. Is Python a good language in a class for beginning programmers?</A>

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</UL>

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<P>

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<HR>

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<H2>3. Building Python and Other Known Bugs</H2>

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<UL>

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<LI><A HREF="#3.1">3.1. Is there a test set?</A>

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<LI><A HREF="#3.2">3.2. When running the test set, I get complaints about floating point operations, but when playing with floating point operations I cannot find anything wrong with them.</A>

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<LI><A HREF="#3.3">3.3. Link errors after rerunning the configure script.</A>

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<LI><A HREF="#3.4">3.4. The python interpreter complains about options passed to a script (after the script name).</A>

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<LI><A HREF="#3.5">3.5. When building on the SGI, make tries to run python to create glmodule.c, but python hasn't been built or installed yet.</A>

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<LI><A HREF="#3.6">3.6. I use VPATH but some targets are built in the source directory.</A>

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<LI><A HREF="#3.7">3.7. Trouble building or linking with the GNU readline library.</A>

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<LI><A HREF="#3.8">3.8. Trouble with socket I/O on older Linux 1.x versions.</A>

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<LI><A HREF="#3.9">3.9. Trouble with prototypes on Ultrix.</A>

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<LI><A HREF="#3.10">3.10. Other trouble building Python on platform X.</A>

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<LI><A HREF="#3.11">3.11. How to configure dynamic loading on Linux.</A>

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<LI><A HREF="#3.12">3.12. I can't get shared modules to work on Linux 2.0 (Slackware96)?</A>

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<LI><A HREF="#3.13">3.13. Trouble when making modules shared on Linux.</A>

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<LI><A HREF="#3.14">3.14. [deleted]</A>

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<LI><A HREF="#3.15">3.15. Errors when linking with a shared library containing C++ code.</A>

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<LI><A HREF="#3.16">3.16. Deleted</A>

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<LI><A HREF="#3.17">3.17. Deleted.</A>

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<LI><A HREF="#3.18">3.18. Compilation or link errors for the _tkinter module</A>

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<LI><A HREF="#3.19">3.19. I configured and built Python for Tcl/Tk but "import Tkinter" fails.</A>

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<LI><A HREF="#3.20">3.20. [deleted]</A>

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<LI><A HREF="#3.21">3.21. Several common system calls are missing from the posix module.</A>

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<LI><A HREF="#3.22">3.22. ImportError: No module named string, on MS Windows.</A>

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<LI><A HREF="#3.23">3.23. Core dump on SGI when using the gl module.</A>

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<LI><A HREF="#3.24">3.24. "Initializer not a constant" while building DLL on MS-Windows</A>

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<LI><A HREF="#3.25">3.25. Output directed to a pipe or file disappears on Linux.</A>

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<LI><A HREF="#3.26">3.26. [deleted]</A>

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<LI><A HREF="#3.27">3.27. [deleted]</A>

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<LI><A HREF="#3.28">3.28. How can I test if Tkinter is working?</A>

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<LI><A HREF="#3.29">3.29. Is there a way to get the interactive mode of the python interpreter to perform function/variable name completion?</A>

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<LI><A HREF="#3.30">3.30. Why is the Python interpreter not built as a shared library?</A>

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<LI><A HREF="#3.31">3.31. Build with GCC on Solaris 2.6 (SunOS 5.6) fails</A>

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<LI><A HREF="#3.32">3.32. Running "make clean" seems to leave problematic files that cause subsequent builds to fail.</A>

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<LI><A HREF="#3.33">3.33. Submitting bug reports and patches</A>

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<LI><A HREF="#3.34">3.34. I can't load shared libraries under Python 1.5.2, Solaris 7, and gcc 2.95.2</A>

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<LI><A HREF="#3.35">3.35. In the regression test, test___all__ fails for the profile module. What's wrong?</A>

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<LI><A HREF="#3.36">3.36. relocations remain against allocatable but non-writable sections</A>

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</UL>

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<P>

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<HR>

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<H2>4. Programming in Python</H2>

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<UL>

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<LI><A HREF="#4.1">4.1. Is there a source code level debugger with breakpoints, step, etc.?</A>

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<LI><A HREF="#4.2">4.2. Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)? (Also phrased as: Can I use a built-in type as base class?)</A>

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<LI><A HREF="#4.3">4.3. Is there a curses/termcap package for Python?</A>

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<LI><A HREF="#4.4">4.4. Is there an equivalent to C's onexit() in Python?</A>

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<LI><A HREF="#4.5">4.5. [deleted]</A>

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<LI><A HREF="#4.6">4.6. How do I iterate over a sequence in reverse order?</A>

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<LI><A HREF="#4.7">4.7. My program is too slow. How do I speed it up?</A>

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<LI><A HREF="#4.8">4.8. When I have imported a module, then edit it, and import it again (into the same Python process), the changes don't seem to take place. What is going on?</A>

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<LI><A HREF="#4.9">4.9. How do I find the current module name?</A>

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<LI><A HREF="#4.10">4.10. I have a module in which I want to execute some extra code when it is run as a script. How do I find out whether I am running as a script?</A>

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<LI><A HREF="#4.11">4.11. I try to run a program from the Demo directory but it fails with ImportError: No module named ...; what gives?</A>

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<LI><A HREF="#4.12">4.12. [deleted]</A>

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<LI><A HREF="#4.13">4.13. What GUI toolkits exist for Python?</A>

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<LI><A HREF="#4.14">4.14. Are there any interfaces to database packages in Python?</A>

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<LI><A HREF="#4.15">4.15. Is it possible to write obfuscated one-liners in Python?</A>

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<LI><A HREF="#4.16">4.16. Is there an equivalent of C's "?:" ternary operator?</A>

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<LI><A HREF="#4.17">4.17. My class defines __del__ but it is not called when I delete the object.</A>

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<LI><A HREF="#4.18">4.18. How do I change the shell environment for programs called using os.popen() or os.system()? Changing os.environ doesn't work.</A>

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<LI><A HREF="#4.19">4.19. What is a class?</A>

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<LI><A HREF="#4.20">4.20. What is a method?</A>

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<LI><A HREF="#4.22">4.22. What is an unbound method?</A>

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<LI><A HREF="#4.23">4.23. How do I call a method defined in a base class from a derived class that overrides it?</A>

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<LI><A HREF="#4.24">4.24. How do I call a method from a base class without using the name of the base class?</A>

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<LI><A HREF="#4.25">4.25. How can I organize my code to make it easier to change the base class?</A>

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<LI><A HREF="#4.26">4.26. How can I find the methods or attributes of an object?</A>

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<LI><A HREF="#4.27">4.27. I can't seem to use os.read() on a pipe created with os.popen().</A>

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<LI><A HREF="#4.28">4.28. How can I create a stand-alone binary from a Python script?</A>

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<LI><A HREF="#4.29">4.29. What WWW tools are there for Python?</A>

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<LI><A HREF="#4.30">4.30. How do I run a subprocess with pipes connected to both input and output?</A>

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<LI><A HREF="#4.31">4.31. How do I call a function if I have the arguments in a tuple?</A>

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<LI><A HREF="#4.32">4.32. How do I enable font-lock-mode for Python in Emacs?</A>

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<LI><A HREF="#4.33">4.33. Is there a scanf() or sscanf() equivalent?</A>

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<LI><A HREF="#4.34">4.34. Can I have Tk events handled while waiting for I/O?</A>

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<LI><A HREF="#4.35">4.35. How do I write a function with output parameters (call by reference)?</A>

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<LI><A HREF="#4.36">4.36. Please explain the rules for local and global variables in Python.</A>

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<LI><A HREF="#4.37">4.37. How can I have modules that mutually import each other?</A>

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<LI><A HREF="#4.38">4.38. How do I copy an object in Python?</A>

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<LI><A HREF="#4.39">4.39. How to implement persistent objects in Python? (Persistent == automatically saved to and restored from disk.)</A>

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<LI><A HREF="#4.40">4.40. I try to use __spam and I get an error about _SomeClassName__spam.</A>

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<LI><A HREF="#4.41">4.41. How do I delete a file? And other file questions.</A>

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<LI><A HREF="#4.42">4.42. How to modify urllib or httplib to support HTTP/1.1?</A>

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<LI><A HREF="#4.43">4.43. Unexplicable syntax errors in compile() or exec.</A>

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<LI><A HREF="#4.44">4.44. How do I convert a string to a number?</A>

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<LI><A HREF="#4.45">4.45. How do I convert a number to a string?</A>

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<LI><A HREF="#4.47">4.47. How do I check if an object is an instance of a given class or of a subclass of it?</A>

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<LI><A HREF="#4.48">4.48. What is delegation?</A>

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<LI><A HREF="#4.49">4.49. How do I test a Python program or component.</A>

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<LI><A HREF="#4.50">4.50. My multidimensional list (array) is broken! What gives?</A>

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<LI><A HREF="#4.51">4.51. I want to do a complicated sort: can you do a Schwartzian Transform in Python?</A>

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<LI><A HREF="#4.52">4.52. How to convert between tuples and lists?</A>

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<LI><A HREF="#4.53">4.53. Files retrieved with urllib contain leading garbage that looks like email headers.</A>

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<LI><A HREF="#4.54">4.54. How do I get a list of all instances of a given class?</A>

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<LI><A HREF="#4.55">4.55. A regular expression fails with regex.error: match failure.</A>

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<LI><A HREF="#4.56">4.56. I can't get signal handlers to work.</A>

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<LI><A HREF="#4.57">4.57. I can't use a global variable in a function? Help!</A>

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<LI><A HREF="#4.58">4.58. What's a negative index? Why doesn't list.insert() use them?</A>

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<LI><A HREF="#4.59">4.59. How can I sort one list by values from another list?</A>

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<LI><A HREF="#4.60">4.60. Why doesn't dir() work on builtin types like files and lists?</A>

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<LI><A HREF="#4.61">4.61. How can I mimic CGI form submission (METHOD=POST)?</A>

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<LI><A HREF="#4.62">4.62. If my program crashes with a bsddb (or anydbm) database open, it gets corrupted. How come?</A>

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<LI><A HREF="#4.63">4.63. How do I make a Python script executable on Unix?</A>

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<LI><A HREF="#4.64">4.64. How do you remove duplicates from a list?</A>

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<LI><A HREF="#4.65">4.65. Are there any known year 2000 problems in Python?</A>

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<LI><A HREF="#4.66">4.66. I want a version of map that applies a method to a sequence of objects! Help!</A>

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<LI><A HREF="#4.67">4.67. How do I generate random numbers in Python?</A>

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<LI><A HREF="#4.68">4.68. How do I access the serial (RS232) port?</A>

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<LI><A HREF="#4.69">4.69. Images on Tk-Buttons don't work in Py15?</A>

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<LI><A HREF="#4.70">4.70. Where is the math.py (socket.py, regex.py, etc.) source file?</A>

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<LI><A HREF="#4.71">4.71. How do I send mail from a Python script?</A>

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<LI><A HREF="#4.72">4.72. How do I avoid blocking in connect() of a socket?</A>

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<LI><A HREF="#4.73">4.73. How do I specify hexadecimal and octal integers?</A>

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<LI><A HREF="#4.74">4.74. How to get a single keypress at a time?</A>

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<LI><A HREF="#4.75">4.75. How can I overload constructors (or methods) in Python?</A>

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<LI><A HREF="#4.76">4.76. How do I pass keyword arguments from one method to another?</A>

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<LI><A HREF="#4.77">4.77. What module should I use to help with generating HTML?</A>

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<LI><A HREF="#4.78">4.78. How do I create documentation from doc strings?</A>

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<LI><A HREF="#4.79">4.79. How do I read (or write) binary data?</A>

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<LI><A HREF="#4.80">4.80. I can't get key bindings to work in Tkinter</A>

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<LI><A HREF="#4.81">4.81. "import crypt" fails</A>

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<LI><A HREF="#4.82">4.82. Are there coding standards or a style guide for Python programs?</A>

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<LI><A HREF="#4.83">4.83. How do I freeze Tkinter applications?</A>

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<LI><A HREF="#4.84">4.84. How do I create static class data and static class methods?</A>

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<LI><A HREF="#4.85">4.85. __import__('x.y.z') returns <module 'x'>; how do I get z?</A>

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<LI><A HREF="#4.86">4.86. Basic thread wisdom</A>

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<LI><A HREF="#4.87">4.87. Why doesn't closing sys.stdout (stdin, stderr) really close it?</A>

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<LI><A HREF="#4.88">4.88. What kinds of global value mutation are thread-safe?</A>

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<LI><A HREF="#4.89">4.89. How do I modify a string in place?</A>

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<LI><A HREF="#4.90">4.90. How to pass on keyword/optional parameters/arguments</A>

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<LI><A HREF="#4.91">4.91. How can I get a dictionary to display its keys in a consistent order?</A>

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<LI><A HREF="#4.92">4.92. Is there a Python tutorial?</A>

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<LI><A HREF="#4.93">4.93. Deleted</A>

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<LI><A HREF="#4.94">4.94. How do I get a single keypress without blocking?</A>

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<LI><A HREF="#4.95">4.95. Is there an equivalent to Perl chomp()? (Remove trailing newline from string)</A>

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<LI><A HREF="#4.96">4.96. Why is join() a string method when I'm really joining the elements of a (list, tuple, sequence)?</A>

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<LI><A HREF="#4.97">4.97. How can my code discover the name of an object?</A>

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<LI><A HREF="#4.98">4.98. Why are floating point calculations so inaccurate?</A>

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<LI><A HREF="#4.99">4.99. I tried to open Berkeley DB file, but bsddb produces bsddb.error: (22, 'Invalid argument'). Help! How can I restore my data?</A>

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<LI><A HREF="#4.100">4.100. What are the "best practices" for using import in a module?</A>

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<LI><A HREF="#4.101">4.101. Is there a tool to help find bugs or perform static analysis?</A>

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<LI><A HREF="#4.102">4.102. UnicodeError: ASCII [decoding,encoding] error: ordinal not in range(128)</A>

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<LI><A HREF="#4.103">4.103. Using strings to call functions/methods</A>

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<LI><A HREF="#4.104">4.104. How fast are exceptions?</A>

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<LI><A HREF="#4.105">4.105. Sharing global variables across modules</A>

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<LI><A HREF="#4.106">4.106. Why is cPickle so slow?</A>

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<LI><A HREF="#4.107">4.107. When importing module XXX, why do I get "undefined symbol: PyUnicodeUCS2_..." ?</A>

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<LI><A HREF="#4.108">4.108. How do I create a .pyc file?</A>

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</UL>

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<P>

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<HR>

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<H2>5. Extending Python</H2>

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<UL>

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<LI><A HREF="#5.1">5.1. Can I create my own functions in C?</A>

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<LI><A HREF="#5.2">5.2. Can I create my own functions in C++?</A>

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<LI><A HREF="#5.3">5.3. How can I execute arbitrary Python statements from C?</A>

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<LI><A HREF="#5.4">5.4. How can I evaluate an arbitrary Python expression from C?</A>

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<LI><A HREF="#5.5">5.5. How do I extract C values from a Python object?</A>

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<LI><A HREF="#5.6">5.6. How do I use Py_BuildValue() to create a tuple of arbitrary length?</A>

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<LI><A HREF="#5.7">5.7. How do I call an object's method from C?</A>

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<LI><A HREF="#5.8">5.8. How do I catch the output from PyErr_Print() (or anything that prints to stdout/stderr)?</A>

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<LI><A HREF="#5.9">5.9. How do I access a module written in Python from C?</A>

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<LI><A HREF="#5.10">5.10. How do I interface to C++ objects from Python?</A>

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<LI><A HREF="#5.11">5.11. mSQLmodule (or other old module) won't build with Python 1.5 (or later)</A>

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<LI><A HREF="#5.12">5.12. I added a module using the Setup file and the make fails! Huh?</A>

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<LI><A HREF="#5.13">5.13. I want to compile a Python module on my Red Hat Linux system, but some files are missing.</A>

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<LI><A HREF="#5.14">5.14. What does "SystemError: _PyImport_FixupExtension: module yourmodule not loaded" mean?</A>

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<LI><A HREF="#5.15">5.15. How to tell "incomplete input" from "invalid input"?</A>

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<LI><A HREF="#5.16">5.16. How do I debug an extension?</A>

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<LI><A HREF="#5.17">5.17. How do I find undefined Linux g++ symbols, __builtin_new or __pure_virtural</A>

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<LI><A HREF="#5.18">5.18. How do I define and create objects corresponding to built-in/extension types</A>

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</UL>

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<P>

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<HR>

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<H2>6. Python's design</H2>

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<UL>

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<LI><A HREF="#6.1">6.1. Why isn't there a switch or case statement in Python?</A>

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<LI><A HREF="#6.2">6.2. Why does Python use indentation for grouping of statements?</A>

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<LI><A HREF="#6.3">6.3. Why are Python strings immutable?</A>

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<LI><A HREF="#6.4">6.4. Delete</A>

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<LI><A HREF="#6.5">6.5. Why does Python use methods for some functionality (e.g. list.index()) but functions for other (e.g. len(list))?</A>

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<LI><A HREF="#6.6">6.6. Why can't I derive a class from built-in types (e.g. lists or files)?</A>

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<LI><A HREF="#6.7">6.7. Why must 'self' be declared and used explicitly in method definitions and calls?</A>

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<LI><A HREF="#6.8">6.8. Can't you emulate threads in the interpreter instead of relying on an OS-specific thread implementation?</A>

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<LI><A HREF="#6.9">6.9. Why can't lambda forms contain statements?</A>

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<LI><A HREF="#6.10">6.10. [deleted]</A>

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<LI><A HREF="#6.11">6.11. [deleted]</A>

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<LI><A HREF="#6.12">6.12. Why is there no more efficient way of iterating over a dictionary than first constructing the list of keys()?</A>

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<LI><A HREF="#6.13">6.13. Can Python be compiled to machine code, C or some other language?</A>

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<LI><A HREF="#6.14">6.14. How does Python manage memory?</A>

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<LI><A HREF="#6.15">6.15. Why are there separate tuple and list data types?</A>

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<LI><A HREF="#6.16">6.16. How are lists implemented?</A>

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<LI><A HREF="#6.17">6.17. How are dictionaries implemented?</A>

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<LI><A HREF="#6.18">6.18. Why must dictionary keys be immutable?</A>

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<LI><A HREF="#6.19">6.19. How the heck do you make an array in Python?</A>

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<LI><A HREF="#6.20">6.20. Why doesn't list.sort() return the sorted list?</A>

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<LI><A HREF="#6.21">6.21. How do you specify and enforce an interface spec in Python?</A>

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<LI><A HREF="#6.22">6.22. Why do all classes have the same type? Why do instances all have the same type?</A>

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<LI><A HREF="#6.23">6.23. Why isn't all memory freed when Python exits?</A>

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<LI><A HREF="#6.24">6.24. Why no class methods or mutable class variables?</A>

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<LI><A HREF="#6.25">6.25. Why are default values sometimes shared between objects?</A>

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<LI><A HREF="#6.27">6.27. How do you make a higher order function in Python?</A>

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<LI><A HREF="#6.28">6.28. Why do I get a SyntaxError for a 'continue' inside a 'try'?</A>

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<LI><A HREF="#6.29">6.29. Why can't raw strings (r-strings) end with a backslash?</A>

281

<LI><A HREF="#6.30">6.30. Why can't I use an assignment in an expression?</A>

282

<LI><A HREF="#6.31">6.31. Why doesn't Python have a "with" statement like some other languages?</A>

283

<LI><A HREF="#6.32">6.32. Why are colons required for if/while/def/class?</A>

284

<LI><A HREF="#6.33">6.33. Can't we get rid of the Global Interpreter Lock?</A>

285

286

</UL>

287

288

<P>

289

<HR>

290

<H2>7. Using Python on non-UNIX platforms</H2>

291

<UL>

292

<LI><A HREF="#7.1">7.1. Is there a Mac version of Python?</A>

293

<LI><A HREF="#7.2">7.2. Are there DOS and Windows versions of Python?</A>

294

<LI><A HREF="#7.3">7.3. Is there an OS/2 version of Python?</A>

295

<LI><A HREF="#7.4">7.4. Is there a VMS version of Python?</A>

296

<LI><A HREF="#7.5">7.5. What about IBM mainframes, or other non-UNIX platforms?</A>

297

<LI><A HREF="#7.6">7.6. Where are the source or Makefiles for the non-UNIX versions?</A>

298

<LI><A HREF="#7.7">7.7. What is the status and support for the non-UNIX versions?</A>

299

<LI><A HREF="#7.8">7.8. I have a PC version but it appears to be only a binary. Where's the library?</A>

300

<LI><A HREF="#7.9">7.9. Where's the documentation for the Mac or PC version?</A>

301

<LI><A HREF="#7.10">7.10. How do I create a Python program file on the Mac or PC?</A>

302

<LI><A HREF="#7.11">7.11. How can I use Tkinter on Windows 95/NT?</A>

303

<LI><A HREF="#7.12">7.12. cgi.py (or other CGI programming) doesn't work sometimes on NT or win95!</A>

304

<LI><A HREF="#7.13">7.13. Why doesn't os.popen() work in PythonWin on NT?</A>

305

<LI><A HREF="#7.14">7.14. How do I use different functionality on different platforms with the same program?</A>

306

<LI><A HREF="#7.15">7.15. Is there an Amiga version of Python?</A>

307

<LI><A HREF="#7.16">7.16. Why doesn't os.popen()/win32pipe.popen() work on Win9x?</A>

308

309

</UL>

310

311

<P>

312

<HR>

313

<H2>8. Python on Windows</H2>

314

<UL>

315

<LI><A HREF="#8.1">8.1. Using Python for CGI on Microsoft Windows</A>

316

<LI><A HREF="#8.2">8.2. How to check for a keypress without blocking?</A>

317

<LI><A HREF="#8.3">8.3. $PYTHONPATH</A>

318

<LI><A HREF="#8.4">8.4. dedent syntax errors</A>

319

<LI><A HREF="#8.5">8.5. How do I emulate os.kill() in Windows?</A>

320

<LI><A HREF="#8.6">8.6. Why does os.path.isdir() fail on NT shared directories?</A>

321

<LI><A HREF="#8.7">8.7. PyRun_SimpleFile() crashes on Windows but not on Unix</A>

322

<LI><A HREF="#8.8">8.8. Import of _tkinter fails on Windows 95/98</A>

323

<LI><A HREF="#8.9">8.9. Can't extract the downloaded documentation on Windows</A>

324

<LI><A HREF="#8.10">8.10. Can't get Py_RunSimpleFile() to work.</A>

325

<LI><A HREF="#8.11">8.11. Where is Freeze for Windows?</A>

326

327

<LI><A HREF="#8.13">8.13. Missing cw3215mt.dll (or missing cw3215.dll)</A>

328

<LI><A HREF="#8.14">8.14. How to make python scripts executable:</A>

329

<LI><A HREF="#8.15">8.15. Warning about CTL3D32 version from installer</A>

330

<LI><A HREF="#8.16">8.16. How can I embed Python into a Windows application?</A>

331

<LI><A HREF="#8.17">8.17. Setting up IIS 5 to use Python for CGI</A>

332

<LI><A HREF="#8.18">8.18. How do I run a Python program under Windows?</A>

333

334

</UL>

335

336

<HR>

337

<H1>1. General information and availability</H1>

338

339

<HR>

340

<H2><A NAME="1.1">1.1. What is Python?</A></H2>

341

Python is an interpreted, interactive, object-oriented programming

342

language. It incorporates modules, exceptions, dynamic typing, very

343

high level dynamic data types, and classes. Python combines

344

remarkable power with very clear syntax. It has interfaces to many

345

system calls and libraries, as well as to various window systems, and

346

is extensible in C or C++. It is also usable as an extension language

347

for applications that need a programmable interface. Finally, Python

348

is portable: it runs on many brands of UNIX, on the Mac, and on PCs

349

under MS-DOS, Windows, Windows NT, and OS/2.

350

<P>

351

To find out more, the best thing to do is to start reading the

352

tutorial from the documentation set (see a few questions further

353

down).

354

<P>

355

See also question 1.17 (what is Python good for).

356

<P>

357

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363

<P>

364

365

<HR>

366

<H2><A NAME="1.2">1.2. Why is it called Python?</A></H2>

367

Apart from being a computer scientist, I'm also a fan of "Monty

368

Python's Flying Circus" (a BBC comedy series from the seventies, in

369

the -- unlikely -- case you didn't know). It occurred to me one day

370

that I needed a name that was short, unique, and slightly mysterious.

371

And I happened to be reading some scripts from the series at the

372

time... So then I decided to call my language Python.

373

<P>

374

By now I don't care any more whether you use a Python, some other

375

snake, a foot or 16-ton weight, or a wood rat as a logo for Python!

376

<P>

377

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383

<P>

384

385

<HR>

386

<H2><A NAME="1.3">1.3. How do I obtain a copy of the Python source?</A></H2>

387

The latest Python source distribution is always available from

388

python.org, at <A HREF="http://www.python.org/download">http://www.python.org/download</A>. The latest development sources can be obtained via anonymous CVS from SourceForge, at <A HREF="http://www.sf.net/projects/python">http://www.sf.net/projects/python</A> .

389

<P>

390

The source distribution is a gzipped tar file containing the complete C source, LaTeX

391

documentation, Python library modules, example programs, and several

392

useful pieces of freely distributable software. This will compile and

393

run out of the box on most UNIX platforms. (See section 7 for

394

non-UNIX information.)

395

<P>

396

Older versions of Python are also available from python.org.

397

<P>

398

399

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/ Last changed on Tue Apr 9 17:06:16 2002 by

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<A HREF="mailto:akuchlin@mems-exchange.org">A.M. Kuchling</A>

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<P>

405

406

<HR>

407

<H2><A NAME="1.4">1.4. How do I get documentation on Python?</A></H2>

408

All documentation is available on-line, starting at <A HREF="http://www.python.org/doc">http://www.python.org/doc</A>/.

409

<P>

410

The LaTeX source for the documentation is part of the source

411

distribution. If you don't have LaTeX, the latest Python

412

documentation set is available, in various formats like postscript

413

and html, by anonymous ftp - visit the above URL for links to the

414

current versions.

415

<P>

416

PostScript for a high-level description of Python is in the file nluug-paper.ps

417

(a separate file on the ftp site).

418

<P>

419

420

<A HREF="faqw.py?req=edit&file=faq01.004.htp">Edit this entry</A> /

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/ Last changed on Wed Jan 21 12:02:55 1998 by

424

<A HREF="mailto:klm@python.org">Ken Manheimer</A>

425

<P>

426

427

<HR>

428

<H2><A NAME="1.5">1.5. Are there other ftp sites that mirror the Python distribution?</A></H2>

429

The following anonymous ftp sites keep mirrors of the Python

430

distribution:

431

<P>

432

USA:

433

<P>

434

<PRE>

435

<A HREF="ftp://ftp.python.org/pub/python">ftp://ftp.python.org/pub/python</A>/

436

<A HREF="ftp://gatekeeper.dec.com/pub/plan/python">ftp://gatekeeper.dec.com/pub/plan/python</A>/

437

<A HREF="ftp://ftp.uu.net/languages/python">ftp://ftp.uu.net/languages/python</A>/

438

<A HREF="ftp://ftp.wustl.edu/graphics/graphics/sgi-stuff/python">ftp://ftp.wustl.edu/graphics/graphics/sgi-stuff/python</A>/

439

<A HREF="ftp://ftp.sterling.com/programming/languages/python">ftp://ftp.sterling.com/programming/languages/python</A>/

440

<A HREF="ftp://uiarchive.cso.uiuc.edu/pub/lang/python">ftp://uiarchive.cso.uiuc.edu/pub/lang/python</A>/

441

<A HREF="ftp://ftp.pht.com/mirrors/python/python">ftp://ftp.pht.com/mirrors/python/python</A>/

442

<A HREF="ftp://ftp.cdrom.com/pub/python">ftp://ftp.cdrom.com/pub/python</A>/

443

</PRE>

444

Europe:

445

<P>

446

<PRE>

447

<A HREF="ftp://ftp.cwi.nl/pub/python">ftp://ftp.cwi.nl/pub/python</A>/

448

<A HREF="ftp://ftp.funet.fi/pub/languages/python">ftp://ftp.funet.fi/pub/languages/python</A>/

449

<A HREF="ftp://ftp.sunet.se/pub/lang/python">ftp://ftp.sunet.se/pub/lang/python</A>/

450

<A HREF="ftp://unix.hensa.ac.uk/mirrors/uunet/languages/python">ftp://unix.hensa.ac.uk/mirrors/uunet/languages/python</A>/

451

<A HREF="ftp://ftp.lip6.fr/pub/python">ftp://ftp.lip6.fr/pub/python</A>/

452

<A HREF="ftp://sunsite.cnlab-switch.ch/mirror/python">ftp://sunsite.cnlab-switch.ch/mirror/python</A>/

453

<A HREF="ftp://ftp.informatik.tu-muenchen.de/pub/comp/programming/languages/python">ftp://ftp.informatik.tu-muenchen.de/pub/comp/programming/languages/python</A>/

454

</PRE>

455

Australia:

456

<P>

457

<PRE>

458

<A HREF="ftp://ftp.dstc.edu.au/pub/python">ftp://ftp.dstc.edu.au/pub/python</A>/

459

</PRE>

460

<P>

461

462

<A HREF="faqw.py?req=edit&file=faq01.005.htp">Edit this entry</A> /

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/ Last changed on Wed Mar 24 09:20:49 1999 by

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<A HREF="mailto:akuchlin@cnri.reston.va.us">A.M. Kuchling</A>

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<P>

468

469

<HR>

470

<H2><A NAME="1.6">1.6. Is there a newsgroup or mailing list devoted to Python?</A></H2>

471

There is a newsgroup, comp.lang.python,

472

and a mailing list. The newsgroup and mailing list are gatewayed into

473

each other -- if you can read news it's unnecessary to subscribe to

474

the mailing list. To subscribe to the mailing list

475

(<A HREF="mailto:python-list@python.org">python-list@python.org</A>) visit its Mailman webpage at

476

<A HREF="http://www.python.org/mailman/listinfo/python-list">http://www.python.org/mailman/listinfo/python-list</A>

477

<P>

478

More info about the newsgroup and mailing list, and about other lists,

479

can be found at

480

<A HREF="http://www.python.org/psa/MailingLists.html">http://www.python.org/psa/MailingLists.html</A>.

481

<P>

482

Archives of the newsgroup are kept by Deja News and accessible

483

through the "Python newsgroup search" web page,

484

<A HREF="http://www.python.org/search/search_news.html">http://www.python.org/search/search_news.html</A>.

485

This page also contains pointer to other archival collections.

486

<P>

487

488

<A HREF="faqw.py?req=edit&file=faq01.006.htp">Edit this entry</A> /

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/ Last changed on Wed Jun 23 09:29:36 1999 by

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493

<P>

494

495

<HR>

496

<H2><A NAME="1.7">1.7. Is there a WWW page devoted to Python?</A></H2>

497

Yes, <A HREF="http://www.python.org">http://www.python.org</A>/ is the official Python home page.

498

<P>

499

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<A HREF="faqw.py?req=edit&file=faq01.007.htp">Edit this entry</A> /

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/ Last changed on Fri May 23 14:42:59 1997 by

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<A HREF="mailto:klm@python.org">Ken Manheimer</A>

505

<P>

506

507

<HR>

508

<H2><A NAME="1.8">1.8. Is the Python documentation available on the WWW?</A></H2>

509

Yes. Python 2.0 documentation is available from

510

<A HREF="http://www.pythonlabs.com/tech/python2.0/doc">http://www.pythonlabs.com/tech/python2.0/doc</A>/ and from

511

<A HREF="http://www.python.org/doc">http://www.python.org/doc</A>/. Note that most documentation

512

is available for on-line browsing as well as for downloading.

513

<P>

514

515

<A HREF="faqw.py?req=edit&file=faq01.008.htp">Edit this entry</A> /

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/ Last changed on Tue Jan 2 03:14:08 2001 by

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<A HREF="mailto:moshez@zadka.site.co.il">Moshe Zadka</A>

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<P>

521

522

<HR>

523

<H2><A NAME="1.9">1.9. Are there any books on Python?</A></H2>

524

Yes, many, and more are being published. See

525

the python.org Wiki at <A HREF="http://www.python.org/cgi-bin/moinmoin/PythonBooks">http://www.python.org/cgi-bin/moinmoin/PythonBooks</A> for a list.

526

<P>

527

You can also search online bookstores for "Python"

528

(and filter out the Monty Python references; or

529

perhaps search for "Python" and "language").

530

<P>

531

532

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/ Last changed on Mon Aug 5 19:08:49 2002 by

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537

<P>

538

539

<HR>

540

<H2><A NAME="1.10">1.10. Are there any published articles about Python that I can reference?</A></H2>

541

If you can't reference the web site, and you don't want to reference the books

542

(see previous question), there are several articles on Python that you could

543

reference.

544

<P>

545

Most publications about Python are collected on the Python web site:

546

<P>

547

<PRE>

548

<A HREF="http://www.python.org/doc/Publications.html">http://www.python.org/doc/Publications.html</A>

549

</PRE>

550

It is no longer recommended to reference this

551

very old article by Python's author:

552

<P>

553

<PRE>

554

Guido van Rossum and Jelke de Boer, "Interactively Testing Remote

555

Servers Using the Python Programming Language", CWI Quarterly, Volume

556

4, Issue 4 (December 1991), Amsterdam, pp 283-303.

557

</PRE>

558

<P>

559

560

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/ Last changed on Sat Jul 4 20:52:31 1998 by

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565

<P>

566

567

<HR>

568

<H2><A NAME="1.11">1.11. Are there short introductory papers or talks on Python?</A></H2>

569

There are several - you can find links to some of them collected at

570

<A HREF="http://www.python.org/doc/Hints.html#intros">http://www.python.org/doc/Hints.html#intros</A>.

571

<P>

572

573

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/ Last changed on Fri May 23 15:04:05 1997 by

577

<A HREF="mailto:klm@python.org">Ken Manheimer</A>

578

<P>

579

580

<HR>

581

<H2><A NAME="1.12">1.12. How does the Python version numbering scheme work?</A></H2>

582

Python versions are numbered A.B.C or A.B. A is the major version

583

number -- it is only incremented for really major changes in the

584

language. B is the minor version number, incremented for less

585

earth-shattering changes. C is the micro-level -- it is

586

incremented for each bugfix release. See PEP 6 for more information

587

about bugfix releases.

588

<P>

589

Not all releases have bugfix releases.

590

Note that in the past (ending with 1.5.2),

591

micro releases have added significant changes;

592

in fact the changeover from 0.9.9 to 1.0.0 was the first time

593

that either A or B changed!

594

<P>

595

Alpha, beta and release candidate versions have an additional suffixes.

596

The suffix for an alpha version is "aN" for some small number N, the

597

suffix for a beta version is "bN" for some small number N, and the

598

suffix for a release candidate version is "cN" for some small number N.

599

<P>

600

Note that (for instance) all versions labeled 2.0aN precede the

601

versions labeled 2.0bN, which precede versions labeled 2.0cN, and

602

<I>those</I> precede 2.0.

603

<P>

604

As a rule, no changes are made between release candidates and the final

605

release unless there are show-stopper bugs.

606

<P>

607

You may also find version numbers with a "+" suffix, e.g. "2.2+".

608

These are unreleased versions, built directly from the CVS trunk.

609

<P>

610

See also the documentation for sys.version, sys.hexversion, and

611

sys.version_info.

612

<P>

613

614

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/ Last changed on Mon Jan 14 06:34:17 2002 by

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619

<P>

620

621

<HR>

622

<H2><A NAME="1.13">1.13. How do I get a beta test version of Python?</A></H2>

623

All releases, including alphas, betas and release candidates, are announced on

624

comp.lang.python and comp.lang.python.announce newsgroups,

625

which are gatewayed into the <A HREF="mailto:python-list@python.org">python-list@python.org</A> and

626

<A HREF="mailto:python-announce@python.org">python-announce@python.org</A>. In addition, all these announcements appear on

627

the Python home page, at <A HREF="http://www.python.org">http://www.python.org</A>.

628

<P>

629

You can also access the development version of Python through CVS. See <A HREF="http://sourceforge.net/cvs/?group_id=5470">http://sourceforge.net/cvs/?group_id=5470</A> for details. If you're not familiar with CVS, documents like <A HREF="http://linux.oreillynet.com/pub/a/linux/2002/01/03/cvs_intro.html">http://linux.oreillynet.com/pub/a/linux/2002/01/03/cvs_intro.html</A>

630

provide an introduction.

631

<P>

632

633

<A HREF="faqw.py?req=edit&file=faq01.013.htp">Edit this entry</A> /

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635

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/ Last changed on Mon Jun 3 00:57:08 2002 by

637

<A HREF="mailto:neal@metaslash.com">Neal Norwitz</A>

638

<P>

639

640

<HR>

641

<H2><A NAME="1.14">1.14. Are there copyright restrictions on the use of Python?</A></H2>

642

Hardly. You can do anything you want with the source, as long as

643

you leave the copyrights in, and display those copyrights in any

644

documentation about Python that you produce. Also, don't use the

645

author's institute's name in publicity without prior written

646

permission, and don't hold them responsible for anything (read the

647

actual copyright for a precise legal wording).

648

<P>

649

In particular, if you honor the copyright rules, it's OK to use Python

650

for commercial use, to sell copies of Python in source or binary form,

651

or to sell products that enhance Python or incorporate Python (or part

652

of it) in some form. I would still like to know about all commercial

653

use of Python!

654

<P>

655

656

<A HREF="faqw.py?req=edit&file=faq01.014.htp">Edit this entry</A> /

657

658

<P>

659

660

<HR>

661

<H2><A NAME="1.15">1.15. Why was Python created in the first place?</A></H2>

662

Here's a <I>very</I> brief summary of what got me started:

663

<P>

664

I had extensive experience with implementing an interpreted language

665

in the ABC group at CWI, and from working with this group I had

666

learned a lot about language design. This is the origin of many

667

Python features, including the use of indentation for statement

668

grouping and the inclusion of very-high-level data types (although the

669

details are all different in Python).

670

<P>

671

I had a number of gripes about the ABC language, but also liked many

672

of its features. It was impossible to extend the ABC language (or its

673

implementation) to remedy my complaints -- in fact its lack of

674

extensibility was one of its biggest problems.

675

I had some experience with using Modula-2+ and talked with the

676

designers of Modula-3 (and read the M3 report). M3 is the origin of

677

the syntax and semantics used for exceptions, and some other Python

678

features.

679

<P>

680

I was working in the Amoeba distributed operating system group at

681

CWI. We needed a better way to do system administration than by

682

writing either C programs or Bourne shell scripts, since Amoeba had

683

its own system call interface which wasn't easily accessible from the

684

Bourne shell. My experience with error handling in Amoeba made me

685

acutely aware of the importance of exceptions as a programming

686

language feature.

687

<P>

688

It occurred to me that a scripting language with a syntax like ABC

689

but with access to the Amoeba system calls would fill the need. I

690

realized that it would be foolish to write an Amoeba-specific

691

language, so I decided that I needed a language that was generally

692

extensible.

693

<P>

694

During the 1989 Christmas holidays, I had a lot of time on my hand,

695

so I decided to give it a try. During the next year, while still

696

mostly working on it in my own time, Python was used in the Amoeba

697

project with increasing success, and the feedback from colleagues made

698

me add many early improvements.

699

<P>

700

In February 1991, after just over a year of development, I decided

701

to post to USENET. The rest is in the Misc/HISTORY file.

702

<P>

703

704

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/ Last changed on Fri May 23 00:06:23 1997 by

708

709

<P>

710

711

<HR>

712

<H2><A NAME="1.16">1.16. Do I have to like "Monty Python's Flying Circus"?</A></H2>

713

No, but it helps. Pythonistas like the occasional reference to SPAM,

714

and of course, nobody expects the Spanish Inquisition

715

<P>

716

The two main reasons to use Python are:

717

<P>

718

<PRE>

719

- Portable

720

- Easy to learn

721

</PRE>

722

The <I>three</I> main reasons to use Python are:

723

<P>

724

<PRE>

725

- Portable

726

- Easy to learn

727

- Powerful standard library

728

</PRE>

729

(And nice red uniforms.)

730

<P>

731

And remember, there is <I>no</I> rule six.

732

<P>

733

734

<A HREF="faqw.py?req=edit&file=faq01.016.htp">Edit this entry</A> /

735

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737

/ Last changed on Wed May 28 10:39:21 1997 by

738

739

<P>

740

741

<HR>

742

<H2><A NAME="1.17">1.17. What is Python good for?</A></H2>

743

Python is used in many situations where a great deal of dynamism,

744

ease of use, power, and flexibility are required.

745

<P>

746

In the area of basic text

747

manipulation core Python (without any non-core extensions) is easier

748

to use and is roughly as fast as just about any language, and this makes Python

749

good for many system administration type tasks and for CGI programming

750

and other application areas that manipulate text and strings and such.

751

<P>

752

When augmented with

753

standard extensions (such as PIL, COM, Numeric, oracledb, kjbuckets,

754

tkinter, win32api, etc.)

755

or special purpose extensions (that you write, perhaps using helper tools such

756

as SWIG, or using object protocols such as ILU/CORBA or COM) Python

757

becomes a very convenient "glue" or "steering"

758

language that helps make heterogeneous collections of unrelated

759

software packages work together.

760

For example by combining Numeric with oracledb you can help your

761

SQL database do statistical analysis, or even Fourier transforms.

762

One of the features that makes Python excel in the "glue language" role

763

is Python's simple, usable, and powerful C language runtime API.

764

<P>

765

Many developers also use Python extensively as a graphical user

766

interface development aide.

767

<P>

768

769

<A HREF="faqw.py?req=edit&file=faq01.017.htp">Edit this entry</A> /

770

771

772

/ Last changed on Sat May 24 10:13:11 1997 by

773

<A HREF="mailto:aaron_watters@msn.com">Aaron Watters</A>

774

<P>

775

776

<HR>

777

<H2><A NAME="1.18">1.18. Can I use the FAQ Wizard software to maintain my own FAQ?</A></H2>

778

Sure. It's in Tools/faqwiz/ of the python source tree.

779

<P>

780

781

<A HREF="faqw.py?req=edit&file=faq01.018.htp">Edit this entry</A> /

782

783

784

/ Last changed on Fri Mar 29 06:50:32 2002 by

785

786

<P>

787

788

<HR>

789

<H2><A NAME="1.19">1.19. Which editor has good support for editing Python source code?</A></H2>

790

On Unix, the first choice is Emacs/XEmacs. There's an elaborate

791

mode for editing Python code, which is available from the Python

792

source distribution (Misc/python-mode.el). It's also bundled

793

with XEmacs (we're still working on legal details to make it possible

794

to bundle it with FSF Emacs). And it has its own web page:

795

<P>

796

<PRE>

797

<A HREF="http://www.python.org/emacs/python-mode/index.html">http://www.python.org/emacs/python-mode/index.html</A>

798

</PRE>

799

There are many other choices, for Unix, Windows or Macintosh.

800

Richard Jones compiled a table from postings on the Python newsgroup:

801

<P>

802

<PRE>

803

<A HREF="http://www.bofh.asn.au/~richard/editors.html">http://www.bofh.asn.au/~richard/editors.html</A>

804

</PRE>

805

See also FAQ question 7.10 for some more Mac and Win options.

806

<P>

807

808

<A HREF="faqw.py?req=edit&file=faq01.019.htp">Edit this entry</A> /

809

810

811

/ Last changed on Mon Jun 15 23:21:04 1998 by

812

813

<P>

814

815

<HR>

816

<H2><A NAME="1.20">1.20. I've never programmed before. Is there a Python tutorial?</A></H2>

817

There are several, and at least one book.

818

All information for beginning Python programmers is collected here:

819

<P>

820

<PRE>

821

<A HREF="http://www.python.org/doc/Newbies.html">http://www.python.org/doc/Newbies.html</A>

822

</PRE>

823

<P>

824

825

<A HREF="faqw.py?req=edit&file=faq01.020.htp">Edit this entry</A> /

826

827

828

/ Last changed on Wed Sep 5 05:34:07 2001 by

829

830

<P>

831

832

<HR>

833

<H2><A NAME="1.21">1.21. Where in the world is www.python.org located?</A></H2>

834

It's currently in Amsterdam, graciously hosted by XS4ALL:

835

<P>

836

<PRE>

837

838

</PRE>

839

Thanks to Thomas Wouters for setting this up!!!!

840

<P>

841

842

<A HREF="faqw.py?req=edit&file=faq01.021.htp">Edit this entry</A> /

843

844

845

/ Last changed on Fri Aug 3 21:49:27 2001 by

846

847

<P>

848

849

<HR>

850

<H1>2. Python in the real world</H1>

851

852

<HR>

853

<H2><A NAME="2.1">2.1. How many people are using Python?</A></H2>

854

Certainly thousands, and quite probably tens of thousands of users.

855

More are seeing the light each day. The comp.lang.python newsgroup is

856

very active, but overall there is no accurate estimate of the number of subscribers or Python users.

857

<P>

858

Jacek Artymiak has created a Python Users Counter; you can see the

859

current count by visiting

860

<A HREF="http://www.wszechnica.safenet.pl/cgi-bin/checkpythonuserscounter.py">http://www.wszechnica.safenet.pl/cgi-bin/checkpythonuserscounter.py</A>

861

(this will not increment the counter; use the link there if you haven't

862

added yourself already). Most Python users appear not to have registered themselves.

863

<P>

864

865

<A HREF="faqw.py?req=edit&file=faq02.001.htp">Edit this entry</A> /

866

867

868

/ Last changed on Thu Feb 21 23:29:18 2002 by

869

870

<P>

871

872

<HR>

873

<H2><A NAME="2.2">2.2. Have any significant projects been done in Python?</A></H2>

874

At CWI (the former home of Python), we have written a 20,000 line

875

authoring environment for transportable hypermedia presentations, a

876

5,000 line multimedia teleconferencing tool, as well as many many

877

smaller programs.

878

<P>

879

At CNRI (Python's new home), we have written two large applications:

880

Grail, a fully featured web browser (see

881

<A HREF="http://grail.cnri.reston.va.us">http://grail.cnri.reston.va.us</A>),

882

and the Knowbot Operating Environment,

883

a distributed environment for mobile code.

884

<P>

885

The University of Virginia uses Python to control a virtual reality

886

engine. See <A HREF="http://alice.cs.cmu.edu">http://alice.cs.cmu.edu</A>.

887

<P>

888

The ILU project at Xerox PARC can generate Python glue for ILU

889

interfaces. See <A HREF="ftp://ftp.parc.xerox.com/pub/ilu/ilu.html">ftp://ftp.parc.xerox.com/pub/ilu/ilu.html</A>. ILU

890

is a free CORBA compliant ORB which supplies distributed object

891

connectivity to a host of platforms using a host of languages.

892

<P>

893

Mark Hammond and Greg Stein and others are interfacing Python to

894

Microsoft's COM and ActiveX architectures. This means, among other

895

things, that Python may be used in active server pages or as a COM

896

controller (for example to automatically extract from or insert information

897

into Excel or MSAccess or any other COM aware application).

898

Mark claims Python can even be a ActiveX scripting host (which

899

means you could embed JScript inside a Python application, if you

900

had a strange sense of humor). Python/AX/COM is distributed as part

901

of the PythonWin distribution.

902

<P>

903

The University of California, Irvine uses a student administration

904

system called TELE-Vision written entirely in Python. Contact: Ray

905

Price <A HREF="mailto:rlprice@uci.edu">rlprice@uci.edu</A>.

906

<P>

907

The Melbourne Cricket Ground (MCG) in Australia (a 100,000+ person venue)

908

has it's scoreboard system written largely in Python on MS Windows.

909

Python expressions are used to create almost every scoring entry that

910

appears on the board. The move to Python/C++ away from exclusive C++

911

has provided a level of functionality that would simply not have been

912

viable otherwise.

913

<P>

914

See also the next question.

915

<P>

916

Note: this FAQ entry is really old.

917

See <A HREF="http://www.python.org/psa/Users.html">http://www.python.org/psa/Users.html</A> for a more recent list.

918

<P>

919

920

<A HREF="faqw.py?req=edit&file=faq02.002.htp">Edit this entry</A> /

921

922

923

/ Last changed on Wed Oct 25 13:24:15 2000 by

924

925

<P>

926

927

<HR>

928

<H2><A NAME="2.3">2.3. Are there any commercial projects going on using Python?</A></H2>

929

Yes, there's lots of commercial activity using Python. See

930

<A HREF="http://www.python.org/psa/Users.html">http://www.python.org/psa/Users.html</A> for a list.

931

<P>

932

933

<A HREF="faqw.py?req=edit&file=faq02.003.htp">Edit this entry</A> /

934

935

936

/ Last changed on Wed Oct 14 18:17:33 1998 by

937

938

<P>

939

940

<HR>

941

<H2><A NAME="2.4">2.4. How stable is Python?</A></H2>

942

Very stable. New, stable releases have been coming out roughly every 3 to 12 months since 1991, and this seems likely to continue.

943

<P>

944

With the introduction of retrospective "bugfix" releases the stability of the language implementations can be, and is being, improved independently of the new features offered by more recent major or minor releases. Bugfix releases, indicated by a third component of the version number, only fix known problems and do not gratuitously introduce new and possibly incompatible features or modified library functionality.

945

<P>

946

Release 2.2 got its first bugfix on April 10, 2002. The new version

947

number is now 2.2.1. The 2.1 release, at 2.1.3, can probably be

948

considered the "most stable" platform because it has been bugfixed

949

twice.

950

<P>

951

952

<A HREF="faqw.py?req=edit&file=faq02.004.htp">Edit this entry</A> /

953

954

955

/ Last changed on Tue Jul 23 10:20:04 2002 by

956

<A HREF="mailto:kubieziel@gmx.de">Jens Kubieziel</A>

957

<P>

958

959

<HR>

960

<H2><A NAME="2.5">2.5. What new developments are expected for Python in the future?</A></H2>

961

See <A HREF="http://www.python.org/peps">http://www.python.org/peps</A>/ for the Python Enhancement

962

Proposals (PEPs). PEPs are design

963

documents

964

describing a suggested new feature for Python, providing

965

a concise technical specification and a rationale.

966

<P>

967

Also, follow the discussions on the python-dev mailing list.

968

<P>

969

970

<A HREF="faqw.py?req=edit&file=faq02.005.htp">Edit this entry</A> /

971

972

973

/ Last changed on Tue Apr 9 17:09:51 2002 by

974

<A HREF="mailto:akuchlin@mems-exchange.org">A.M. Kuchling</A>

975

<P>

976

977

<HR>

978

<H2><A NAME="2.6">2.6. Is it reasonable to propose incompatible changes to Python?</A></H2>

979

In general, no. There are already millions of lines of Python code

980

around the world, so any changes in the language that invalidates more

981

than a very small fraction of existing programs has to be frowned

982

upon. Even if you can provide a conversion program, there still is

983

the problem of updating all documentation. Providing a gradual

984

upgrade path is the only way if a feature has to be changed.

985

<P>

986

See <A HREF="http://www.python.org/peps/pep-0005.html">http://www.python.org/peps/pep-0005.html</A> for the proposed

987

mechanism for creating backwards-incompatibilities.

988

<P>

989

990

<A HREF="faqw.py?req=edit&file=faq02.006.htp">Edit this entry</A> /

991

992

993

/ Last changed on Mon Apr 1 22:13:47 2002 by

994

<A HREF="mailto:fdrake@acm.org">Fred Drake</A>

995

<P>

996

997

<HR>

998

<H2><A NAME="2.7">2.7. What is the future of Python?</A></H2>

999

Please see <A HREF="http://www.python.org/peps">http://www.python.org/peps</A>/ for proposals of future

1000

activities. One of the PEPs (Python Enhancement Proposals) deals

1001

with the PEP process and PEP format -- see

1002

<A HREF="http://www.python.org/peps/pep-0001.html">http://www.python.org/peps/pep-0001.html</A> if you want to

1003

submit a PEP. In <A HREF="http://www.python.org/peps/pep-0042.html">http://www.python.org/peps/pep-0042.html</A> there

1004

is a list of wishlists the Python Development team plans to tackle.

1005

<P>

1006

1007

<A HREF="faqw.py?req=edit&file=faq02.007.htp">Edit this entry</A> /

1008

1009

1010

/ Last changed on Mon Apr 1 22:15:46 2002 by

1011

<A HREF="mailto:fdrake@acm.org">Fred Drake</A>

1012

<P>

1013

1014

<HR>

1015

<H2><A NAME="2.8">2.8. What was the PSA, anyway?</A></H2>

1016

The Python Software Activity was

1017

created by a number of Python aficionados who want Python to be more

1018

than the product and responsibility of a single individual.

1019

The PSA was not an independent organization, but lived

1020

under the umbrealla of CNRI.

1021

<P>

1022

The PSA has been superseded by the Python Software Foundation,

1023

an independent non-profit organization. The PSF's home page

1024

is at <A HREF="http://www.python.org/psf">http://www.python.org/psf</A>/.

1025

<P>

1026

Some pages created by the PSA still live at

1027

<A HREF="http://www.python.org/psa">http://www.python.org/psa</A>/

1028

<P>

1029

1030

<A HREF="faqw.py?req=edit&file=faq02.008.htp">Edit this entry</A> /

1031

1032

1033

/ Last changed on Thu Jul 25 18:19:44 2002 by

1034

1035

<P>

1036

1037

<HR>

1038

<H2><A NAME="2.9">2.9. Deleted</A></H2>

1039

<P>

1040

<P>

1041

1042

<A HREF="faqw.py?req=edit&file=faq02.009.htp">Edit this entry</A> /

1043

1044

1045

/ Last changed on Tue Jan 2 02:51:30 2001 by

1046

<A HREF="mailto:moshez@zadka.site.co.il">Moshe Zadka</A>

1047

<P>

1048

1049

<HR>

1050

<H2><A NAME="2.10">2.10. Deleted</A></H2>

1051

<P>

1052

<P>

1053

1054

<A HREF="faqw.py?req=edit&file=faq02.010.htp">Edit this entry</A> /

1055

1056

1057

/ Last changed on Tue Jan 2 02:52:19 2001 by

1058

<A HREF="mailto:moshez@zadka.site.co.il">Moshe Zadka</A>

1059

<P>

1060

1061

<HR>

1062

<H2><A NAME="2.11">2.11. Is Python Y2K (Year 2000) Compliant?</A></H2>

1063

As of January, 2001 no major problems have been reported and Y2K

1064

compliance seems to be a non-issue.

1065

<P>

1066

Since Python is available free of charge, there are no absolute

1067

guarantees. If there <I>are</I> unforeseen problems, liability is the

1068

user's rather than the developers', and there is nobody you can sue for damages.

1069

<P>

1070

Python does few

1071

date manipulations, and what it does is all based on the Unix

1072

representation for time (even on non-Unix systems) which uses seconds

1073

since 1970 and won't overflow until 2038.

1074

<P>

1075

1076

<A HREF="faqw.py?req=edit&file=faq02.011.htp">Edit this entry</A> /

1077

1078

1079

/ Last changed on Mon Jan 8 17:19:32 2001 by

1080

<A HREF="mailto:sholden@holdenweb.com">Steve Holden</A>

1081

<P>

1082

1083

<HR>

1084

<H2><A NAME="2.12">2.12. Is Python a good language in a class for beginning programmers?</A></H2>

1085

Yes. This long answer attempts to address any concerns you might

1086

have with teaching Python as a programmer's first language.

1087

(If you want to discuss Python's use in education, then

1088

you may be interested in joining the edu-sig mailinglist.

1089

See <A HREF="http://www.python.org/sigs/edu-sig">http://www.python.org/sigs/edu-sig</A>/ )

1090

<P>

1091

It is still common to start students with a procedural

1092

(subset of a) statically typed language such as Pascal, C, or

1093

a subset of C++ or Java. I think that students may be better

1094

served by learning Python as their first language. Python has

1095

a very simple and consistent syntax and a large standard library.

1096

Most importantly, using Python in a beginning programming course

1097

permits students to concentrate on important programming skills,

1098

such as problem decomposition and data type design.

1099

<P>

1100

With Python, students can be quickly introduced to basic concepts

1101

such as loops and procedures. They can even probably work with

1102

user-defined objects in their very first course. They could

1103

implement a tree structure as nested Python lists, for example.

1104

They could be introduced to objects in their first course if

1105

desired. For a student who has never programmed before, using

1106

a statically typed language seems unnatural. It presents

1107

additional complexity that the student must master and slows

1108

the pace of the course. The students are trying to learn to

1109

think like a computer, decompose problems, design consistent

1110

interfaces, and encapsulate data. While learning to use a

1111

statically typed language is important, it is not necessarily the

1112

best topic to address in the students' first programming course.

1113

<P>

1114

Many other aspects of Python make it a good first language.

1115

Python has a large standard library (like Java) so that

1116

students can be assigned programming projects very early in the

1117

course that <I>do</I> something. Assignments aren't restricted to the

1118

standard four-function calculator and check balancing programs.

1119

By using the standard library, students can gain the satisfaction

1120

of working on realistic applications as they learn the fundamentals

1121

of programming. Using the standard library also teaches students

1122

about code reuse.

1123

<P>

1124

Python's interactive interpreter also enables students to

1125

test language features while they're programming. They can keep

1126

a window with the interpreter running while they enter their

1127

programs' source in another window. If they can't remember the

1128

methods for a list, they can do something like this:

1129

<P>

1130

<PRE>

1131

>>> L = []

1132

>>> dir(L)

1133

['append', 'count', 'extend', 'index', 'insert', 'pop', 'remove',

1134

'reverse', 'sort']

1135

>>> print L.append.__doc__

1136

L.append(object) -- append object to end

1137

>>> L.append(1)

1138

>>> L

1139

[1]

1140

</PRE>

1141

With the interpreter, documentation is never far from the

1142

student as he's programming.

1143

<P>

1144

There are also good IDEs for Python. Guido van Rossum's IDLE

1145

is a cross-platform IDE for Python that is written in Python

1146

using Tk. There is also a Windows specific IDE called PythonWin.

1147

Emacs users will be happy to know that there is a very good Python

1148

mode for Emacs. All of these programming environments provide

1149

syntax highlighting, auto-indenting, and access to the interactive

1150

interpreter while coding. For more information about IDEs, see XXX.

1151

<P>

1152

If your department is currently using Pascal because it was

1153

designed to be a teaching language, then you'll be happy to

1154

know that Guido van Rossum designed Python to be simple to

1155

teach to everyone but powerful enough to implement real world

1156

applications. Python makes a good language for first time

1157

programmers because that was one of Python's design goals.

1158

There are papers at <A HREF="http://www.python.org/doc/essays">http://www.python.org/doc/essays</A>/ on the Python website

1159

by Python's creator explaining his objectives for the language.

1160

One that may interest you is titled "Computer Programming for Everybody"

1161

<A HREF="http://www.python.org/doc/essays/cp4e.html">http://www.python.org/doc/essays/cp4e.html</A>

1162

<P>

1163

If you're seriously considering Python as a language for your

1164

school, Guido van Rossum may even be willing to correspond with

1165

you about how the language would fit in your curriculum.

1166

See <A HREF="http://www.python.org/doc/FAQ.html#2.2">http://www.python.org/doc/FAQ.html#2.2</A> for examples of

1167

Python's use in the "real world."

1168

<P>

1169

While Python, its source code, and its IDEs are freely

1170

available, this consideration should not rule

1171

out other languages. There are other free languages (Java,

1172

free C compilers), and many companies are willing to waive some

1173

or all of their fees for student programming tools if it

1174

guarantees that a whole graduating class will know how to

1175

use their tools. That is, if one of the requirements for

1176

the language that will be taught is that it be freely

1177

available, then Python qualifies, but this requirement

1178

does not preclude other languages.

1179

<P>

1180

While Python jobs may not be as prevalent as C/C++/Java jobs,

1181

teachers should not worry about teaching students critical job

1182

skills in their first course. The skills that win students a

1183

job are those they learn in their senior classes and internships.

1184

Their first programming courses are there to lay a solid

1185

foundation in programming fundamentals. The primary question

1186

in choosing the language for such a course should be which

1187

language permits the students to learn this material without

1188

hindering or limiting them.

1189

<P>

1190

Another argument for Python is that there are many tasks for

1191

which something like C++ is overkill. That's where languages

1192

like Python, Perl, Tcl, and Visual Basic thrive. It's critical

1193

for students to know something about these languages. (Every

1194

employer for whom I've worked used at least one such language.)

1195

Of the languages listed above, Python probably makes the best

1196

language in a programming curriculum since its syntax is simple,

1197

consistent, and not unlike other languages (C/C++/Java) that

1198

are probably in the curriculum. By starting students with

1199

Python, a department simultaneously lays the foundations for

1200

other programming courses and introduces students to the type

1201

of language that is often used as a "glue" language. As an

1202

added bonus, Python can be used to interface with Microsoft's

1203

COM components (thanks to Mark Hammond). There is also Jython,

1204

a Java implementation of the Python interpreter, that can be

1205

used to connect Java components.

1206

<P>

1207

If you currently start students with Pascal or C/C++ or Java,

1208

you may be worried they will have trouble learning a statically

1209

typed language after starting with Python. I think that this

1210

fear most often stems from the fact that the teacher started

1211

with a statically typed language, and we tend to like to teach

1212

others in the same way we were taught. In reality, the

1213

transition from Python to one of these other languages is

1214

quite simple.

1215

<P>

1216

To motivate a statically typed language such as C++, begin the

1217

course by explaining that unlike Python, their first language,

1218

C++ is compiled to a machine dependent executable. Explain

1219

that the point is to make a very fast executable. To permit

1220

the compiler to make optimizations, programmers must help it

1221

by specifying the "types" of variables. By restricting each

1222

variable to a specific type, the compiler can reduce the

1223

book-keeping it has to do to permit dynamic types. The compiler

1224

also has to resolve references at compile time. Thus, the

1225

language gains speed by sacrificing some of Python's dynamic

1226

features. Then again, the C++ compiler provides type safety

1227

and catches many bugs at compile time instead of run time (a

1228

critical consideration for many commercial applications). C++

1229

is also designed for very large programs where one may want to

1230

guarantee that others don't touch an object's implementation.

1231

C++ provides very strong language features to separate an object's

1232

implementation from its interface. Explain why this separation

1233

is a good thing.

1234

<P>

1235

The first day of a C++ course could then be a whirlwind introduction

1236

to what C++ requires and provides. The point here is that after

1237

a semester or two of Python, students are hopefully competent

1238

programmers. They know how to handle loops and write procedures.

1239

They've also worked with objects, thought about the benefits of

1240

consistent interfaces, and used the technique of subclassing to

1241

specialize behavior. Thus, a whirlwind introduction to C++ could

1242

show them how objects and subclassing looks in C++. The

1243

potentially difficult concepts of object-oriented design were

1244

taught without the additional obstacles presented by a language

1245

such as C++ or Java. When learning one of these languages,

1246

the students would already understand the "road map." They

1247

understand objects; they would just be learning how objects

1248

fit in a statically typed languages. Language requirements

1249

and compiler errors that seem unnatural to beginning programmers

1250

make sense in this new context. Many students will find it

1251

helpful to be able to write a fast prototype of their algorithms

1252

in Python. Thus, they can test and debug their ideas before

1253

they attempt to write the code in the new language, saving the

1254

effort of working with C++ types for when they've discovered a

1255

working solution for their assignments. When they get annoyed

1256

with the rigidity of types, they'll be happy to learn about

1257

containers and templates to regain some of the lost flexibility

1258

Python afforded them. Students may also gain an appreciation

1259

for the fact that no language is best for every task. They'll

1260

see that C++ is faster, but they'll know that they can gain

1261

flexibility and development speed with a Python when execution

1262

speed isn't critical.

1263

<P>

1264

If you have any concerns that weren't addressed here, try

1265

posting to the Python newsgroup. Others there have done some

1266

work with using Python as an instructional tool. Good luck.

1267

We'd love to hear about it if you choose Python for your course.

1268

<P>

1269

1270

<A HREF="faqw.py?req=edit&file=faq02.012.htp">Edit this entry</A> /

1271

1272

1273

/ Last changed on Mon Dec 2 19:32:35 2002 by

1274

<A HREF="mailto:sconce@in-spec-inc.com">Bill Sconce</A>

1275

<P>

1276

1277

<HR>

1278

<H1>3. Building Python and Other Known Bugs</H1>

1279

1280

<HR>

1281

<H2><A NAME="3.1">3.1. Is there a test set?</A></H2>

1282

Sure. You can run it after building with "make test", or you can

1283

run it manually with this command at the Python prompt:

1284

<P>

1285

<PRE>

1286

import test.autotest

1287

</PRE>

1288

In Python 1.4 or earlier, use

1289

<P>

1290

<PRE>

1291

import autotest

1292

</PRE>

1293

The test set doesn't test <I>all</I> features of Python,

1294

but it goes a long way to confirm that Python is actually working.

1295

<P>

1296

NOTE: if "make test" fails, don't just mail the output to the

1297

newsgroup -- this doesn't give enough information to debug the

1298

problem. Instead, find out which test fails, and run that test

1299

manually from an interactive interpreter. For example, if

1300

"make test" reports that test_spam fails, try this interactively:

1301

<P>

1302

<PRE>

1303

import test.test_spam

1304

</PRE>

1305

This generally produces more verbose output which can be diagnosed

1306

to debug the problem. If you find a bug in Python or the libraries, or in the tests, please report this in the Python bug tracker at SourceForge:

1307

<P>

1308

<A HREF="http://sourceforge.net/tracker/?func=add&group_id=5470&atid=105470">http://sourceforge.net/tracker/?func=add&group_id=5470&atid=105470</A>

1309

<P>

1310

1311

<A HREF="faqw.py?req=edit&file=faq03.001.htp">Edit this entry</A> /

1312

1313

1314

/ Last changed on Fri Apr 27 10:29:36 2001 by

1315

<A HREF="mailto:fdrake@acm.org">Fred Drake</A>

1316

<P>

1317

1318

<HR>

1319

<H2><A NAME="3.2">3.2. When running the test set, I get complaints about floating point operations, but when playing with floating point operations I cannot find anything wrong with them.</A></H2>

1320

The test set makes occasional unwarranted assumptions about the

1321

semantics of C floating point operations. Until someone donates a

1322

better floating point test set, you will have to comment out the

1323

offending floating point tests and execute similar tests manually.

1324

<P>

1325

1326

<A HREF="faqw.py?req=edit&file=faq03.002.htp">Edit this entry</A> /

1327

1328

<P>

1329

1330

<HR>

1331

<H2><A NAME="3.3">3.3. Link errors after rerunning the configure script.</A></H2>

1332

It is generally necessary to run "make clean" after a configuration

1333

change.

1334

<P>

1335

1336

<A HREF="faqw.py?req=edit&file=faq03.003.htp">Edit this entry</A> /

1337

1338

<P>

1339

1340

<HR>

1341

<H2><A NAME="3.4">3.4. The python interpreter complains about options passed to a script (after the script name).</A></H2>

1342

You are probably linking with GNU getopt, e.g. through -liberty.

1343

Don't. The reason for the complaint is that GNU getopt, unlike System

1344

V getopt and other getopt implementations, doesn't consider a

1345

non-option to be the end of the option list. A quick (and compatible)

1346

fix for scripts is to add "--" to the interpreter, like this:

1347

<P>

1348

<PRE>

1349

#! /usr/local/bin/python --

1350

</PRE>

1351

You can also use this interactively:

1352

<P>

1353

<PRE>

1354

python -- script.py [options]

1355

</PRE>

1356

Note that a working getopt implementation is provided in the Python

1357

distribution (in Python/getopt.c) but not automatically used.

1358

<P>

1359

1360

<A HREF="faqw.py?req=edit&file=faq03.004.htp">Edit this entry</A> /

1361

1362

<P>

1363

1364

<HR>

1365

<H2><A NAME="3.5">3.5. When building on the SGI, make tries to run python to create glmodule.c, but python hasn't been built or installed yet.</A></H2>

1366

Comment out the line mentioning glmodule.c in Setup and build a

1367

python without gl first; install it or make sure it is in your $PATH,

1368

then edit the Setup file again to turn on the gl module, and make

1369

again. You don't need to do "make clean"; you do need to run "make

1370

Makefile" in the Modules subdirectory (or just run "make" at the

1371

toplevel).

1372

<P>

1373

1374

<A HREF="faqw.py?req=edit&file=faq03.005.htp">Edit this entry</A> /

1375

1376

<P>

1377

1378

<HR>

1379

<H2><A NAME="3.6">3.6. I use VPATH but some targets are built in the source directory.</A></H2>

1380

On some systems (e.g. Sun), if the target already exists in the

1381

source directory, it is created there instead of in the build

1382

directory. This is usually because you have previously built without

1383

VPATH. Try running "make clobber" in the source directory.

1384

<P>

1385

1386

<A HREF="faqw.py?req=edit&file=faq03.006.htp">Edit this entry</A> /

1387

1388

<P>

1389

1390

<HR>

1391

<H2><A NAME="3.7">3.7. Trouble building or linking with the GNU readline library.</A></H2>

1392

You can use the GNU readline library to improve the interactive user

1393

interface: this gives you line editing and command history when

1394

calling python interactively. Its sources are distributed with

1395

Python (at least for 2.0). Uncomment the line

1396

<P>

1397

#readline readline.c -lreadline -ltermcap

1398

<P>

1399

in Modules/Setup. The configuration option --with-readline

1400

is no longer supported, at least in Python 2.0. Some hints on

1401

building and using the readline library:

1402

On SGI IRIX 5, you may have to add the following

1403

to rldefs.h:

1404

<P>

1405

<PRE>

1406

#ifndef sigmask

1407

#define sigmask(sig) (1L << ((sig)-1))

1408

#endif

1409

</PRE>

1410

On some systems, you will have to add #include "rldefs.h" to the

1411

top of several source files, and if you use the VPATH feature, you

1412

will have to add dependencies of the form foo.o: foo.c to the

1413

Makefile for several values of foo.

1414

The readline library requires use of the termcap library. A

1415

known problem with this is that it contains entry points which

1416

cause conflicts with the STDWIN and SGI GL libraries. The STDWIN

1417

conflict can be solved by adding a line saying '#define werase w_erase' to the

1418

stdwin.h file (in the STDWIN distribution, subdirectory H). The

1419

GL conflict has been solved in the Python configure script by a

1420

hack that forces use of the static version of the termcap library.

1421

Check the newsgroup gnu.bash.bug news:gnu.bash.bug for

1422

specific problems with the readline library (I don't read this group

1423

but I've been told that it is the place for readline bugs).

1424

<P>

1425

1426

<A HREF="faqw.py?req=edit&file=faq03.007.htp">Edit this entry</A> /

1427

1428

1429

/ Last changed on Sat Dec 2 18:23:48 2000 by

1430

<A HREF="mailto:trotts@llnl.gov">Issac Trotts</A>

1431

<P>

1432

1433

<HR>

1434

<H2><A NAME="3.8">3.8. Trouble with socket I/O on older Linux 1.x versions.</A></H2>

1435

Once you've built Python, use it to run the regen script in the

1436

Lib/plat-linux2 directory. Apparently the files as distributed don't match the system headers on some Linux versions.

1437

<P>

1438

Note that this FAQ entry only applies to Linux kernel versions 1.x.y;

1439

these are hardly around any more.

1440

<P>

1441

1442

<A HREF="faqw.py?req=edit&file=faq03.008.htp">Edit this entry</A> /

1443

1444

1445

/ Last changed on Tue Jul 30 20:05:52 2002 by

1446

<A HREF="mailto:kubieziel@gmx.de">Jens Kubieziel</A>

1447

<P>

1448

1449

<HR>

1450

<H2><A NAME="3.9">3.9. Trouble with prototypes on Ultrix.</A></H2>

1451

Ultrix cc seems broken -- use gcc, or edit config.h to #undef

1452

HAVE_PROTOTYPES.

1453

<P>

1454

1455

<A HREF="faqw.py?req=edit&file=faq03.009.htp">Edit this entry</A> /

1456

1457

<P>

1458

1459

<HR>

1460

<H2><A NAME="3.10">3.10. Other trouble building Python on platform X.</A></H2>

1461

Please submit the details to the SourceForge bug tracker:

1462

<P>

1463

<PRE>

1464

<A HREF="http://sourceforge.net/tracker/?group_id=5470&atid=105470">http://sourceforge.net/tracker/?group_id=5470&atid=105470</A>

1465

</PRE>

1466

and we'll look

1467

into it. Please provide as many details as possible. In particular,

1468

if you don't tell us what type of computer and what operating system

1469

(and version) you are using it will be difficult for us to figure out

1470

what is the matter. If you have compilation output logs,

1471

please use file uploads -- don't paste everything in the message box.

1472

<P>

1473

In many cases, we won't have access to the same hardware or operating system version, so <I>please</I>, if you have a SourceForge account, log in before filing your report, or if you don't have an account, include an email address at which we can reach you for further questions. Logging in to SourceForge first will also cause SourceForge to send you updates as we act on your report.

1474

<P>

1475

1476

<A HREF="faqw.py?req=edit&file=faq03.010.htp">Edit this entry</A> /

1477

1478

1479

/ Last changed on Fri Apr 27 10:53:18 2001 by

1480

<A HREF="mailto:fdrake@acm.org">Fred Drake</A>

1481

<P>

1482

1483

<HR>

1484

<H2><A NAME="3.11">3.11. How to configure dynamic loading on Linux.</A></H2>

1485

This is now automatic as long as your Linux version uses the ELF

1486

object format (all recent Linuxes do).

1487

<P>

1488

1489

<A HREF="faqw.py?req=edit&file=faq03.011.htp">Edit this entry</A> /

1490

1491

<P>

1492

1493

<HR>

1494

<H2><A NAME="3.12">3.12. I can't get shared modules to work on Linux 2.0 (Slackware96)?</A></H2>

1495

This is a bug in the Slackware96 release. The fix is simple: Make sure

1496

that there is a link from /lib/libdl.so to /lib/libdl.so.1 so that the

1497

following links are setup: /lib/libdl.so -> /lib/libdl.so.1

1498

/lib/libdl.so.1 -> /lib/libdl.so.1.7.14 You may have to rerun the

1499

configure script, after rm'ing the config.cache file, before you

1500

attempt to rebuild python after this fix.

1501

<P>

1502

1503

<A HREF="faqw.py?req=edit&file=faq03.012.htp">Edit this entry</A> /

1504

1505

1506

/ Last changed on Wed May 21 15:45:03 1997 by

1507

1508

<P>

1509

1510

<HR>

1511

<H2><A NAME="3.13">3.13. Trouble when making modules shared on Linux.</A></H2>

1512

This happens when you have built Python for static linking and then

1513

enable

1514

<PRE>

1515

*shared*

1516

</PRE>

1517

in the Setup file. Shared library code must be

1518

compiled with "-fpic". If a .o file for the module already exist that

1519

was compiled for static linking, you must remove it or do "make clean"

1520

in the Modules directory.

1521

<P>

1522

1523

<A HREF="faqw.py?req=edit&file=faq03.013.htp">Edit this entry</A> /

1524

1525

1526

/ Last changed on Fri May 23 13:42:30 1997 by

1527

1528

<P>

1529

1530

<HR>

1531

<H2><A NAME="3.14">3.14. [deleted]</A></H2>

1532

[ancient information on threads on linux (when thread support

1533

was not standard) used to be here]

1534

<P>

1535

1536

<A HREF="faqw.py?req=edit&file=faq03.014.htp">Edit this entry</A> /

1537

1538

1539

/ Last changed on Sun Jun 2 17:27:13 2002 by

1540

<A HREF="mailto:erno-pyfaq@erno.iki.fi">Erno Kuusela</A>

1541

<P>

1542

1543

<HR>

1544

<H2><A NAME="3.15">3.15. Errors when linking with a shared library containing C++ code.</A></H2>

1545

Link the main Python binary with C++. Change the definition of

1546

LINKCC in Modules/Makefile to be your C++ compiler. You may have to

1547

edit config.c slightly to make it compilable with C++.

1548

<P>

1549

1550

<A HREF="faqw.py?req=edit&file=faq03.015.htp">Edit this entry</A> /

1551

1552

<P>

1553

1554

<HR>

1555

<H2><A NAME="3.16">3.16. Deleted</A></H2>

1556

<P>

1557

<P>

1558

1559

<A HREF="faqw.py?req=edit&file=faq03.016.htp">Edit this entry</A> /

1560

1561

1562

/ Last changed on Tue Sep 11 16:02:22 2001 by

1563

1564

<P>

1565

1566

<HR>

1567

<H2><A NAME="3.17">3.17. Deleted.</A></H2>

1568

<P>

1569

<P>

1570

1571

<A HREF="faqw.py?req=edit&file=faq03.017.htp">Edit this entry</A> /

1572

1573

1574

/ Last changed on Tue Sep 11 15:54:57 2001 by

1575

1576

<P>

1577

1578

<HR>

1579

<H2><A NAME="3.18">3.18. Compilation or link errors for the _tkinter module</A></H2>

1580

Most likely, there's a version mismatch between the Tcl/Tk header

1581

files (tcl.h and tk.h) and the Tcl/Tk libraries you are using e.g.

1582

"-ltk8.0" and "-ltcl8.0" arguments for _tkinter in the Setup file).

1583

It is possible to install several versions of the Tcl/Tk libraries,

1584

but there can only be one version of the tcl.h and tk.h header

1585

files. If the library doesn't match the header, you'll get

1586

problems, either when linking the module, or when importing it.

1587

Fortunately, the version number is clearly stated in each file,

1588

so this is easy to find. Reinstalling and using the latest

1589

version usually fixes the problem.

1590

<P>

1591

(Also note that when compiling unpatched Python 1.5.1 against

1592

Tcl/Tk 7.6/4.2 or older, you get an error on Tcl_Finalize. See

1593

the 1.5.1 patch page at <A HREF="http://www.python.org/1.5/patches-1.5.1">http://www.python.org/1.5/patches-1.5.1</A>/.)

1594

<P>

1595

1596

<A HREF="faqw.py?req=edit&file=faq03.018.htp">Edit this entry</A> /

1597

1598

1599

/ Last changed on Thu Jun 11 00:49:14 1998 by

1600

1601

<P>

1602

1603

<HR>

1604

<H2><A NAME="3.19">3.19. I configured and built Python for Tcl/Tk but "import Tkinter" fails.</A></H2>

1605

Most likely, you forgot to enable the line in Setup that says

1606

"TKPATH=:$(DESTLIB)/tkinter".

1607

<P>

1608

1609

<A HREF="faqw.py?req=edit&file=faq03.019.htp">Edit this entry</A> /

1610

1611

<P>

1612

1613

<HR>

1614

<H2><A NAME="3.20">3.20. [deleted]</A></H2>

1615

[ancient information on a gcc+tkinter bug on alpha was here]

1616

<P>

1617

1618

<A HREF="faqw.py?req=edit&file=faq03.020.htp">Edit this entry</A> /

1619

1620

1621

/ Last changed on Mon Jun 3 16:46:23 2002 by

1622

<A HREF="mailto:erno-pyfaq@erno.iki.fi">Erno Kuusela</A>

1623

<P>

1624

1625

<HR>

1626

<H2><A NAME="3.21">3.21. Several common system calls are missing from the posix module.</A></H2>

1627

Most likely, <I>all</I> test compilations run by the configure script

1628

are failing for some reason or another. Have a look in config.log to

1629

see what could be the reason. A common reason is specifying a

1630

directory to the --with-readline option that doesn't contain the

1631

libreadline.a file.

1632

<P>

1633

1634

<A HREF="faqw.py?req=edit&file=faq03.021.htp">Edit this entry</A> /

1635

1636

<P>

1637

1638

<HR>

1639

<H2><A NAME="3.22">3.22. ImportError: No module named string, on MS Windows.</A></H2>

1640

Most likely, your PYTHONPATH environment variable should be set to

1641

something like:

1642

<P>

1643

set PYTHONPATH=c:\python;c:\python\lib;c:\python\scripts

1644

<P>

1645

(assuming Python was installed in c:\python)

1646

<P>

1647

1648

<A HREF="faqw.py?req=edit&file=faq03.022.htp">Edit this entry</A> /

1649

1650

<P>

1651

1652

<HR>

1653

<H2><A NAME="3.23">3.23. Core dump on SGI when using the gl module.</A></H2>

1654

There are conflicts between entry points in the termcap and curses

1655

libraries and an entry point in the GL library. There's a hack of a

1656

fix for the termcap library if it's needed for the GNU readline

1657

library, but it doesn't work when you're using curses. Concluding,

1658

you can't build a Python binary containing both the curses and gl

1659

modules.

1660

<P>

1661

1662

<A HREF="faqw.py?req=edit&file=faq03.023.htp">Edit this entry</A> /

1663

1664

<P>

1665

1666

<HR>

1667

<H2><A NAME="3.24">3.24. "Initializer not a constant" while building DLL on MS-Windows</A></H2>

1668

Static type object initializers in extension modules may cause compiles to

1669

fail with an error message like "initializer not a constant".

1670

Fredrik Lundh <<A HREF="mailto:Fredrik.Lundh@image.combitech.se">Fredrik.Lundh@image.combitech.se</A>> explains:

1671

<P>

1672

This shows up when building DLL under MSVC. There's two ways to

1673

address this: either compile the module as C++, or change your code to

1674

something like:

1675

<P>

1676

<PRE>

1677

statichere PyTypeObject bstreamtype = {

1678

PyObject_HEAD_INIT(NULL) /* must be set by init function */

1679

0,

1680

"bstream",

1681

sizeof(bstreamobject),

1682

</PRE>

1683

<PRE>

1684

...

1685

</PRE>

1686

<PRE>

1687

void

1688

initbstream()

1689

{

1690

/* Patch object type */

1691

bstreamtype.ob_type = &PyType_Type;

1692

Py_InitModule("bstream", functions);

1693

...

1694

}

1695

</PRE>

1696

<P>

1697

1698

<A HREF="faqw.py?req=edit&file=faq03.024.htp">Edit this entry</A> /

1699

1700

1701

/ Last changed on Sun May 25 14:58:05 1997 by

1702

<A HREF="mailto:aaron_watters@msn.com">Aaron Watters</A>

1703

<P>

1704

1705

<HR>

1706

<H2><A NAME="3.25">3.25. Output directed to a pipe or file disappears on Linux.</A></H2>

1707

Some people have reported that when they run their script

1708

interactively, it runs great, but that when they redirect it

1709

to a pipe or file, no output appears.

1710

<P>

1711

<PRE>

1712

% python script.py

1713

...some output...

1714

% python script.py >file

1715

% cat file

1716

% # no output

1717

% python script.py | cat

1718

% # no output

1719

%

1720

</PRE>

1721

This was a bug in Linux kernel. It is fixed and should not appear anymore. So most Linux users are <I>not</I> affected by this.

1722

<P>

1723

If redirection doesn't work on your Linux system, check what shell you are using. Shells like (t)csh doesn't support redirection.

1724

<P>

1725

1726

<A HREF="faqw.py?req=edit&file=faq03.025.htp">Edit this entry</A> /

1727

1728

1729

/ Last changed on Thu Jan 16 13:38:30 2003 by

1730

<A HREF="mailto:kubieziel@gmx.de">Jens Kubieziel</A>

1731

<P>

1732

1733

<HR>

1734

<H2><A NAME="3.26">3.26. [deleted]</A></H2>

1735

[ancient libc/linux problem was here]

1736

<P>

1737

1738

<A HREF="faqw.py?req=edit&file=faq03.026.htp">Edit this entry</A> /

1739

1740

1741

/ Last changed on Mon Jun 3 16:48:08 2002 by

1742

<A HREF="mailto:erno-pyfaq@erno.iki.fi">Erno Kuusela</A>

1743

<P>

1744

1745

<HR>

1746

<H2><A NAME="3.27">3.27. [deleted]</A></H2>

1747

[ancient linux + threads + tk problem was described here]

1748

<P>

1749

1750

<A HREF="faqw.py?req=edit&file=faq03.027.htp">Edit this entry</A> /

1751

1752

1753

/ Last changed on Mon Jun 3 16:49:08 2002 by

1754

<A HREF="mailto:erno-pyfaq@erno.iki.fi">Erno Kuusela</A>

1755

<P>

1756

1757

<HR>

1758

<H2><A NAME="3.28">3.28. How can I test if Tkinter is working?</A></H2>

1759

Try the following:

1760

<P>

1761

<PRE>

1762

python

1763

>>> import _tkinter

1764

>>> import Tkinter

1765

>>> Tkinter._test()

1766

</PRE>

1767

This should pop up a window with two buttons,

1768

one "Click me" and one "Quit".

1769

<P>

1770

If the first statement (import _tkinter) fails, your Python

1771

installation probably has not been configured to support Tcl/Tk.

1772

On Unix, if you have installed Tcl/Tk, you have to rebuild Python

1773

after editing the Modules/Setup file to enable the _tkinter module

1774

and the TKPATH environment variable.

1775

<P>

1776

It is also possible to get complaints about Tcl/Tk version

1777

number mismatches or missing TCL_LIBRARY or TK_LIBRARY

1778

environment variables. These have to do with Tcl/Tk installation

1779

problems.

1780

<P>

1781

A common problem is to have installed versions of tcl.h and tk.h

1782

that don't match the installed version of the Tcl/Tk libraries;

1783

this usually results in linker errors or (when using dynamic

1784

loading) complaints about missing symbols during loading

1785

the shared library.

1786

<P>

1787

1788

<A HREF="faqw.py?req=edit&file=faq03.028.htp">Edit this entry</A> /

1789

1790

1791

/ Last changed on Thu Aug 28 17:01:46 1997 by

1792

<A HREF="mailto:guido@python.org">Guido van Rossum</A>

1793

<P>

1794

1795

<HR>

1796

<H2><A NAME="3.29">3.29. Is there a way to get the interactive mode of the python interpreter to perform function/variable name completion?</A></H2>

1797

(From a posting by Guido van Rossum)

1798

<P>

1799

On Unix, if you have enabled the readline module (i.e. if Emacs-style

1800

command line editing and bash-style history works for you), you can

1801

add this by importing the undocumented standard library module

1802

"rlcompleter". When completing a simple identifier, it

1803

completes keywords, built-ins and globals in __main__; when completing

1804

NAME.NAME..., it evaluates (!) the expression up to the last dot and

1805

completes its attributes.

1806

<P>

1807

This way, you can do "import string", type "string.", hit the

1808

completion key twice, and see the list of names defined by the

1809

string module.

1810

<P>

1811

Tip: to use the tab key as the completion key, call

1812

<P>

1813

<PRE>

1814

readline.parse_and_bind("tab: complete")

1815

</PRE>

1816

You can put this in a ~/.pythonrc file, and set the PYTHONSTARTUP

1817

environment variable to ~/.pythonrc. This will cause the completion to be enabled

1818

whenever you run Python interactively.

1819

<P>

1820

Notes (see the docstring for rlcompleter.py for more information):

1821

<P>

1822

* The evaluation of the NAME.NAME... form may cause arbitrary

1823

application defined code to be executed if an object with a

1824

__getattr__ hook is found. Since it is the responsibility of the

1825

application (or the user) to enable this feature, I consider this an

1826

acceptable risk. More complicated expressions (e.g. function calls or

1827

indexing operations) are <I>not</I> evaluated.

1828

<P>

1829

* GNU readline is also used by the built-in functions input() and

1830

raw_input(), and thus these also benefit/suffer from the complete

1831

features. Clearly an interactive application can benefit by

1832

specifying its own completer function and using raw_input() for all

1833

its input.

1834

<P>

1835

* When stdin is not a tty device, GNU readline is never

1836

used, and this module (and the readline module) are silently inactive.

1837

<P>

1838

1839

<A HREF="faqw.py?req=edit&file=faq03.029.htp">Edit this entry</A> /

1840

1841

1842

/ Last changed on Fri Jun 12 09:55:24 1998 by

1843

<A HREF="mailto:akuchlin@cnri.reston.va.us">A.M. Kuchling</A>

1844

<P>

1845

1846

<HR>

1847

<H2><A NAME="3.30">3.30. Why is the Python interpreter not built as a shared library?</A></H2>

1848

(This is a Unix question; on Mac and Windows, it <I>is</I> a shared

1849

library.)

1850

<P>

1851

It's just a nightmare to get this to work on all different platforms.

1852

Shared library portability is a pain. And yes, I know about GNU libtool

1853

-- but it requires me to use its conventions for filenames etc, and it

1854

would require a complete and utter rewrite of all the makefile and

1855

config tools I'm currently using.

1856

<P>

1857

In practice, few applications embed Python -- it's much more common to

1858

have Python extensions, which already are shared libraries. Also,

1859

serious embedders often want total control over which Python version

1860

and configuration they use so they wouldn't want to use a standard

1861

shared library anyway. So while the motivation of saving space

1862

when lots of apps embed Python is nice in theory, I

1863

doubt that it will save much in practice. (Hence the low priority I

1864

give to making a shared library.)

1865

<P>

1866

For Linux systems, the simplest method of producing libpython1.5.so seems to

1867

be (originally from the Minotaur project web page,

1868

<A HREF="http://www.equi4.com/minotaur/minotaur.html">http://www.equi4.com/minotaur/minotaur.html</A>):

1869

<P>

1870

<PRE>

1871

make distclean

1872

./configure

1873

make OPT="-fpic -O2"

1874

mkdir .extract

1875

(cd .extract; ar xv ../libpython1.5.a)

1876

gcc -shared -o libpython1.5.so .extract/*.o

1877

rm -rf .extract

1878

</PRE>

1879

In Python 2.3 this will be supported by the standard build routine

1880

(at least on Linux) with --enable-shared. Note however that there

1881

is little advantage, and it slows down Python because of the need

1882

for PIC code and the extra cost at startup time to find the library.

1883

<P>

1884

1885

<A HREF="faqw.py?req=edit&file=faq03.030.htp">Edit this entry</A> /

1886

1887

1888

/ Last changed on Thu May 30 13:36:55 2002 by

1889

1890

<P>

1891

1892

<HR>

1893

<H2><A NAME="3.31">3.31. Build with GCC on Solaris 2.6 (SunOS 5.6) fails</A></H2>

1894

If you have upgraded Solaris 2.5 or 2.5.1 to Solaris 2.6,

1895

but you have not upgraded

1896

your GCC installation, the compile may fail, e.g. like this:

1897

<P>

1898

<PRE>

1899

In file included from /usr/include/sys/stream.h:26,

1900

from /usr/include/netinet/in.h:38,

1901

from /usr/include/netdb.h:96,

1902

from ./socketmodule.c:121:

1903

/usr/include/sys/model.h:32: #error "No DATAMODEL_NATIVE specified"

1904

</PRE>

1905

Solution: rebuild GCC for Solaris 2.6.

1906

You might be able to simply re-run fixincludes, but

1907

people have had mixed success with doing that.

1908

<P>

1909

1910

<A HREF="faqw.py?req=edit&file=faq03.031.htp">Edit this entry</A> /

1911

1912

1913

/ Last changed on Wed Oct 21 11:18:46 1998 by

1914

1915

<P>

1916

1917

<HR>

1918

<H2><A NAME="3.32">3.32. Running "make clean" seems to leave problematic files that cause subsequent builds to fail.</A></H2>

1919

Use "make clobber" instead.

1920

<P>

1921

Use "make clean" to reduce the size of the source/build directory

1922

after you're happy with your build and installation.

1923

If you have already tried to build python and you'd like to start

1924

over, you should use "make clobber". It does a "make clean" and also

1925

removes files such as the partially built Python library from a previous build.

1926

<P>

1927

1928

<A HREF="faqw.py?req=edit&file=faq03.032.htp">Edit this entry</A> /

1929

1930

1931

/ Last changed on Thu Jun 24 20:39:26 1999 by

1932

1933

<P>

1934

1935

<HR>

1936

<H2><A NAME="3.33">3.33. Submitting bug reports and patches</A></H2>

1937

To report a bug or submit a patch, please use the relevant service

1938

from the Python project at SourceForge.

1939

<P>

1940

Bugs: <A HREF="http://sourceforge.net/tracker/?group_id=5470&atid=105470">http://sourceforge.net/tracker/?group_id=5470&atid=105470</A>

1941

<P>

1942

Patches: <A HREF="http://sourceforge.net/tracker/?group_id=5470&atid=305470">http://sourceforge.net/tracker/?group_id=5470&atid=305470</A>

1943

<P>

1944

If you have a SourceForge account, please log in before submitting your bug report; this will make it easier for us to contact you regarding your report in the event we have follow-up questions. It will also enable SourceForge to send you update information as we act on your bug. If you do not have a SourceForge account, please consider leaving your name and email address as part of the report.

1945

<P>

1946

1947

<A HREF="faqw.py?req=edit&file=faq03.033.htp">Edit this entry</A> /

1948

1949

1950

/ Last changed on Fri Apr 27 10:58:26 2001 by

1951

<A HREF="mailto:fdrake@acm.org">Fred Drake</A>

1952

<P>

1953

1954

<HR>

1955

<H2><A NAME="3.34">3.34. I can't load shared libraries under Python 1.5.2, Solaris 7, and gcc 2.95.2</A></H2>

1956

When trying to load shared libraries, you may see errors like:

1957

ImportError: ld.so.1: python: fatal: relocation error: file /usr/local/lib/python1.5/site-packages/Perp/util/du_SweepUtilc.so:

1958

<PRE>

1959

symbol PyExc_RuntimeError: referenced symbol not found

1960

</PRE>

1961

<P>

1962

There is a problem with the configure script for Python 1.5.2

1963

under Solaris 7 with gcc 2.95 . configure should set the make variable

1964

LINKFORSHARED=-Xlinker -export-dynamic

1965

<P>

1966

<P>

1967

in Modules/Makefile,

1968

<P>

1969

Manually add this line to the Modules/Makefile.

1970

This builds a Python executable that can load shared library extensions (xxx.so) .

1971

<P>

1972

1973

<A HREF="faqw.py?req=edit&file=faq03.034.htp">Edit this entry</A> /

1974

1975

1976

/ Last changed on Mon Feb 19 10:37:05 2001 by

1977

1978

<P>

1979

1980

<HR>

1981

<H2><A NAME="3.35">3.35. In the regression test, test___all__ fails for the profile module. What's wrong?</A></H2>

1982

If you have been using the profile module, and have properly calibrated a copy of the module as described in the documentation for the profiler:

1983

<P>

1984

<A HREF="http://www.python.org/doc/current/lib/profile-calibration.html">http://www.python.org/doc/current/lib/profile-calibration.html</A>

1985

<P>

1986

then it is possible that the regression test "test___all__" will fail if you run the regression test manually rather than using "make test" in the Python source directory. This will happen if you have set your PYTHONPATH environment variable to include the directory containing your calibrated profile module. You have probably calibrated the profiler using an older version of the profile module which does not define the __all__ value, added to the module as of Python 2.1.

1987

<P>

1988

The problem can be fixed by removing the old calibrated version of the profile module and using the latest version to do a fresh calibration. In general, you will need to re-calibrate for each version of Python anyway, since the performance characteristics can change in subtle ways that impact profiling.

1989

<P>

1990

1991

<A HREF="faqw.py?req=edit&file=faq03.035.htp">Edit this entry</A> /

1992

1993

1994

/ Last changed on Fri Apr 27 10:44:10 2001 by

1995

<A HREF="mailto:fdrake@acm.org">Fred Drake</A>

1996

<P>

1997

1998

<HR>

1999

<H2><A NAME="3.36">3.36. relocations remain against allocatable but non-writable sections</A></H2>

2000

This linker error occurs on Solaris if you attempt to build an extension module which incorporates position-dependent (non-PIC) code. A common source of problems is that a static library (.a file), such as libreadline.a or libcrypto.a is linked with the extension module. The error specifically occurs when using gcc as the compiler, but /usr/ccs/bin/ld as the linker.

2001

<P>

2002

The following solutions and work-arounds are known:

2003

<P>

2004

1. Rebuild the libraries (libreadline, libcrypto) with -fPIC (-KPIC if using the system compiler). This is recommended; all object files in a shared library should be position-independent.

2005

<P>

2006

2. Statically link the extension module and its libraries into the Python interpreter, by editing Modules/Setup.

2007

<P>

2008

3. Use GNU ld instead of /usr/ccs/bin/ld; GNU ld will accept non-PIC code in shared libraries (and mark the section writable)

2009

<P>

2010

4. Pass -mimpure-text to GCC when linking the module. This will force gcc to not pass -z text to ld; in turn, ld will make all text sections writable.

2011

<P>

2012

Options 3 and 4 are not recommended, since the ability to share code across processes is lost.

2013

<P>

2014

2015

<A HREF="faqw.py?req=edit&file=faq03.036.htp">Edit this entry</A> /

2016

2017

2018

/ Last changed on Tue Jan 29 12:05:11 2002 by

2019

<A HREF="mailto:loewis@informatik.hu-berlin.de">Martin v. L�wis</A>

2020

<P>

2021

2022

<HR>

2023

<H1>4. Programming in Python</H1>

2024

2025

<HR>

2026

<H2><A NAME="4.1">4.1. Is there a source code level debugger with breakpoints, step, etc.?</A></H2>

2027

Yes.

2028

<P>

2029

Module pdb is a rudimentary but adequate console-mode debugger for Python. It is part of the standard Python library, and is documented in the Library Reference Manual. (You can also write your own debugger by using the code for pdb as an example.)

2030

<P>

2031

The IDLE interactive development environment, which is part of the standard Python distribution (normally available in Tools/idle), includes a graphical debugger. There is documentation for the IDLE debugger at <A HREF="http://www.python.org/idle/doc/idle2.html#Debugger">http://www.python.org/idle/doc/idle2.html#Debugger</A>

2032

<P>

2033

Pythonwin is a Python IDE that includes a GUI debugger based on bdb. The Pythonwin debugger colors breakpoints and has quite a few cool features (including debugging non-Pythonwin programs). A reference can be found at <A HREF="http://www.python.org/ftp/python/pythonwin/pwindex.html">http://www.python.org/ftp/python/pythonwin/pwindex.html</A>

2034

More recent versions of PythonWin are available as a part of the ActivePython distribution (see <A HREF="http://www.activestate.com/Products/ActivePython/index.html">http://www.activestate.com/Products/ActivePython/index.html</A>).

2035

<P>

2036

Pydb is a version of the standard Python debugger pdb, modified for use with DDD (Data Display Debugger), a popular graphical debugger front end. Pydb can be found at <A HREF="http://packages.debian.org/unstable/devel/pydb.html">http://packages.debian.org/unstable/devel/pydb.html</A>

2037

and DDD can be found at <A HREF="http://www.gnu.org/software/ddd">http://www.gnu.org/software/ddd</A>/

2038

<P>

2039

There are a number of commmercial Python IDEs that include graphical debuggers. They include:

2040

<P>

2041

<PRE>

2042

* Wing IDE (<A HREF="http://wingide.com">http://wingide.com</A>/)

2043

* Komodo IDE (<A HREF="http://www.activestate.com/Products/Komodo">http://www.activestate.com/Products/Komodo</A>/)

2044

</PRE>

2045

<P>

2046

2047

<A HREF="faqw.py?req=edit&file=faq04.001.htp">Edit this entry</A> /

2048

2049

2050

/ Last changed on Tue Jan 28 01:43:41 2003 by

2051

<A HREF="mailto:steve@ferg.org">Stephen Ferg</A>

2052

<P>

2053

2054

<HR>

2055

<H2><A NAME="4.2">4.2. Can I create an object class with some methods implemented in C and others in Python (e.g. through inheritance)? (Also phrased as: Can I use a built-in type as base class?)</A></H2>

2056

In Python 2.2, you can inherit from builtin classes such as int, list, dict, etc.

2057

<P>

2058

In previous versions of Python, you can easily create a Python class which serves as a wrapper around a built-in object, e.g. (for dictionaries):

2059

<P>

2060

<PRE>

2061

# A user-defined class behaving almost identical

2062

# to a built-in dictionary.

2063

class UserDict:

2064

def __init__(self): self.data = {}

2065

def __repr__(self): return repr(self.data)

2066

def __cmp__(self, dict):

2067

if type(dict) == type(self.data):

2068

return cmp(self.data, dict)

2069

else:

2070

return cmp(self.data, dict.data)

2071

def __len__(self): return len(self.data)

2072

def __getitem__(self, key): return self.data[key]

2073

def __setitem__(self, key, item): self.data[key] = item

2074

def __delitem__(self, key): del self.data[key]

2075

def keys(self): return self.data.keys()

2076

def items(self): return self.data.items()

2077

def values(self): return self.data.values()

2078

def has_key(self, key): return self.data.has_key(key)

2079

</PRE>

2080

A2. See Jim Fulton's ExtensionClass for an example of a mechanism

2081

which allows you to have superclasses which you can inherit from in

2082

Python -- that way you can have some methods from a C superclass (call

2083

it a mixin) and some methods from either a Python superclass or your

2084

subclass. ExtensionClass is distributed as a part of Zope (see

2085

<A HREF="http://www.zope.org">http://www.zope.org</A>), but will be phased out with Zope 3, since

2086

Zope 3 uses Python 2.2 or later which supports direct inheritance

2087

from built-in types. Here's a link to the original paper about

2088

ExtensionClass:

2089

<A HREF="http://debian.acm.ndsu.nodak.edu/doc/python-extclass/ExtensionClass.html">http://debian.acm.ndsu.nodak.edu/doc/python-extclass/ExtensionClass.html</A>

2090

<P>

2091

A3. The Boost Python Library (BPL, <A HREF="http://www.boost.org/libs/python/doc/index.html">http://www.boost.org/libs/python/doc/index.html</A>)

2092

provides a way of doing this from C++ (i.e. you can inherit from an

2093

extension class written in C++ using the BPL).

2094

<P>

2095

2096

<A HREF="faqw.py?req=edit&file=faq04.002.htp">Edit this entry</A> /

2097

2098

2099

/ Last changed on Tue May 28 21:09:52 2002 by

2100

2101

<P>

2102

2103

<HR>

2104

<H2><A NAME="4.3">4.3. Is there a curses/termcap package for Python?</A></H2>

2105

The standard Python source distribution comes with a curses module in

2106

the Modules/ subdirectory, though it's not compiled by default (note

2107

that this is not available in the Windows distribution -- there is

2108

no curses module for Windows).

2109

<P>

2110

In Python versions before 2.0 the module only supported plain curses;

2111

you couldn't use ncurses features like colors with it (though it would

2112

link with ncurses).

2113

<P>

2114

In Python 2.0, the curses module has been greatly extended, starting

2115

from Oliver Andrich's enhanced version, to provide many additional

2116

functions from ncurses and SYSV curses, such as colour, alternative

2117

character set support, pads, and mouse support. This means the

2118

module is no longer compatible with operating systems that only

2119

have BSD curses, but there don't seem to be any currently

2120

maintained OSes that fall into this category.

2121

<P>

2122

2123

<A HREF="faqw.py?req=edit&file=faq04.003.htp">Edit this entry</A> /

2124

2125

2126

/ Last changed on Sun Jun 23 20:24:06 2002 by

2127

<A HREF="mailto:tim.one@comcast.net">Tim Peters</A>

2128

<P>

2129

2130

<HR>

2131

<H2><A NAME="4.4">4.4. Is there an equivalent to C's onexit() in Python?</A></H2>

2132

For Python 2.0: The new atexit module provides a register function that

2133

is similar to C's onexit. See the Library Reference for details. For

2134

2.0 you should <I>not</I> assign to sys.exitfunc!

2135

<P>

2136

For Python 1.5.2: You need to import sys and assign a function to

2137

sys.exitfunc, it will be called when your program exits, is

2138

killed by an unhandled exception, or (on UNIX) receives a

2139

SIGHUP or SIGTERM signal.

2140

<P>

2141

2142

<A HREF="faqw.py?req=edit&file=faq04.004.htp">Edit this entry</A> /

2143

2144

2145

/ Last changed on Thu Dec 28 12:14:55 2000 by

2146

<A HREF="mailto:pbjorn@uswest.net">Bjorn Pettersen</A>

2147

<P>

2148

2149

<HR>

2150

<H2><A NAME="4.5">4.5. [deleted]</A></H2>

2151

[python used to lack nested scopes, it was explained here]

2152

<P>

2153

2154

<A HREF="faqw.py?req=edit&file=faq04.005.htp">Edit this entry</A> /

2155

2156

2157

/ Last changed on Thu Mar 21 05:18:22 2002 by

2158

<A HREF="mailto:erno-pyfaq@erno.iki.fi">Erno Kuusela</A>

2159

<P>

2160

2161

<HR>

2162

<H2><A NAME="4.6">4.6. How do I iterate over a sequence in reverse order?</A></H2>

2163

If it is a list, the fastest solution is

2164

<P>

2165

<PRE>

2166

list.reverse()

2167

try:

2168

for x in list:

2169

"do something with x"

2170

finally:

2171

list.reverse()

2172

</PRE>

2173

This has the disadvantage that while you are in the loop, the list

2174

is temporarily reversed. If you don't like this, you can make a copy.

2175

This appears expensive but is actually faster than other solutions:

2176

<P>

2177

<PRE>

2178

rev = list[:]

2179

rev.reverse()

2180

for x in rev:

2181

2182

</PRE>

2183

If it's not a list, a more general but slower solution is:

2184

<P>

2185

<PRE>

2186

for i in range(len(sequence)-1, -1, -1):

2187

x = sequence[i]

2188

2189

</PRE>

2190

A more elegant solution, is to define a class which acts as a sequence

2191

and yields the elements in reverse order (solution due to Steve

2192

Majewski):

2193

<P>

2194

<PRE>

2195

class Rev:

2196

def __init__(self, seq):

2197

self.forw = seq

2198

def __len__(self):

2199

return len(self.forw)

2200

def __getitem__(self, i):

2201

return self.forw[-(i + 1)]

2202

</PRE>

2203

You can now simply write:

2204

<P>

2205

<PRE>

2206

for x in Rev(list):

2207

2208

</PRE>

2209

Unfortunately, this solution is slowest of all, due to the method

2210

call overhead...

2211

<P>

2212

2213

<A HREF="faqw.py?req=edit&file=faq04.006.htp">Edit this entry</A> /

2214

2215

2216

/ Last changed on Sun May 25 21:10:50 1997 by

2217

2218

<P>

2219

2220

<HR>

2221

<H2><A NAME="4.7">4.7. My program is too slow. How do I speed it up?</A></H2>

2222

That's a tough one, in general. There are many tricks to speed up

2223

Python code; I would consider rewriting parts in C only as a last

2224

resort. One thing to notice is that function and (especially) method

2225

calls are rather expensive; if you have designed a purely OO interface

2226

with lots of tiny functions that don't do much more than get or set an

2227

instance variable or call another method, you may consider using a

2228

more direct way, e.g. directly accessing instance variables. Also see

2229

the standard module "profile" (described in the Library Reference

2230

manual) which makes it possible to find out where

2231

your program is spending most of its time (if you have some patience

2232

-- the profiling itself can slow your program down by an order of

2233

magnitude).

2234

<P>

2235

Remember that many standard optimization heuristics you

2236

may know from other programming experience may well apply

2237

to Python. For example it may be faster to send output to output

2238

devices using larger writes rather than smaller ones in order to

2239

avoid the overhead of kernel system calls. Thus CGI scripts

2240

that write all output in "one shot" may be notably faster than

2241

those that write lots of small pieces of output.

2242

<P>

2243

Also, be sure to use "aggregate" operations where appropriate.

2244

For example the "slicing" feature allows programs to chop up

2245

lists and other sequence objects in a single tick of the interpreter

2246

mainloop using highly optimized C implementations. Thus to

2247

get the same effect as

2248

<P>

2249

<PRE>

2250

L2 = []

2251

for i in range[3]:

2252

L2.append(L1[i])

2253

</PRE>

2254

it is much shorter and far faster to use

2255

<P>

2256

<PRE>

2257

L2 = list(L1[:3]) # "list" is redundant if L1 is a list.

2258

</PRE>

2259

Note that the map() function, particularly used with

2260

builtin methods or builtin functions can be a convenient

2261

accelerator. For example to pair the elements of two

2262

lists together:

2263

<P>

2264

<PRE>

2265

>>> map(None, [1,2,3], [4,5,6])

2266

[(1, 4), (2, 5), (3, 6)]

2267

</PRE>

2268

or to compute a number of sines:

2269

<P>

2270

<PRE>

2271

>>> map( math.sin, (1,2,3,4))

2272

[0.841470984808, 0.909297426826, 0.14112000806, -0.756802495308]

2273

</PRE>

2274

The map operation completes very quickly in such cases.

2275

<P>

2276

Other examples of aggregate operations include the join and split

2277

methods of string objects. For example if s1..s7 are large (10K+) strings then

2278

"".join([s1,s2,s3,s4,s5,s6,s7]) may be far faster than

2279

the more obvious s1+s2+s3+s4+s5+s6+s7, since the "summation"

2280

will compute many subexpressions, whereas join does all

2281

copying in one pass. For manipulating strings also consider the

2282

regular expression libraries and the "substitution" operations

2283

String % tuple and String % dictionary. Also be sure to use

2284

the list.sort builtin method to do sorting, and see FAQ's 4.51

2285

and 4.59 for examples of moderately advanced usage -- list.sort beats

2286

other techniques for sorting in all but the most extreme

2287

circumstances.

2288

<P>

2289

There are many other aggregate operations

2290

available in the standard libraries and in contributed libraries

2291

and extensions.

2292

<P>

2293

Another common trick is to "push loops into functions or methods."

2294

For example suppose you have a program that runs slowly and you

2295

use the profiler (profile.run) to determine that a Python function ff

2296

is being called lots of times. If you notice that ff

2297

<P>

2298

<PRE>

2299

def ff(x):

2300

...do something with x computing result...

2301

return result

2302

</PRE>

2303

tends to be called in loops like (A)

2304

<P>

2305

<PRE>

2306

list = map(ff, oldlist)

2307

</PRE>

2308

or (B)

2309

<P>

2310

<PRE>

2311

for x in sequence:

2312

value = ff(x)

2313

...do something with value...

2314

</PRE>

2315

then you can often eliminate function call overhead by rewriting

2316

ff to

2317

<P>

2318

<PRE>

2319

def ffseq(seq):

2320

resultseq = []

2321

for x in seq:

2322

...do something with x computing result...

2323

resultseq.append(result)

2324

return resultseq

2325

</PRE>

2326

and rewrite (A) to

2327

<P>

2328

<PRE>

2329

list = ffseq(oldlist)

2330

</PRE>

2331

and (B) to

2332

<P>

2333

<PRE>

2334

for value in ffseq(sequence):

2335

...do something with value...

2336

</PRE>

2337

Other single calls ff(x) translate to ffseq([x])[0] with little

2338

penalty. Of course this technique is not always appropriate

2339

and there are other variants, which you can figure out.

2340

<P>

2341

You can gain some performance by explicitly storing the results of

2342

a function or method lookup into a local variable. A loop like

2343

<P>

2344

<PRE>

2345

for key in token:

2346

dict[key] = dict.get(key, 0) + 1

2347

</PRE>

2348

resolves dict.get every iteration. If the method isn't going to

2349

change, a faster implementation is

2350

<P>

2351

<PRE>

2352

dict_get = dict.get # look up the method once

2353

for key in token:

2354

dict[key] = dict_get(key, 0) + 1

2355

</PRE>

2356

Default arguments can be used to determine values once, at

2357

compile time instead of at run time. This can only be done for

2358

functions or objects which will not be changed during program

2359

execution, such as replacing

2360

<P>

2361

<PRE>

2362

def degree_sin(deg):

2363

return math.sin(deg * math.pi / 180.0)

2364

</PRE>

2365

with

2366

<P>

2367

<PRE>

2368

def degree_sin(deg, factor = math.pi/180.0, sin = math.sin):

2369

return sin(deg * factor)

2370

</PRE>

2371

Because this trick uses default arguments for terms which should

2372

not be changed, it should only be used when you are not concerned

2373

with presenting a possibly confusing API to your users.

2374

<P>

2375

<P>

2376

For an anecdote related to optimization, see

2377

<P>

2378

<PRE>

2379

<A HREF="http://www.python.org/doc/essays/list2str.html">http://www.python.org/doc/essays/list2str.html</A>

2380

</PRE>

2381

<P>

2382

2383

<A HREF="faqw.py?req=edit&file=faq04.007.htp">Edit this entry</A> /

2384

2385

2386

/ Last changed on Mon Jun 3 01:03:54 2002 by

2387

<A HREF="mailto:neal@metaslash.com">Neal Norwitz</A>

2388

<P>

2389

2390

<HR>

2391

<H2><A NAME="4.8">4.8. When I have imported a module, then edit it, and import it again (into the same Python process), the changes don't seem to take place. What is going on?</A></H2>

2392

For reasons of efficiency as well as consistency, Python only reads

2393

the module file on the first time a module is imported. (Otherwise a

2394

program consisting of many modules, each of which imports the same

2395

basic module, would read the basic module over and over again.) To

2396

force rereading of a changed module, do this:

2397

<P>

2398

<PRE>

2399

import modname

2400

reload(modname)

2401

</PRE>

2402

Warning: this technique is not 100% fool-proof. In particular,

2403

modules containing statements like

2404

<P>

2405

<PRE>

2406

from modname import some_objects

2407

</PRE>

2408

will continue to work with the old version of the imported objects.

2409

<P>

2410

2411

<A HREF="faqw.py?req=edit&file=faq04.008.htp">Edit this entry</A> /

2412

2413

<P>

2414

2415

<HR>

2416

<H2><A NAME="4.9">4.9. How do I find the current module name?</A></H2>

2417

A module can find out its own module name by looking at the

2418

(predefined) global variable __name__. If this has the value

2419

'__main__' you are running as a script.

2420

<P>

2421

2422

<A HREF="faqw.py?req=edit&file=faq04.009.htp">Edit this entry</A> /

2423

2424

<P>

2425

2426

<HR>

2427

<H2><A NAME="4.10">4.10. I have a module in which I want to execute some extra code when it is run as a script. How do I find out whether I am running as a script?</A></H2>

2428

See the previous question. E.g. if you put the following on the

2429

last line of your module, main() is called only when your module is

2430

running as a script:

2431

<P>

2432

<PRE>

2433

if __name__ == '__main__': main()

2434

</PRE>

2435

<P>

2436

2437

<A HREF="faqw.py?req=edit&file=faq04.010.htp">Edit this entry</A> /

2438

2439

<P>

2440

2441

<HR>

2442

<H2><A NAME="4.11">4.11. I try to run a program from the Demo directory but it fails with ImportError: No module named ...; what gives?</A></H2>

2443

This is probably an optional module (written in C!) which hasn't

2444

been configured on your system. This especially happens with modules

2445

like "Tkinter", "stdwin", "gl", "Xt" or "Xm". For Tkinter, STDWIN and

2446

many other modules, see Modules/Setup.in for info on how to add these

2447

modules to your Python, if it is possible at all. Sometimes you will

2448

have to ftp and build another package first (e.g. Tcl and Tk for Tkinter).

2449

Sometimes the module only works on specific platforms (e.g. gl only works

2450

on SGI machines).

2451

<P>

2452

NOTE: if the complaint is about "Tkinter" (upper case T) and you have

2453

already configured module "tkinter" (lower case t), the solution is

2454

<I>not</I> to rename tkinter to Tkinter or vice versa. There is probably

2455

something wrong with your module search path. Check out the value of

2456

sys.path.

2457

<P>

2458

For X-related modules (Xt and Xm) you will have to do more work: they

2459

are currently not part of the standard Python distribution. You will

2460

have to ftp the Extensions tar file, i.e.

2461

<A HREF="ftp://ftp.python.org/pub/python/src/X-extension.tar.gz">ftp://ftp.python.org/pub/python/src/X-extension.tar.gz</A> and follow

2462

the instructions there.

2463

<P>

2464

2465

<A HREF="faqw.py?req=edit&file=faq04.011.htp">Edit this entry</A> /

2466

2467

2468

/ Last changed on Wed Feb 12 21:31:08 2003 by

2469

<A HREF="mailto:kubieziel@gmx.de">Jens Kubieziel</A>

2470

<P>

2471

2472

<HR>

2473

<H2><A NAME="4.12">4.12. [deleted]</A></H2>

2474

[stdwin (long dead windowing library) entry deleted]

2475

<P>

2476

2477

<A HREF="faqw.py?req=edit&file=faq04.012.htp">Edit this entry</A> /

2478

2479

2480

/ Last changed on Thu Mar 21 08:30:13 2002 by

2481

<A HREF="mailto:erno-pyfaq@erno.iki.fi">Erno Kuusela</A>

2482

<P>

2483

2484

<HR>

2485

<H2><A NAME="4.13">4.13. What GUI toolkits exist for Python?</A></H2>

2486

Depending on what platform(s) you are aiming at, there are several.

2487

<P>

2488

Currently supported solutions:

2489

<P>

2490

Cross-platform:

2491

<P>

2492

Tk:

2493

<P>

2494

There's a neat object-oriented interface to the Tcl/Tk widget set,

2495

called Tkinter. It is part of the standard Python distribution and

2496

well-supported -- all you need to do is build and install Tcl/Tk and

2497

enable the _tkinter module and the TKPATH definition in Modules/Setup

2498

when building Python. This is probably the easiest to install and

2499

use, and the most complete widget set. It is also very likely that in

2500

the future the standard Python GUI API will be based on or at least

2501

look very much like the Tkinter interface. For more info about Tk,

2502

including pointers to the source, see the Tcl/Tk home page at

2503

<A HREF="http://www.scriptics.com">http://www.scriptics.com</A>. Tcl/Tk is now fully

2504

portable to the Mac and Windows platforms (NT and 95 only); you need

2505

Python 1.4beta3 or later and Tk 4.1patch1 or later.

2506

<P>

2507

wxWindows:

2508

<P>

2509

There's an interface to wxWindows called wxPython. wxWindows is a

2510

portable GUI class library written in C++. It supports GTK, Motif,

2511

MS-Windows and Mac as targets. Ports to other platforms are being

2512

contemplated or have already had some work done on them. wxWindows

2513

preserves the look and feel of the underlying graphics toolkit, and

2514

there is quite a rich widget set and collection of GDI classes.

2515

See the wxWindows page at <A HREF="http://www.wxwindows.org">http://www.wxwindows.org</A>/ for more details.

2516

wxPython is a python extension module that wraps many of the wxWindows

2517

C++ classes, and is quickly gaining popularity amongst Python

2518

developers. You can get wxPython as part of the source or CVS

2519

distribution of wxWindows, or directly from its home page at

2520

<A HREF="http://alldunn.com/wxPython">http://alldunn.com/wxPython</A>/.

2521

<P>

2522

Gtk+:

2523

<P>

2524

PyGtk bindings for the Gtk+ Toolkit by James Henstridge exist; see <A HREF="ftp://ftp.daa.com.au/pub/james/python">ftp://ftp.daa.com.au/pub/james/python</A>/. Note that there are two incompatible bindings. If you are using Gtk+ 1.2.x you should get the 0.6.x PyGtk bindings from

2525

<P>

2526

<PRE>

2527

<A HREF="ftp://ftp.gtk.org/pub/python/v1.2">ftp://ftp.gtk.org/pub/python/v1.2</A>

2528

</PRE>

2529

If you plan to use Gtk+ 2.0 with Python (highly recommended if you are just starting with Gtk), get the most recent distribution from

2530

<P>

2531

<PRE>

2532

<A HREF="ftp://ftp.gtk.org/pub/python/v2.0">ftp://ftp.gtk.org/pub/python/v2.0</A>

2533

</PRE>

2534

If you are adventurous, you can also check out the source from the Gnome CVS repository. Set your CVS directory to :pserver:<A HREF="mailto:anonymous@anoncvs.gnome.org">anonymous@anoncvs.gnome.org</A>:/cvs/gnome and check the gnome-python module out from the repository.

2535

<P>

2536

Other:

2537

<P>

2538

There are also bindings available for the Qt toolkit (PyQt), and for KDE (PyKDE); see <A HREF="http://www.thekompany.com/projects/pykde">http://www.thekompany.com/projects/pykde</A>/.

2539

<P>

2540

For OpenGL bindings, see <A HREF="http://starship.python.net/~da/PyOpenGL">http://starship.python.net/~da/PyOpenGL</A>.

2541

<P>

2542

Platform specific:

2543

<P>

2544

The Mac port has a rich and ever-growing set of modules that support

2545

the native Mac toolbox calls. See the documentation that comes with

2546

the Mac port. See <A HREF="ftp://ftp.python.org/pub/python/mac">ftp://ftp.python.org/pub/python/mac</A>. Support

2547

by Jack Jansen <A HREF="mailto:jack@cwi.nl">jack@cwi.nl</A>.

2548

<P>

2549

Pythonwin by Mark Hammond (<A HREF="mailto:MHammond@skippinet.com.au">MHammond@skippinet.com.au</A>)

2550

includes an interface to the Microsoft Foundation

2551

Classes and a Python programming environment using it that's written

2552

mostly in Python. See <A HREF="http://www.python.org/windows">http://www.python.org/windows</A>/.

2553

<P>

2554

There's an object-oriented GUI based on the Microsoft Foundation

2555

Classes model called WPY, supported by Jim Ahlstrom <A HREF="mailto:jim@interet.com">jim@interet.com</A>.

2556

Programs written in WPY run unchanged and with native look and feel on

2557

Windows NT/95, Windows 3.1 (using win32s), and on Unix (using Tk).

2558

Source and binaries for Windows and Linux are available in

2559

<A HREF="ftp://ftp.python.org/pub/python/wpy">ftp://ftp.python.org/pub/python/wpy</A>/.

2560

<P>

2561

Obsolete or minority solutions:

2562

<P>

2563

There's an interface to X11, including the Athena and Motif widget

2564

sets (and a few individual widgets, like Mosaic's HTML widget and

2565

SGI's GL widget) available from

2566

<A HREF="ftp://ftp.python.org/pub/python/src/X-extension.tar.gz">ftp://ftp.python.org/pub/python/src/X-extension.tar.gz</A>.

2567

Support by Sjoerd Mullender <A HREF="mailto:sjoerd@cwi.nl">sjoerd@cwi.nl</A>.

2568

<P>

2569

On top of the X11 interface there's the vpApp

2570

toolkit by Per Spilling, now also maintained by Sjoerd Mullender

2571

<A HREF="mailto:sjoerd@cwi.nl">sjoerd@cwi.nl</A>. See <A HREF="ftp://ftp.cwi.nl/pub/sjoerd/vpApp.tar.gz">ftp://ftp.cwi.nl/pub/sjoerd/vpApp.tar.gz</A>.

2572

<P>

2573

For SGI IRIX only, there are unsupported interfaces to the complete

2574

GL (Graphics Library -- low level but very good 3D capabilities) as

2575

well as to FORMS (a buttons-and-sliders-etc package built on top of GL

2576

by Mark Overmars -- ftp'able from

2577

<A HREF="ftp://ftp.cs.ruu.nl/pub/SGI/FORMS">ftp://ftp.cs.ruu.nl/pub/SGI/FORMS</A>/). This is probably also

2578

becoming obsolete, as OpenGL takes over (see above).

2579

<P>

2580

There's an interface to STDWIN, a platform-independent low-level

2581

windowing interface for Mac and X11. This is totally unsupported and

2582

rapidly becoming obsolete. The STDWIN sources are at

2583

<A HREF="ftp://ftp.cwi.nl/pub/stdwin">ftp://ftp.cwi.nl/pub/stdwin</A>/.

2584

<P>

2585

There is an interface to WAFE, a Tcl interface to the X11

2586

Motif and Athena widget sets. WAFE is at

2587

<A HREF="http://www.wu-wien.ac.at/wafe/wafe.html">http://www.wu-wien.ac.at/wafe/wafe.html</A>.

2588

<P>

2589

2590

<A HREF="faqw.py?req=edit&file=faq04.013.htp">Edit this entry</A> /

2591

2592

2593

/ Last changed on Mon May 13 21:40:39 2002 by

2594

<A HREF="mailto:skip@pobox.com">Skip Montanaro</A>

2595

<P>

2596

2597

<HR>

2598

<H2><A NAME="4.14">4.14. Are there any interfaces to database packages in Python?</A></H2>

2599

Yes! See the Database Topic Guide at

2600

<A HREF="http://www.python.org/topics/database">http://www.python.org/topics/database</A>/ for details.

2601

<P>

2602

2603

<A HREF="faqw.py?req=edit&file=faq04.014.htp">Edit this entry</A> /

2604

2605

2606

/ Last changed on Tue Jan 4 20:12:19 2000 by

2607

<A HREF="mailto:bwarsaw@python.org">Barney Warplug</A>

2608

<P>

2609

2610

<HR>

2611

<H2><A NAME="4.15">4.15. Is it possible to write obfuscated one-liners in Python?</A></H2>

2612

Yes. See the following three examples, due to Ulf Bartelt:

2613

<P>

2614

<PRE>

2615

# Primes < 1000

2616

print filter(None,map(lambda y:y*reduce(lambda x,y:x*y!=0,

2617

map(lambda x,y=y:y%x,range(2,int(pow(y,0.5)+1))),1),range(2,1000)))

2618

</PRE>

2619

<PRE>

2620

# First 10 Fibonacci numbers

2621

print map(lambda x,f=lambda x,f:(x<=1) or (f(x-1,f)+f(x-2,f)): f(x,f),

2622

range(10))

2623

</PRE>

2624

<PRE>

2625

# Mandelbrot set

2626

print (lambda Ru,Ro,Iu,Io,IM,Sx,Sy:reduce(lambda x,y:x+y,map(lambda y,

2627

Iu=Iu,Io=Io,Ru=Ru,Ro=Ro,Sy=Sy,L=lambda yc,Iu=Iu,Io=Io,Ru=Ru,Ro=Ro,i=IM,

2628

Sx=Sx,Sy=Sy:reduce(lambda x,y:x+y,map(lambda x,xc=Ru,yc=yc,Ru=Ru,Ro=Ro,

2629

i=i,Sx=Sx,F=lambda xc,yc,x,y,k,f=lambda xc,yc,x,y,k,f:(k<=0)or (x*x+y*y

2630

>=4.0) or 1+f(xc,yc,x*x-y*y+xc,2.0*x*y+yc,k-1,f):f(xc,yc,x,y,k,f):chr(

2631

64+F(Ru+x*(Ro-Ru)/Sx,yc,0,0,i)),range(Sx))):L(Iu+y*(Io-Iu)/Sy),range(Sy

2632

))))(-2.1, 0.7, -1.2, 1.2, 30, 80, 24)

2633

# \___ ___/ \___ ___/ | | |__ lines on screen

2634

# V V | |______ columns on screen

2635

# | | |__________ maximum of "iterations"

2636

# | |_________________ range on y axis

2637

# |____________________________ range on x axis

2638

</PRE>

2639

Don't try this at home, kids!

2640

<P>

2641

2642

<A HREF="faqw.py?req=edit&file=faq04.015.htp">Edit this entry</A> /

2643

2644

2645

/ Last changed on Wed May 21 15:48:33 1997 by

2646

2647

<P>

2648

2649

<HR>

2650

<H2><A NAME="4.16">4.16. Is there an equivalent of C's "?:" ternary operator?</A></H2>

2651

Not directly. In many cases you can mimic a?b:c with "a and b or

2652

c", but there's a flaw: if b is zero (or empty, or None -- anything

2653

that tests false) then c will be selected instead. In many cases you

2654

can prove by looking at the code that this can't happen (e.g. because

2655

b is a constant or has a type that can never be false), but in general

2656

this can be a problem.

2657

<P>

2658

Tim Peters (who wishes it was Steve Majewski) suggested the following

2659

solution: (a and [b] or [c])[0]. Because [b] is a singleton list it

2660

is never false, so the wrong path is never taken; then applying [0] to

2661

the whole thing gets the b or c that you really wanted. Ugly, but it

2662

gets you there in the rare cases where it is really inconvenient to

2663

rewrite your code using 'if'.

2664

<P>

2665

As a last resort it is possible to implement the "?:" operator as a function:

2666

<P>

2667

<PRE>

2668

def q(cond,on_true,on_false):

2669

from inspect import isfunction

2670

</PRE>

2671

<PRE>

2672

if cond:

2673

if not isfunction(on_true): return on_true

2674

else: return apply(on_true)

2675

else:

2676

if not isfunction(on_false): return on_false

2677

else: return apply(on_false)

2678

</PRE>

2679

In most cases you'll pass b and c directly: q(a,b,c). To avoid evaluating b

2680

or c when they shouldn't be, encapsulate them

2681

within a lambda function, e.g.: q(a,lambda: b, lambda: c).

2682

<P>

2683

<P>

2684

<P>

2685

It has been asked <I>why</I> Python has no if-then-else expression,

2686

since most language have one; it is a frequently requested feature.

2687

<P>

2688

There are several possible answers: just as many languages do

2689

just fine without one; it can easily lead to less readable code;

2690

no sufficiently "Pythonic" syntax has been discovered; a search

2691

of the standard library found remarkably few places where using an

2692

if-then-else expression would make the code more understandable.

2693

<P>

2694

Nevertheless, in an effort to decide once and for all whether

2695

an if-then-else expression should be added to the language,

2696

PEP 308 (<A HREF="http://www.python.org/peps/pep-0308.html">http://www.python.org/peps/pep-0308.html</A>) has been

2697

put forward, proposing a specific syntax. The community can

2698

now vote on this issue.

2699

<P>

2700

2701

<A HREF="faqw.py?req=edit&file=faq04.016.htp">Edit this entry</A> /

2702

2703

2704

/ Last changed on Fri Feb 7 19:41:13 2003 by

2705

<A HREF="mailto:goodger@python.org">David Goodger</A>

2706

<P>

2707

2708

<HR>

2709

<H2><A NAME="4.17">4.17. My class defines __del__ but it is not called when I delete the object.</A></H2>

2710

There are several possible reasons for this.

2711

<P>

2712

The del statement does not necessarily call __del__ -- it simply

2713

decrements the object's reference count, and if this reaches zero

2714

__del__ is called.

2715

<P>

2716

If your data structures contain circular links (e.g. a tree where

2717

each child has a parent pointer and each parent has a list of

2718

children) the reference counts will never go back to zero. You'll

2719

have to define an explicit close() method which removes those

2720

pointers. Please don't ever call __del__ directly -- __del__ should

2721

call close() and close() should make sure that it can be called more

2722

than once for the same object.

2723

<P>

2724

If the object has ever been a local variable (or argument, which is

2725

really the same thing) to a function that caught an expression in an

2726

except clause, chances are that a reference to the object still exists

2727

in that function's stack frame as contained in the stack trace.

2728

Normally, deleting (better: assigning None to) sys.exc_traceback will

2729

take care of this. If a stack was printed for an unhandled

2730

exception in an interactive interpreter, delete sys.last_traceback

2731

instead.

2732

<P>

2733

There is code that deletes all objects when the interpreter exits,

2734

but it is not called if your Python has been configured to support

2735

threads (because other threads may still be active). You can define

2736

your own cleanup function using sys.exitfunc (see question 4.4).

2737

<P>

2738

Finally, if your __del__ method raises an exception, a warning message is printed to sys.stderr.

2739

<P>

2740

<P>

2741

Starting with Python 2.0, a garbage collector periodically reclaims the space used by most cycles with no external references. (See the "gc" module documentation for details.) There <I>are</I>, however, pathological cases where it can be expected to fail. Moreover, the garbage collector runs some time after the last reference to your data structure vanishes, so your __del__ method may be called at an inconvenient and random time. This is inconvenient if you're trying to reproduce a problem. Worse, the order in which object's __del__ methods are executed is arbitrary.

2742

<P>

2743

Another way to avoid cyclical references is to use the "weakref" module, which allows you to point to objects without incrementing their reference count. Tree data structures, for instance, should use weak references for their parent and sibling pointers (if they need them!).

2744

<P>

2745

Question 6.14 is intended to explain the new garbage collection algorithm.

2746

<P>

2747

2748

<A HREF="faqw.py?req=edit&file=faq04.017.htp">Edit this entry</A> /

2749

2750

2751

/ Last changed on Mon Jun 10 15:27:28 2002 by

2752

<A HREF="mailto:smurf@noris.de">Matthias Urlichs</A>

2753

<P>

2754

2755

<HR>

2756

<H2><A NAME="4.18">4.18. How do I change the shell environment for programs called using os.popen() or os.system()? Changing os.environ doesn't work.</A></H2>

2757

You must be using either a version of python before 1.4, or on a

2758

(rare) system that doesn't have the putenv() library function.

2759

<P>

2760

Before Python 1.4, modifying the environment passed to subshells was

2761

left out of the interpreter because there seemed to be no

2762

well-established portable way to do it (in particular, some systems,

2763

have putenv(), others have setenv(), and some have none at all). As

2764

of Python 1.4, almost all Unix systems <I>do</I> have putenv(), and so does

2765

the Win32 API, and thus the os module was modified so that changes to

2766

os.environ are trapped and the corresponding putenv() call is made.

2767

<P>

2768

2769

<A HREF="faqw.py?req=edit&file=faq04.018.htp">Edit this entry</A> /

2770

2771

<P>

2772

2773

<HR>

2774

<H2><A NAME="4.19">4.19. What is a class?</A></H2>

2775

A class is the particular object type created by executing

2776

a class statement. Class objects are used as templates, to create

2777

instance objects, which embody both the data structure

2778

(attributes) and program routines (methods) specific to a datatype.

2779

<P>

2780

A class can be based on one or more other classes, called its base

2781

class(es). It then inherits the attributes and methods of its base classes. This allows an object model to be successively refined

2782

by inheritance.

2783

<P>

2784

The term "classic class" is used to refer to the original

2785

class implementation in Python. One problem with classic

2786

classes is their inability to use the built-in data types

2787

(such as list and dictionary) as base classes. Starting

2788

with Python 2.2 an attempt is in progress to unify user-defined

2789

classes and built-in types. It is now possible to declare classes

2790

that inherit from built-in types.

2791

<P>

2792

2793

<A HREF="faqw.py?req=edit&file=faq04.019.htp">Edit this entry</A> /

2794

2795

2796

/ Last changed on Mon May 27 01:31:21 2002 by

2797

<A HREF="mailto:sholden@holdenweb.com">Steve Holden</A>

2798

<P>

2799

2800

<HR>

2801

<H2><A NAME="4.20">4.20. What is a method?</A></H2>

2802

A method is a function that you normally call as

2803

x.name(arguments...) for some object x. The term is used for methods

2804

of classes and class instances as well as for methods of built-in

2805

objects. (The latter have a completely different implementation and

2806

only share the way their calls look in Python code.) Methods of

2807

classes (and class instances) are defined as functions inside the

2808

class definition.

2809

<P>

2810

2811

<A HREF="faqw.py?req=edit&file=faq04.020.htp">Edit this entry</A> /

2812

2813

<P>

2814

2815

<HR>

2816

2817

Self is merely a conventional name for the first argument of a

2818

method -- i.e. a function defined inside a class definition. A method

2819

defined as meth(self, a, b, c) should be called as x.meth(a, b, c) for

2820

some instance x of the class in which the definition occurs;

2821

the called method will think it is called as meth(x, a, b, c).

2822

<P>

2823

2824

<A HREF="faqw.py?req=edit&file=faq04.021.htp">Edit this entry</A> /

2825

2826

<P>

2827

2828

<HR>

2829

<H2><A NAME="4.22">4.22. What is an unbound method?</A></H2>

2830

An unbound method is a method defined in a class that is not yet

2831

bound to an instance. You get an unbound method if you ask for a

2832

class attribute that happens to be a function. You get a bound method

2833

if you ask for an instance attribute. A bound method knows which

2834

instance it belongs to and calling it supplies the instance automatically;

2835

an unbound method only knows which class it wants for its first

2836

argument (a derived class is also OK). Calling an unbound method

2837

doesn't "magically" derive the first argument from the context -- you

2838

have to provide it explicitly.

2839

<P>

2840

Trivia note regarding bound methods: each reference to a bound

2841

method of a particular object creates a bound method object. If you

2842

have two such references (a = inst.meth; b = inst.meth), they will

2843

compare equal (a == b) but are not the same (a is not b).

2844

<P>

2845

2846

<A HREF="faqw.py?req=edit&file=faq04.022.htp">Edit this entry</A> /

2847

2848

2849

/ Last changed on Wed May 6 18:07:25 1998 by

2850

<A HREF="mailto:clarence@avtel.com">Clarence Gardner</A>

2851

<P>

2852

2853

<HR>

2854

<H2><A NAME="4.23">4.23. How do I call a method defined in a base class from a derived class that overrides it?</A></H2>

2855

If your class definition starts with "class Derived(Base): ..."

2856

then you can call method meth defined in Base (or one of Base's base

2857

classes) as Base.meth(self, arguments...). Here, Base.meth is an

2858

unbound method (see previous question).

2859

<P>

2860

2861

<A HREF="faqw.py?req=edit&file=faq04.023.htp">Edit this entry</A> /

2862

2863

<P>

2864

2865

<HR>

2866

<H2><A NAME="4.24">4.24. How do I call a method from a base class without using the name of the base class?</A></H2>

2867

DON'T DO THIS. REALLY. I MEAN IT. It appears that you could call

2868

self.__class__.__bases__[0].meth(self, arguments...) but this fails when

2869

a doubly-derived method is derived from your class: for its instances,

2870

self.__class__.__bases__[0] is your class, not its base class -- so

2871

(assuming you are doing this from within Derived.meth) you would start

2872

a recursive call.

2873

<P>

2874

Often when you want to do this you are forgetting that classes

2875

are first class in Python. You can "point to" the class you want

2876

to delegate an operation to either at the instance or at the

2877

subclass level. For example if you want to use a "glorp"

2878

operation of a superclass you can point to the right superclass

2879

to use.

2880

<P>

2881

<PRE>

2882

class subclass(superclass1, superclass2, superclass3):

2883

delegate_glorp = superclass2

2884

...

2885

def glorp(self, arg1, arg2):

2886

... subclass specific stuff ...

2887

self.delegate_glorp.glorp(self, arg1, arg2)

2888

...

2889

</PRE>

2890

<PRE>

2891

class subsubclass(subclass):

2892

delegate_glorp = superclass3

2893

...

2894

</PRE>

2895

Note, however that setting delegate_glorp to subclass in

2896

subsubclass would cause an infinite recursion on subclass.delegate_glorp. Careful! Maybe you are getting too fancy for your own good. Consider simplifying the design (?).

2897

<P>

2898

2899

<A HREF="faqw.py?req=edit&file=faq04.024.htp">Edit this entry</A> /

2900

2901

2902

/ Last changed on Mon Jul 28 13:58:22 1997 by

2903

<A HREF="mailto:aaron_watters@msn.com">aaron watters</A>

2904

<P>

2905

2906

<HR>

2907

<H2><A NAME="4.25">4.25. How can I organize my code to make it easier to change the base class?</A></H2>

2908

You could define an alias for the base class, assign the real base

2909

class to it before your class definition, and use the alias throughout

2910

your class. Then all you have to change is the value assigned to the

2911

alias. Incidentally, this trick is also handy if you want to decide

2912

dynamically (e.g. depending on availability of resources) which base

2913

class to use. Example:

2914

<P>

2915

<PRE>

2916

BaseAlias = <real base class>

2917

class Derived(BaseAlias):

2918

def meth(self):

2919

BaseAlias.meth(self)

2920

...

2921

</PRE>

2922

<P>

2923

2924

<A HREF="faqw.py?req=edit&file=faq04.025.htp">Edit this entry</A> /

2925

2926

2927

/ Last changed on Wed May 21 15:49:57 1997 by

2928

2929

<P>

2930

2931

<HR>

2932

<H2><A NAME="4.26">4.26. How can I find the methods or attributes of an object?</A></H2>

2933

This depends on the object type.

2934

<P>

2935

For an instance x of a user-defined class, instance attributes are

2936

found in the dictionary x.__dict__, and methods and attributes defined

2937

by its class are found in x.__class__.__bases__[i].__dict__ (for i in

2938

range(len(x.__class__.__bases__))). You'll have to walk the tree of

2939

base classes to find <I>all</I> class methods and attributes.

2940

<P>

2941

Many, but not all built-in types define a list of their method names

2942

in x.__methods__, and if they have data attributes, their names may be

2943

found in x.__members__. However this is only a convention.

2944

<P>

2945

For more information, read the source of the standard (but

2946

undocumented) module newdir.

2947

<P>

2948

2949

<A HREF="faqw.py?req=edit&file=faq04.026.htp">Edit this entry</A> /

2950

2951

<P>

2952

2953

<HR>

2954

<H2><A NAME="4.27">4.27. I can't seem to use os.read() on a pipe created with os.popen().</A></H2>

2955

os.read() is a low-level function which takes a file descriptor (a

2956

small integer). os.popen() creates a high-level file object -- the

2957

same type used for sys.std{in,out,err} and returned by the builtin

2958

open() function. Thus, to read n bytes from a pipe p created with

2959

os.popen(), you need to use p.read(n).

2960

<P>

2961

2962

<A HREF="faqw.py?req=edit&file=faq04.027.htp">Edit this entry</A> /

2963

2964

<P>

2965

2966

<HR>

2967

<H2><A NAME="4.28">4.28. How can I create a stand-alone binary from a Python script?</A></H2>

2968

Even though there are Python compilers being developed,

2969

you probably don't need a <I>real</I> compiler, if all you want

2970

is a stand-alone program. There are three solutions to that.

2971

<P>

2972

One is to use the freeze tool, which is included in the Python

2973

source tree as Tools/freeze. It converts Python byte

2974

code to C arrays. Using a C compiler, you can embed all

2975

your modules into a new program, which is then linked

2976

with the standard Python modules.

2977

<P>

2978

It works by scanning your source recursively for import statements

2979

(in both forms) and looking for the modules in the standard Python path

2980

as well as in the source directory (for built-in modules). It then

2981

1 the modules written in Python to C code (array initializers

2982

that can be turned into code objects using the marshal module) and

2983

creates a custom-made config file that only contains those built-in

2984

modules which are actually used in the program. It then compiles the

2985

generated C code and links it with the rest of the Python interpreter

2986

to form a self-contained binary which acts exactly like your script.

2987

<P>

2988

(Hint: the freeze program only works if your script's filename ends in

2989

".py".)

2990

<P>

2991

There are several utilities which may be helpful. The first is Gordon McMillan's installer at

2992

<P>

2993

<PRE>

2994

<A HREF="http://www.mcmillan-inc.com/install1.html">http://www.mcmillan-inc.com/install1.html</A>

2995

</PRE>

2996

which works on Windows, Linux and at least some forms of Unix.

2997

<P>

2998

Another is Thomas Heller's py2exe (Windows only) at

2999

<P>

3000

<PRE>

3001

<A HREF="http://starship.python.net/crew/theller/py2exe">http://starship.python.net/crew/theller/py2exe</A>/

3002

</PRE>

3003

A third is Christian Tismer's SQFREEZE

3004

(<A HREF="http://starship.python.net/crew/pirx">http://starship.python.net/crew/pirx</A>/) which appends the byte code

3005

to a specially-prepared Python interpreter, which

3006

will find the byte code in executable.

3007

<P>

3008

A fourth is Fredrik Lundh's Squeeze

3009

(<A HREF="http://www.pythonware.com/products/python/squeeze">http://www.pythonware.com/products/python/squeeze</A>/).

3010

<P>

3011

3012

<A HREF="faqw.py?req=edit&file=faq04.028.htp">Edit this entry</A> /

3013

3014

3015

/ Last changed on Wed Jun 19 14:01:30 2002 by

3016

<A HREF="mailto:gmcm@hypernet.com">Gordon McMillan</A>

3017

<P>

3018

3019

<HR>

3020

<H2><A NAME="4.29">4.29. What WWW tools are there for Python?</A></H2>

3021

See the chapters titled "Internet Protocols and Support" and

3022

"Internet Data Handling" in the Library Reference

3023

Manual. Python is full of good things which will help you build server-side and client-side web systems.

3024

<P>

3025

A summary of available frameworks is maintained by Paul Boddie at

3026

<P>

3027

<PRE>

3028

<A HREF="http://thor.prohosting.com/~pboddie/Python/web_modules.html">http://thor.prohosting.com/~pboddie/Python/web_modules.html</A>

3029

</PRE>

3030

Cameron Laird maintains a useful set of pages about Python web technologies at

3031

<P>

3032

<PRE>

3033

<A HREF="http://starbase.neosoft.com/~claird/comp.lang.python/web_python.html">http://starbase.neosoft.com/~claird/comp.lang.python/web_python.html</A>/

3034

</PRE>

3035

There was a web browser written in Python, called Grail --

3036

see <A HREF="http://sourceforge.net/project/grail">http://sourceforge.net/project/grail</A>/. This project has been terminated; <A HREF="http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/grail/grail/README">http://cvs.sourceforge.net/cgi-bin/viewcvs.cgi/grail/grail/README</A> gives more details.

3037

<P>

3038

3039

<A HREF="faqw.py?req=edit&file=faq04.029.htp">Edit this entry</A> /

3040

3041

3042

/ Last changed on Mon Nov 11 22:48:25 2002 by

3043

3044

<P>

3045

3046

<HR>

3047

<H2><A NAME="4.30">4.30. How do I run a subprocess with pipes connected to both input and output?</A></H2>

3048

Use the standard popen2 module. For example:

3049

<P>

3050

<PRE>

3051

import popen2

3052

fromchild, tochild = popen2.popen2("command")

3053

tochild.write("input\n")

3054

tochild.flush()

3055

output = fromchild.readline()

3056

</PRE>

3057

Warning: in general, it is unwise to

3058

do this, because you can easily cause a deadlock where your

3059

process is blocked waiting for output from the child, while the child

3060

is blocked waiting for input from you. This can be caused

3061

because the parent expects the child to output more text than it does,

3062

or it can be caused by data being stuck in stdio buffers due to lack

3063

of flushing. The Python parent can of course explicitly flush the data

3064

it sends to the child before it reads any output, but if the child is

3065

a naive C program it can easily have been written to never explicitly

3066

flush its output, even if it is interactive, since flushing is

3067

normally automatic.

3068

<P>

3069

Note that a deadlock is also possible if you use popen3 to read

3070

stdout and stderr. If one of the two is too large for the internal

3071

buffer (increasing the buffersize does not help) and you read()

3072

the other one first, there is a deadlock, too.

3073

<P>

3074

Note on a bug in popen2: unless your program calls wait()

3075

or waitpid(), finished child processes are never removed,

3076

and eventually calls to popen2 will fail because of a limit on

3077

the number of child processes. Calling os.waitpid with the

3078

os.WNOHANG option can prevent this; a good place to insert such

3079

a call would be before calling popen2 again.

3080

<P>

3081

Another way to produce a deadlock: Call a wait() and there is

3082

still more output from the program than what fits into the

3083

internal buffers.

3084

<P>

3085

In many cases, all you really need is to run some data through a

3086

command and get the result back. Unless the data is infinite in size,

3087

the easiest (and often the most efficient!) way to do this is to write

3088

it to a temporary file and run the command with that temporary file as

3089

input. The standard module tempfile exports a function mktemp() which

3090

generates unique temporary file names.

3091

<P>

3092

<PRE>

3093

import tempfile

3094

import os

3095

class Popen3:

3096

"""

3097

This is a deadlock-save version of popen, that returns

3098

an object with errorlevel, out (a string) and err (a string).

3099

(capturestderr may not work under windows.)

3100

Example: print Popen3('grep spam','\n\nhere spam\n\n').out

3101

"""

3102

def __init__(self,command,input=None,capturestderr=None):

3103

outfile=tempfile.mktemp()

3104

command="( %s ) > %s" % (command,outfile)

3105

if input:

3106

infile=tempfile.mktemp()

3107

open(infile,"w").write(input)

3108

command=command+" <"+infile

3109

if capturestderr:

3110

errfile=tempfile.mktemp()

3111

command=command+" 2>"+errfile

3112

self.errorlevel=os.system(command) >> 8

3113

self.out=open(outfile,"r").read()

3114

os.remove(outfile)

3115

if input:

3116

os.remove(infile)

3117

if capturestderr:

3118

self.err=open(errfile,"r").read()

3119

os.remove(errfile)

3120

</PRE>

3121

Note that many interactive programs (e.g. vi) don't work well with

3122

pipes substituted for standard input and output. You will have to use

3123

pseudo ttys ("ptys") instead of pipes. There is some undocumented

3124

code to use these in the library module pty.py -- I'm afraid you're on

3125

your own here.

3126

<P>

3127

A different answer is a Python interface to Don Libes' "expect"

3128

library. A Python extension that interfaces to expect is called "expy"

3129

and available from

3130

<A HREF="http://expectpy.sourceforge.net">http://expectpy.sourceforge.net</A>/.

3131

<P>

3132

A pure Python solution that works like expect is pexpect of Noah Spurrier.

3133

A beta version is available from

3134

<A HREF="http://pexpect.sourceforge.net">http://pexpect.sourceforge.net</A>/

3135

<P>

3136

3137

<A HREF="faqw.py?req=edit&file=faq04.030.htp">Edit this entry</A> /

3138

3139

3140

/ Last changed on Tue Sep 3 16:31:31 2002 by

3141

<A HREF="mailto:polzin@gmx.de">Tobias Polzin</A>

3142

<P>

3143

3144

<HR>

3145

<H2><A NAME="4.31">4.31. How do I call a function if I have the arguments in a tuple?</A></H2>

3146

Use the built-in function apply(). For instance,

3147

<P>

3148

<PRE>

3149

func(1, 2, 3)

3150

</PRE>

3151

is equivalent to

3152

<P>

3153

<PRE>

3154

args = (1, 2, 3)

3155

apply(func, args)

3156

</PRE>

3157

Note that func(args) is not the same -- it calls func() with exactly

3158

one argument, the tuple args, instead of three arguments, the integers

3159

1, 2 and 3.

3160

<P>

3161

In Python 2.0, you can also use extended call syntax:

3162

<P>

3163

f(*args) is equivalent to apply(f, args)

3164

<P>

3165

3166

<A HREF="faqw.py?req=edit&file=faq04.031.htp">Edit this entry</A> /

3167

3168

3169

/ Last changed on Tue Jan 2 03:42:50 2001 by

3170

<A HREF="mailto:moshez@zadka.site.co.il">Moshe Zadka</A>

3171

<P>

3172

3173

<HR>

3174

<H2><A NAME="4.32">4.32. How do I enable font-lock-mode for Python in Emacs?</A></H2>

3175

If you are using XEmacs 19.14 or later, any XEmacs 20, FSF Emacs 19.34

3176

or any Emacs 20, font-lock should work automatically for you if you

3177

are using the latest python-mode.el.

3178

<P>

3179

If you are using an older version of XEmacs or Emacs you will need

3180

to put this in your .emacs file:

3181

<P>

3182

<PRE>

3183

(defun my-python-mode-hook ()

3184

(setq font-lock-keywords python-font-lock-keywords)

3185

(font-lock-mode 1))

3186

(add-hook 'python-mode-hook 'my-python-mode-hook)

3187

</PRE>

3188

<P>

3189

3190

<A HREF="faqw.py?req=edit&file=faq04.032.htp">Edit this entry</A> /

3191

3192

3193

/ Last changed on Mon Apr 6 16:18:46 1998 by

3194

<A HREF="mailto:bwarsaw@python.org">Barry Warsaw</A>

3195

<P>

3196

3197

<HR>

3198

<H2><A NAME="4.33">4.33. Is there a scanf() or sscanf() equivalent?</A></H2>

3199

Not as such.

3200

<P>

3201

For simple input parsing, the easiest approach is usually to split

3202

the line into whitespace-delimited words using string.split(), and to

3203

convert decimal strings to numeric values using int(),

3204

long() or float(). (Python's int() is 32-bit and its

3205

long() is arbitrary precision.) string.split supports an optional

3206

"sep" parameter which is useful if the line uses something other

3207

than whitespace as a delimiter.

3208

<P>

3209

For more complicated input parsing, regular expressions (see module re)

3210

are better suited and more powerful than C's sscanf().

3211

<P>

3212

There's a contributed module that emulates sscanf(), by Steve Clift;

3213

see contrib/Misc/sscanfmodule.c of the ftp site:

3214

<P>

3215

<PRE>

3216

<A HREF="http://www.python.org/ftp/python/contrib-09-Dec-1999/Misc">http://www.python.org/ftp/python/contrib-09-Dec-1999/Misc</A>/

3217

</PRE>

3218

<P>

3219

3220

<A HREF="faqw.py?req=edit&file=faq04.033.htp">Edit this entry</A> /

3221

3222

3223

/ Last changed on Mon Jun 3 01:07:51 2002 by

3224

<A HREF="mailto:neal@metaslash.com">Neal Norwitz</A>

3225

<P>

3226

3227

<HR>

3228

<H2><A NAME="4.34">4.34. Can I have Tk events handled while waiting for I/O?</A></H2>

3229

Yes, and you don't even need threads! But you'll have to

3230

restructure your I/O code a bit. Tk has the equivalent of Xt's

3231

XtAddInput() call, which allows you to register a callback function

3232

which will be called from the Tk mainloop when I/O is possible on a

3233

file descriptor. Here's what you need:

3234

<P>

3235

<PRE>

3236

from Tkinter import tkinter

3237

tkinter.createfilehandler(file, mask, callback)

3238

</PRE>

3239

The file may be a Python file or socket object (actually, anything

3240

with a fileno() method), or an integer file descriptor. The mask is

3241

one of the constants tkinter.READABLE or tkinter.WRITABLE. The

3242

callback is called as follows:

3243

<P>

3244

<PRE>

3245

callback(file, mask)

3246

</PRE>

3247

You must unregister the callback when you're done, using

3248

<P>

3249

<PRE>

3250

tkinter.deletefilehandler(file)

3251

</PRE>

3252

Note: since you don't know *how many bytes* are available for reading,

3253

you can't use the Python file object's read or readline methods, since

3254

these will insist on reading a predefined number of bytes. For

3255

sockets, the recv() or recvfrom() methods will work fine; for other

3256

files, use os.read(file.fileno(), maxbytecount).

3257

<P>

3258

3259

<A HREF="faqw.py?req=edit&file=faq04.034.htp">Edit this entry</A> /

3260

3261

<P>

3262

3263

<HR>

3264

<H2><A NAME="4.35">4.35. How do I write a function with output parameters (call by reference)?</A></H2>

3265

[Mark Lutz] The thing to remember is that arguments are passed by

3266

assignment in Python. Since assignment just creates references to

3267

objects, there's no alias between an argument name in the caller and

3268

callee, and so no call-by-reference per se. But you can simulate it

3269

in a number of ways:

3270

<P>

3271

1) By using global variables; but you probably shouldn't :-)

3272

<P>

3273

2) By passing a mutable (changeable in-place) object:

3274

<P>

3275

<PRE>

3276

def func1(a):

3277

a[0] = 'new-value' # 'a' references a mutable list

3278

a[1] = a[1] + 1 # changes a shared object

3279

</PRE>

3280

<PRE>

3281

args = ['old-value', 99]

3282

func1(args)

3283

print args[0], args[1] # output: new-value 100

3284

</PRE>

3285

3) By returning a tuple, holding the final values of arguments:

3286

<P>

3287

<PRE>

3288

def func2(a, b):

3289

a = 'new-value' # a and b are local names

3290

b = b + 1 # assigned to new objects

3291

return a, b # return new values

3292

</PRE>

3293

<PRE>

3294

x, y = 'old-value', 99

3295

x, y = func2(x, y)

3296

print x, y # output: new-value 100

3297

</PRE>

3298

4) And other ideas that fall-out from Python's object model. For instance, it might be clearer to pass in a mutable dictionary:

3299

<P>

3300

<PRE>

3301

def func3(args):

3302

args['a'] = 'new-value' # args is a mutable dictionary

3303

args['b'] = args['b'] + 1 # change it in-place

3304

</PRE>

3305

<PRE>

3306

args = {'a':' old-value', 'b': 99}

3307

func3(args)

3308

print args['a'], args['b']

3309

</PRE>

3310

5) Or bundle-up values in a class instance:

3311

<P>

3312

<PRE>

3313

class callByRef:

3314

def __init__(self, **args):

3315

for (key, value) in args.items():

3316

setattr(self, key, value)

3317

</PRE>

3318

<PRE>

3319

def func4(args):

3320

args.a = 'new-value' # args is a mutable callByRef

3321

args.b = args.b + 1 # change object in-place

3322

</PRE>

3323

<PRE>

3324

args = callByRef(a='old-value', b=99)

3325

func4(args)

3326

print args.a, args.b

3327

</PRE>

3328

<PRE>

3329

But there's probably no good reason to get this complicated :-).

3330

</PRE>

3331

[Python's author favors solution 3 in most cases.]

3332

<P>

3333

3334

<A HREF="faqw.py?req=edit&file=faq04.035.htp">Edit this entry</A> /

3335

3336

3337

/ Last changed on Sun Jun 8 23:49:46 1997 by

3338

<A HREF="mailto:david_ascher@brown.edu">David Ascher</A>

3339

<P>

3340

3341

<HR>

3342

<H2><A NAME="4.36">4.36. Please explain the rules for local and global variables in Python.</A></H2>

3343

[Ken Manheimer] In Python, procedure variables are implicitly

3344

global, unless they are assigned anywhere within the block.

3345

In that case

3346

they are implicitly local, and you need to explicitly declare them as

3347

'global'.

3348

<P>

3349

Though a bit surprising at first, a moment's consideration explains

3350

this. On one hand, requirement of 'global' for assigned vars provides

3351

a bar against unintended side-effects. On the other hand, if global

3352

were required for all global references, you'd be using global all the

3353

time. Eg, you'd have to declare as global every reference to a

3354

builtin function, or to a component of an imported module. This

3355

clutter would defeat the usefulness of the 'global' declaration for

3356

identifying side-effects.

3357

<P>

3358

3359

<A HREF="faqw.py?req=edit&file=faq04.036.htp">Edit this entry</A> /

3360

3361

3362

/ Last changed on Fri Aug 28 09:53:27 1998 by

3363

3364

<P>

3365

3366

<HR>

3367

<H2><A NAME="4.37">4.37. How can I have modules that mutually import each other?</A></H2>

3368

Suppose you have the following modules:

3369

<P>

3370

foo.py:

3371

<P>

3372

<PRE>

3373

from bar import bar_var

3374

foo_var=1

3375

</PRE>

3376

bar.py:

3377

<P>

3378

<PRE>

3379

from foo import foo_var

3380

bar_var=2

3381

</PRE>

3382

The problem is that the above is processed by the interpreter thus:

3383

<P>

3384

<PRE>

3385

main imports foo

3386

Empty globals for foo are created

3387

foo is compiled and starts executing

3388

foo imports bar

3389

Empty globals for bar are created

3390

bar is compiled and starts executing

3391

bar imports foo (which is a no-op since there already is a module named foo)

3392

bar.foo_var = foo.foo_var

3393

...

3394

</PRE>

3395

The last step fails, because Python isn't done with interpreting foo yet and the global symbol dict for foo is still empty.

3396

<P>

3397

The same thing happens when you use "import foo", and then try to access "foo.one" in global code.

3398

<P>

3399

<P>

3400

There are (at least) three possible workarounds for this problem.

3401

<P>

3402

Guido van Rossum recommends to avoid all uses of "from <module> import ..." (so everything from an imported module is referenced as <module>.<name>) and to place all code inside functions. Initializations of global variables and class variables should use constants or built-in functions only.

3403

<P>

3404

<P>

3405

Jim Roskind suggests the following order in each module:

3406

<P>

3407

<PRE>

3408

exports (globals, functions, and classes that don't need imported base classes)

3409

import statements

3410

active code (including globals that are initialized from imported values).

3411

</PRE>

3412

Python's author doesn't like this approach much because the imports

3413

appear in a strange place, but has to admit that it works.

3414

<P>

3415

<P>

3416

<P>

3417

Matthias Urlichs recommends to restructure your code so that the recursive import is not necessary in the first place.

3418

<P>

3419

<P>

3420

These solutions are not mutually exclusive.

3421

<P>

3422

3423

<A HREF="faqw.py?req=edit&file=faq04.037.htp">Edit this entry</A> /

3424

3425

3426

/ Last changed on Mon Jun 3 06:52:51 2002 by

3427

<A HREF="mailto:smurf@noris.de">Matthias Urlichs</A>

3428

<P>

3429

3430

<HR>

3431

<H2><A NAME="4.38">4.38. How do I copy an object in Python?</A></H2>

3432

Try copy.copy() or copy.deepcopy() for the general case. Not all objects can be copied, but most can.

3433

<P>

3434

Dictionaries have a copy method. Sequences can be copied by slicing:

3435

<PRE>

3436

new_l = l[:]

3437

</PRE>

3438

<P>

3439

3440

<A HREF="faqw.py?req=edit&file=faq04.038.htp">Edit this entry</A> /

3441

3442

3443

/ Last changed on Thu Mar 21 05:40:26 2002 by

3444

<A HREF="mailto:erno-pyfaq@erno.iki.fi">Erno Kuusela</A>

3445

<P>

3446

3447

<HR>

3448

<H2><A NAME="4.39">4.39. How to implement persistent objects in Python? (Persistent == automatically saved to and restored from disk.)</A></H2>

3449

The library module "pickle" now solves this in a very general way

3450

(though you still can't store things like open files, sockets or

3451

windows), and the library module "shelve" uses pickle and (g)dbm to

3452

create persistent mappings containing arbitrary Python objects.

3453

For possibly better performance also look for the latest version

3454

of the relatively recent cPickle module.

3455

<P>

3456

A more awkward way of doing things is to use pickle's little sister,

3457

marshal. The marshal module provides very fast ways to store

3458

noncircular basic Python types to files and strings, and back again.

3459

Although marshal does not do fancy things like store instances or

3460

handle shared references properly, it does run extremely fast. For

3461

example loading a half megabyte of data may take less than a

3462

third of a second (on some machines). This often beats doing

3463

something more complex and general such as using gdbm with

3464

pickle/shelve.

3465

<P>

3466

3467

<A HREF="faqw.py?req=edit&file=faq04.039.htp">Edit this entry</A> /

3468

3469

3470

/ Last changed on Sun Jun 8 22:59:00 1997 by

3471

<A HREF="mailto:david_ascher@brown.edu">David Ascher</A>

3472

<P>

3473

3474

<HR>

3475

<H2><A NAME="4.40">4.40. I try to use __spam and I get an error about _SomeClassName__spam.</A></H2>

3476

Variables with double leading underscore are "mangled" to provide a

3477

simple but effective way to define class private variables. See the

3478

chapter "New in Release 1.4" in the Python Tutorial.

3479

<P>

3480

3481

<A HREF="faqw.py?req=edit&file=faq04.040.htp">Edit this entry</A> /

3482

3483

<P>

3484

3485

<HR>

3486

<H2><A NAME="4.41">4.41. How do I delete a file? And other file questions.</A></H2>

3487

Use os.remove(filename) or os.unlink(filename); for documentation,

3488

see the posix section of the library manual. They are the same,

3489

unlink() is simply the Unix name for this function. In earlier

3490

versions of Python, only os.unlink() was available.

3491

<P>

3492

To remove a directory, use os.rmdir(); use os.mkdir() to create one.

3493

<P>

3494

To rename a file, use os.rename().

3495

<P>

3496

To truncate a file, open it using f = open(filename, "r+"), and use

3497

f.truncate(offset); offset defaults to the current seek position.

3498

(The "r+" mode opens the file for reading and writing.)

3499

There's also os.ftruncate(fd, offset) for files opened with os.open()

3500

-- for advanced Unix hacks only.

3501

<P>

3502

The shutil module also contains a number of functions to work on files

3503

including copyfile, copytree, and rmtree amongst others.

3504

<P>

3505

3506

<A HREF="faqw.py?req=edit&file=faq04.041.htp">Edit this entry</A> /

3507

3508

3509

/ Last changed on Thu Dec 28 12:30:01 2000 by

3510

<A HREF="mailto:pbjorn@uswest.net">Bjorn Pettersen</A>

3511

<P>

3512

3513

<HR>

3514

<H2><A NAME="4.42">4.42. How to modify urllib or httplib to support HTTP/1.1?</A></H2>

3515

Recent versions of Python (2.0 and onwards) support HTTP/1.1 natively.

3516

<P>

3517

3518

<A HREF="faqw.py?req=edit&file=faq04.042.htp">Edit this entry</A> /

3519

3520

3521

/ Last changed on Tue Jan 2 02:56:56 2001 by

3522

<A HREF="mailto:moshez@zadka.site.co.il">Moshe Zadka</A>

3523

<P>

3524

3525

<HR>

3526

<H2><A NAME="4.43">4.43. Unexplicable syntax errors in compile() or exec.</A></H2>

3527

When a statement suite (as opposed to an expression) is compiled by

3528

compile(), exec or execfile(), it <I>must</I> end in a newline. In some

3529

cases, when the source ends in an indented block it appears that at

3530

least two newlines are required.

3531

<P>

3532

3533

<A HREF="faqw.py?req=edit&file=faq04.043.htp">Edit this entry</A> /

3534

3535

<P>

3536

3537

<HR>

3538

<H2><A NAME="4.44">4.44. How do I convert a string to a number?</A></H2>

3539

For integers, use the built-in int() function, e.g. int('144') == 144. Similarly, long() converts from string to long integer, e.g. long('144') == 144L; and float() to floating-point, e.g. float('144') == 144.0.

3540

<P>

3541

Note that these are restricted to decimal interpretation, so

3542

that int('0144') == 144 and int('0x144') raises ValueError. For Python

3543

2.0 int takes the base to convert from as a second optional argument, so

3544

int('0x144', 16) == 324.

3545

<P>

3546

For greater flexibility, or before Python 1.5, import the module

3547

string and use the string.atoi() function for integers,

3548

string.atol() for long integers, or string.atof() for

3549

floating-point. E.g.,

3550

string.atoi('100', 16) == string.atoi('0x100', 0) == 256.

3551

See the library reference manual section for the string module for

3552

more details.

3553

<P>

3554

While you could use the built-in function eval() instead of

3555

any of those, this is not recommended, because someone could pass you

3556

a Python expression that might have unwanted side effects (like

3557

reformatting your disk). It also has the effect of interpreting numbers

3558

as Python expressions, so that e.g. eval('09') gives a syntax error

3559

since Python regards numbers starting with '0' as octal (base 8).

3560

<P>

3561

3562

<A HREF="faqw.py?req=edit&file=faq04.044.htp">Edit this entry</A> /

3563

3564

3565

/ Last changed on Thu Dec 28 12:37:34 2000 by

3566

<A HREF="mailto:pbjorn@uswest.net">Bjorn Pettersen</A>

3567

<P>

3568

3569

<HR>

3570

<H2><A NAME="4.45">4.45. How do I convert a number to a string?</A></H2>

3571

To convert, e.g., the number 144 to the string '144', use the

3572

built-in function repr() or the backquote notation (these are

3573

equivalent). If you want a hexadecimal or octal representation, use

3574

the built-in functions hex() or oct(), respectively. For fancy

3575

formatting, use the % operator on strings, just like C printf formats,

3576

e.g. "%04d" % 144 yields '0144' and "%.3f" % (1/3.0) yields '0.333'.

3577

See the library reference manual for details.

3578

<P>

3579

3580

<A HREF="faqw.py?req=edit&file=faq04.045.htp">Edit this entry</A> /

3581

3582

<P>

3583

3584

<HR>

3585

3586

There's the shutil module which contains a copyfile()

3587

function that implements a copy loop;

3588

it isn't good enough for the Macintosh, though:

3589

it doesn't copy the resource fork and Finder info.

3590

<P>

3591

3592

<A HREF="faqw.py?req=edit&file=faq04.046.htp">Edit this entry</A> /

3593

3594

3595

/ Last changed on Tue Jan 2 02:59:40 2001 by

3596

<A HREF="mailto:moshez@zadka.site.co.il">Moshe Zadka</A>

3597

<P>

3598

3599

<HR>

3600

<H2><A NAME="4.47">4.47. How do I check if an object is an instance of a given class or of a subclass of it?</A></H2>

3601

If you are developing the classes from scratch it might be better to

3602

program in a more proper object-oriented style -- instead of doing a different

3603

thing based on class membership, why not use a method and define the

3604

method differently in different classes?

3605

<P>

3606

However, there are some legitimate situations

3607

where you need to test for class membership.

3608

<P>

3609

In Python 1.5, you can use the built-in function isinstance(obj, cls).

3610

<P>

3611

The following approaches can be used with earlier Python versions:

3612

<P>

3613

An unobvious method is to raise the object

3614

as an exception and to try to catch the exception with the class you're

3615

testing for:

3616

<P>

3617

<PRE>

3618

def is_instance_of(the_instance, the_class):

3619

try:

3620

raise the_instance

3621

except the_class:

3622

return 1

3623

except:

3624

return 0

3625

</PRE>

3626

This technique can be used to distinguish "subclassness"

3627

from a collection of classes as well

3628

<P>

3629

<PRE>

3630

try:

3631

raise the_instance

3632

except Audible:

3633

the_instance.play(largo)

3634

except Visual:

3635

the_instance.display(gaudy)

3636

except Olfactory:

3637

sniff(the_instance)

3638

except:

3639

raise ValueError, "dunno what to do with this!"

3640

</PRE>

3641

This uses the fact that exception catching tests for class or subclass

3642

membership.

3643

<P>

3644

A different approach is to test for the presence of a class attribute that

3645

is presumably unique for the given class. For instance:

3646

<P>

3647

<PRE>

3648

class MyClass:

3649

ThisIsMyClass = 1

3650

...

3651

</PRE>

3652

<PRE>

3653

def is_a_MyClass(the_instance):

3654

return hasattr(the_instance, 'ThisIsMyClass')

3655

</PRE>

3656

This version is easier to inline, and probably faster (inlined it

3657

is definitely faster). The disadvantage is that someone else could cheat:

3658

<P>

3659

<PRE>

3660

class IntruderClass:

3661

ThisIsMyClass = 1 # Masquerade as MyClass

3662

...

3663

</PRE>

3664

but this may be seen as a feature (anyway, there are plenty of other ways

3665

to cheat in Python). Another disadvantage is that the class must be

3666

prepared for the membership test. If you do not "control the

3667

source code" for the class it may not be advisable to modify the

3668

class to support testability.

3669

<P>

3670

3671

<A HREF="faqw.py?req=edit&file=faq04.047.htp">Edit this entry</A> /

3672

3673

3674

/ Last changed on Fri Jan 2 15:16:04 1998 by

3675

3676

<P>

3677

3678

<HR>

3679

<H2><A NAME="4.48">4.48. What is delegation?</A></H2>

3680

Delegation refers to an object oriented technique Python programmers

3681

may implement with particular ease. Consider the following:

3682

<P>

3683

<PRE>

3684

from string import upper

3685

</PRE>

3686

<PRE>

3687

class UpperOut:

3688

def __init__(self, outfile):

3689

self.__outfile = outfile

3690

def write(self, str):

3691

self.__outfile.write( upper(str) )

3692

def __getattr__(self, name):

3693

return getattr(self.__outfile, name)

3694

</PRE>

3695

Here the UpperOut class redefines the write method

3696

to convert the argument string to upper case before

3697

calling the underlying self.__outfile.write method, but

3698

all other methods are delegated to the underlying

3699

self.__outfile object. The delegation is accomplished

3700

via the "magic" __getattr__ method. Please see the

3701

language reference for more information on the use

3702

of this method.

3703

<P>

3704

Note that for more general cases delegation can

3705

get trickier. Particularly when attributes must be set

3706

as well as gotten the class must define a __settattr__

3707

method too, and it must do so carefully.

3708

<P>

3709

The basic implementation of __setattr__ is roughly

3710

equivalent to the following:

3711

<P>

3712

<PRE>

3713

class X:

3714

...

3715

def __setattr__(self, name, value):

3716

self.__dict__[name] = value

3717

...

3718

</PRE>

3719

Most __setattr__ implementations must modify

3720

self.__dict__ to store local state for self without

3721

causing an infinite recursion.

3722

<P>

3723

3724

<A HREF="faqw.py?req=edit&file=faq04.048.htp">Edit this entry</A> /

3725

3726

3727

/ Last changed on Wed Aug 13 07:11:24 1997 by

3728

<A HREF="mailto:aaron_watters@msn.com">aaron watters</A>

3729

<P>

3730

3731

<HR>

3732

<H2><A NAME="4.49">4.49. How do I test a Python program or component.</A></H2>

3733

We presume for the purposes of this question you are interested

3734

in standalone testing, rather than testing your components inside

3735

a testing framework. The best-known testing framework for Python

3736

is the PyUnit module, maintained at

3737

<P>

3738

<PRE>

3739

<A HREF="http://pyunit.sourceforge.net">http://pyunit.sourceforge.net</A>/

3740

</PRE>

3741

For standalone testing, it helps to write the program so that

3742

it may be easily tested by using good modular design.

3743

In particular your program

3744

should have almost all functionality encapsulated in either functions

3745

or class methods -- and this sometimes has the surprising and

3746

delightful effect of making the program run faster (because

3747

local variable accesses are faster than global accesses).

3748

Furthermore the program should avoid depending on mutating

3749

global variables, since this makes testing much more difficult to do.

3750

<P>

3751

The "global main logic" of your program may be as simple

3752

as

3753

<P>

3754

<PRE>

3755

if __name__=="__main__":

3756

main_logic()

3757

</PRE>

3758

at the bottom of the main module of your program.

3759

<P>

3760

Once your program is organized as a tractable collection

3761

of functions and class behaviours you should write test

3762

functions that exercise the behaviours. A test suite

3763

can be associated with each module which automates

3764

a sequence of tests. This sounds like a lot of work, but

3765

since Python is so terse and flexible it's surprisingly easy.

3766

You can make coding much more pleasant and fun by

3767

writing your test functions in parallel with the "production

3768

code", since this makes it easy to find bugs and even

3769

design flaws earlier.

3770

<P>

3771

"Support modules" that are not intended to be the main

3772

module of a program may include a "test script interpretation"

3773

which invokes a self test of the module.

3774

<P>

3775

<PRE>

3776

if __name__ == "__main__":

3777

self_test()

3778

</PRE>

3779

Even programs that interact with complex external

3780

interfaces may be tested when the external interfaces are

3781

unavailable by using "fake" interfaces implemented in

3782

Python. For an example of a "fake" interface, the following

3783

class defines (part of) a "fake" file interface:

3784

<P>

3785

<PRE>

3786

import string

3787

testdata = "just a random sequence of characters"

3788

</PRE>

3789

<PRE>

3790

class FakeInputFile:

3791

data = testdata

3792

position = 0

3793

closed = 0

3794

</PRE>

3795

<PRE>

3796

def read(self, n=None):

3797

self.testclosed()

3798

p = self.position

3799

if n is None:

3800

result= self.data[p:]

3801

else:

3802

result= self.data[p: p+n]

3803

self.position = p + len(result)

3804

return result

3805

</PRE>

3806

<PRE>

3807

def seek(self, n, m=0):

3808

self.testclosed()

3809

last = len(self.data)

3810

p = self.position

3811

if m==0:

3812

final=n

3813

elif m==1:

3814

final=n+p

3815

elif m==2:

3816

final=len(self.data)+n

3817

else:

3818

raise ValueError, "bad m"

3819

if final<0:

3820

raise IOError, "negative seek"

3821

self.position = final

3822

</PRE>

3823

<PRE>

3824

def isatty(self):

3825

return 0

3826

</PRE>

3827

<PRE>

3828

def tell(self):

3829

return self.position

3830

</PRE>

3831

<PRE>

3832

def close(self):

3833

self.closed = 1

3834

</PRE>

3835

<PRE>

3836

def testclosed(self):

3837

if self.closed:

3838

raise IOError, "file closed"

3839

</PRE>

3840

Try f=FakeInputFile() and test out its operations.

3841

<P>

3842

3843

<A HREF="faqw.py?req=edit&file=faq04.049.htp">Edit this entry</A> /

3844

3845

3846

/ Last changed on Mon Jun 3 01:12:10 2002 by

3847

<A HREF="mailto:neal@metaslash.com">Neal Norwitz</A>

3848

<P>

3849

3850

<HR>

3851

<H2><A NAME="4.50">4.50. My multidimensional list (array) is broken! What gives?</A></H2>

3852

You probably tried to make a multidimensional array like this.

3853

<P>

3854

<PRE>

3855

A = [[None] * 2] * 3

3856

</PRE>

3857

This makes a list containing 3 references to the same list of length

3858

two. Changes to one row will show in all rows, which is probably not

3859

what you want. The following works much better:

3860

<P>

3861

<PRE>

3862

A = [None]*3

3863

for i in range(3):

3864

A[i] = [None] * 2

3865

</PRE>

3866

This generates a list containing 3 different lists of length two.

3867

<P>

3868

If you feel weird, you can also do it in the following way:

3869

<P>

3870

<PRE>

3871

w, h = 2, 3

3872

A = map(lambda i,w=w: [None] * w, range(h))

3873

</PRE>

3874

For Python 2.0 the above can be spelled using a list comprehension:

3875

<P>

3876

<PRE>

3877

w,h = 2,3

3878

A = [ [None]*w for i in range(h) ]

3879

</PRE>

3880

<P>

3881

3882

<A HREF="faqw.py?req=edit&file=faq04.050.htp">Edit this entry</A> /

3883

3884

3885

/ Last changed on Thu Dec 28 12:18:35 2000 by

3886

<A HREF="mailto:pbjorn@uswest.net">Bjorn Pettersen</A>

3887

<P>

3888

3889

<HR>

3890

<H2><A NAME="4.51">4.51. I want to do a complicated sort: can you do a Schwartzian Transform in Python?</A></H2>

3891

Yes, and in Python you only have to write it once:

3892

<P>

3893

<PRE>

3894

def st(List, Metric):

3895

def pairing(element, M = Metric):

3896

return (M(element), element)

3897

paired = map(pairing, List)

3898

paired.sort()

3899

return map(stripit, paired)

3900

</PRE>

3901

<PRE>

3902

def stripit(pair):

3903

return pair[1]

3904

</PRE>

3905

This technique, attributed to Randal Schwartz, sorts the elements

3906

of a list by a metric which maps each element to its "sort value".

3907

For example, if L is a list of string then

3908

<P>

3909

<PRE>

3910

import string

3911

Usorted = st(L, string.upper)

3912

</PRE>

3913

<PRE>

3914

def intfield(s):

3915

return string.atoi( string.strip(s[10:15] ) )

3916

</PRE>

3917

<PRE>

3918

Isorted = st(L, intfield)

3919

</PRE>

3920

Usorted gives the elements of L sorted as if they were upper

3921

case, and Isorted gives the elements of L sorted by the integer

3922

values that appear in the string slices starting at position 10

3923

and ending at position 15. In Python 2.0 this can be done more

3924

naturally with list comprehensions:

3925

<P>

3926

<PRE>

3927

tmp1 = [ (x.upper(), x) for x in L ] # Schwartzian transform

3928

tmp1.sort()

3929

Usorted = [ x[1] for x in tmp1 ]

3930

</PRE>

3931

<PRE>

3932

tmp2 = [ (int(s[10:15]), s) for s in L ] # Schwartzian transform

3933

tmp2.sort()

3934

Isorted = [ x[1] for x in tmp2 ]

3935

</PRE>

3936

<P>

3937

Note that Isorted may also be computed by

3938

<P>

3939

<PRE>

3940

def Icmp(s1, s2):

3941

return cmp( intfield(s1), intfield(s2) )

3942

</PRE>

3943

<PRE>

3944

Isorted = L[:]

3945

Isorted.sort(Icmp)

3946

</PRE>

3947

but since this method computes intfield many times for each

3948

element of L, it is slower than the Schwartzian Transform.

3949

<P>

3950

3951

<A HREF="faqw.py?req=edit&file=faq04.051.htp">Edit this entry</A> /

3952

3953

3954

/ Last changed on Sat Jun 1 19:18:46 2002 by

3955

<A HREF="mailto:neal@metaslash.com">Neal Norwitz</A>

3956

<P>

3957

3958

<HR>

3959

<H2><A NAME="4.52">4.52. How to convert between tuples and lists?</A></H2>

3960

The function tuple(seq) converts any sequence into a tuple with

3961

the same items in the same order.

3962

For example, tuple([1, 2, 3]) yields (1, 2, 3) and tuple('abc')

3963

yields ('a', 'b', 'c'). If the argument is

3964

a tuple, it does not make a copy but returns the same object, so

3965

it is cheap to call tuple() when you aren't sure that an object

3966

is already a tuple.

3967

<P>

3968

The function list(seq) converts any sequence into a list with

3969

the same items in the same order.

3970

For example, list((1, 2, 3)) yields [1, 2, 3] and list('abc')

3971

yields ['a', 'b', 'c']. If the argument is a list,

3972

it makes a copy just like seq[:] would.

3973

<P>

3974

3975

<A HREF="faqw.py?req=edit&file=faq04.052.htp">Edit this entry</A> /

3976

3977

3978

/ Last changed on Sun Jun 14 14:18:53 1998 by

3979

<A HREF="mailto:tim_one@email.msn.com">Tim Peters</A>

3980

<P>

3981

3982

<HR>

3983

<H2><A NAME="4.53">4.53. Files retrieved with urllib contain leading garbage that looks like email headers.</A></H2>

3984

<I>Extremely</I> old versions of Python supplied libraries which

3985

did not support HTTP/1.1; the vanilla httplib in Python 1.4

3986

only recognized HTTP/1.0. In Python 2.0 full HTTP/1.1 support is included.

3987

<P>

3988

3989

<A HREF="faqw.py?req=edit&file=faq04.053.htp">Edit this entry</A> /

3990

3991

3992

/ Last changed on Mon Jan 8 17:26:18 2001 by

3993

<A HREF="mailto:sholden@holdenweb.com">Steve Holden</A>

3994

<P>

3995

3996

<HR>

3997

<H2><A NAME="4.54">4.54. How do I get a list of all instances of a given class?</A></H2>

3998

Python does not keep track of all instances of a class (or of a

3999

built-in type).

4000

<P>

4001

You can program the class's constructor to keep track of all

4002

instances, but unless you're very clever, this has the disadvantage

4003

that the instances never get deleted,because your list of all

4004

instances keeps a reference to them.

4005

<P>

4006

(The trick is to regularly inspect the reference counts of the

4007

instances you've retained, and if the reference count is below a

4008

certain level, remove it from the list. Determining that level is

4009

tricky -- it's definitely larger than 1.)

4010

<P>

4011

4012

<A HREF="faqw.py?req=edit&file=faq04.054.htp">Edit this entry</A> /

4013

4014

4015

/ Last changed on Tue May 27 23:52:16 1997 by

4016

4017

<P>

4018

4019

<HR>

4020

<H2><A NAME="4.55">4.55. A regular expression fails with regex.error: match failure.</A></H2>

4021

This is usually caused by too much backtracking; the regular

4022

expression engine has a fixed size stack which holds at most 4000

4023

backtrack points. Every character matched by e.g. ".*" accounts for a

4024

backtrack point, so even a simple search like

4025

<P>

4026

<PRE>

4027

regex.match('.*x',"x"*5000)

4028

</PRE>

4029

will fail.

4030

<P>

4031

This is fixed in the re module introduced with

4032

Python 1.5; consult the Library Reference section on re for more information.

4033

<P>

4034

4035

<A HREF="faqw.py?req=edit&file=faq04.055.htp">Edit this entry</A> /

4036

4037

4038

/ Last changed on Thu Jul 30 12:35:49 1998 by

4039

<A HREF="mailto:akuchlin@cnri.reston.va.us">A.M. Kuchling</A>

4040

<P>

4041

4042

<HR>

4043

<H2><A NAME="4.56">4.56. I can't get signal handlers to work.</A></H2>

4044

The most common problem is that the signal handler is declared

4045

with the wrong argument list. It is called as

4046

<P>

4047

<PRE>

4048

handler(signum, frame)

4049

</PRE>

4050

so it should be declared with two arguments:

4051

<P>

4052

<PRE>

4053

def handler(signum, frame):

4054

...

4055

</PRE>

4056

<P>

4057

4058

<A HREF="faqw.py?req=edit&file=faq04.056.htp">Edit this entry</A> /

4059

4060

4061

/ Last changed on Wed May 28 09:29:08 1997 by

4062

4063

<P>

4064

4065

<HR>

4066

<H2><A NAME="4.57">4.57. I can't use a global variable in a function? Help!</A></H2>

4067

Did you do something like this?

4068

<P>

4069

<PRE>

4070

x = 1 # make a global

4071

</PRE>

4072

<PRE>

4073

def f():

4074

print x # try to print the global

4075

...

4076

for j in range(100):

4077

if q>3:

4078

x=4

4079

</PRE>

4080

Any variable assigned in a function is local to that function.

4081

unless it is specifically declared global. Since a value is bound

4082

to x as the last statement of the function body, the compiler

4083

assumes that x is local. Consequently the "print x"

4084

attempts to print an uninitialized local variable and will

4085

trigger a NameError.

4086

<P>

4087

In such cases the solution is to insert an explicit global

4088

declaration at the start of the function, making it

4089

<P>

4090

<P>

4091

<PRE>

4092

def f():

4093

global x

4094

print x # try to print the global

4095

...

4096

for j in range(100):

4097

if q>3:

4098

x=4

4099

</PRE>

4100

<P>

4101

In this case, all references to x are interpreted as references

4102

to the x from the module namespace.

4103

<P>

4104

4105

<A HREF="faqw.py?req=edit&file=faq04.057.htp">Edit this entry</A> /

4106

4107

4108

/ Last changed on Mon Feb 12 15:52:12 2001 by

4109

<A HREF="mailto:sholden@holdenweb.com">Steve Holden</A>

4110

<P>

4111

4112

<HR>

4113

<H2><A NAME="4.58">4.58. What's a negative index? Why doesn't list.insert() use them?</A></H2>

4114

Python sequences are indexed with positive numbers and

4115

negative numbers. For positive numbers 0 is the first index

4116

1 is the second index and so forth. For negative indices -1

4117

is the last index and -2 is the pentultimate (next to last) index

4118

and so forth. Think of seq[-n] as the same as seq[len(seq)-n].

4119

<P>

4120

Using negative indices can be very convenient. For example

4121

if the string Line ends in a newline then Line[:-1] is all of Line except

4122

the newline.

4123

<P>

4124

Sadly the list builtin method L.insert does not observe negative

4125

indices. This feature could be considered a mistake but since

4126

existing programs depend on this feature it may stay around

4127

forever. L.insert for negative indices inserts at the start of the

4128

list. To get "proper" negative index behaviour use L[n:n] = [x]

4129

in place of the insert method.

4130

<P>

4131

4132

<A HREF="faqw.py?req=edit&file=faq04.058.htp">Edit this entry</A> /

4133

4134

4135

/ Last changed on Wed Aug 13 07:03:18 1997 by

4136

<A HREF="mailto:aaron_watters@msn.com">aaron watters</A>

4137

<P>

4138

4139

<HR>

4140

<H2><A NAME="4.59">4.59. How can I sort one list by values from another list?</A></H2>

4141

You can sort lists of tuples.

4142

<P>

4143

<PRE>

4144

>>> list1 = ["what", "I'm", "sorting", "by"]

4145

>>> list2 = ["something", "else", "to", "sort"]

4146

>>> pairs = map(None, list1, list2)

4147

>>> pairs

4148

[('what', 'something'), ("I'm", 'else'), ('sorting', 'to'), ('by', 'sort')]

4149

>>> pairs.sort()

4150

>>> pairs

4151

[("I'm", 'else'), ('by', 'sort'), ('sorting', 'to'), ('what', 'something')]

4152

>>> result = pairs[:]

4153

>>> for i in xrange(len(result)): result[i] = result[i][1]

4154

...

4155

>>> result

4156

['else', 'sort', 'to', 'something']

4157

</PRE>

4158

And if you didn't understand the question, please see the

4159

example above ;c). Note that "I'm" sorts before "by" because

4160

uppercase "I" comes before lowercase "b" in the ascii order.

4161

Also see 4.51.

4162

<P>

4163

In Python 2.0 this can be done like:

4164

<P>

4165

<PRE>

4166

>>> list1 = ["what", "I'm", "sorting", "by"]

4167

>>> list2 = ["something", "else", "to", "sort"]

4168

>>> pairs = zip(list1, list2)

4169

>>> pairs

4170

[('what', 'something'), ("I'm", 'else'), ('sorting', 'to'), ('by', 'sort')]

4171

>>> pairs.sort()

4172

>>> result = [ x[1] for x in pairs ]

4173

>>> result

4174

['else', 'sort', 'to', 'something']

4175

</PRE>

4176

[Followup]

4177

<P>

4178

Someone asked, why not this for the last steps:

4179

<P>

4180

<PRE>

4181

result = []

4182

for p in pairs: result.append(p[1])

4183

</PRE>

4184

This is much more legible. However, a quick test shows that

4185

it is almost twice as slow for long lists. Why? First of all,

4186

the append() operation has to reallocate memory, and while it

4187

uses some tricks to avoid doing that each time, it still has

4188

to do it occasionally, and apparently that costs quite a bit.

4189

Second, the expression "result.append" requires an extra

4190

attribute lookup. The attribute lookup could be done away

4191

with by rewriting as follows:

4192

<P>

4193

<PRE>

4194

result = []

4195

append = result.append

4196

for p in pairs: append(p[1])

4197

</PRE>

4198

which gains back some speed, but is still considerably slower

4199

than the original solution, and hardly less convoluted.

4200

<P>

4201

4202

<A HREF="faqw.py?req=edit&file=faq04.059.htp">Edit this entry</A> /

4203

4204

4205

/ Last changed on Thu Dec 28 12:56:35 2000 by

4206

<A HREF="mailto:pbjorn@uswest.net">Bjorn Pettersen</A>

4207

<P>

4208

4209

<HR>

4210

<H2><A NAME="4.60">4.60. Why doesn't dir() work on builtin types like files and lists?</A></H2>

4211

It does starting with Python 1.5.

4212

<P>

4213

Using 1.4, you can find out which methods a given object supports

4214

by looking at its __methods__ attribute:

4215

<P>

4216

<PRE>

4217

>>> List = []

4218

>>> List.__methods__

4219

['append', 'count', 'index', 'insert', 'remove', 'reverse', 'sort']

4220

</PRE>

4221

<P>

4222

4223

<A HREF="faqw.py?req=edit&file=faq04.060.htp">Edit this entry</A> /

4224

4225

4226

/ Last changed on Thu Sep 16 14:56:42 1999 by

4227

<A HREF="mailto:skip@mojam.com">Skip Montanaro</A>

4228

<P>

4229

4230

<HR>

4231

<H2><A NAME="4.61">4.61. How can I mimic CGI form submission (METHOD=POST)?</A></H2>

4232

I would like to retrieve web pages that are the result of POSTing a

4233

form. Is there existing code that would let me do this easily?

4234

<P>

4235

Yes. Here's a simple example that uses httplib.

4236

<P>

4237

<PRE>

4238

#!/usr/local/bin/python

4239

</PRE>

4240

<PRE>

4241

import httplib, sys, time

4242

</PRE>

4243

<PRE>

4244

### build the query string

4245

qs = "First=Josephine&MI=Q&Last=Public"

4246

</PRE>

4247

<PRE>

4248

### connect and send the server a path

4249

httpobj = httplib.HTTP('www.some-server.out-there', 80)

4250

httpobj.putrequest('POST', '/cgi-bin/some-cgi-script')

4251

### now generate the rest of the HTTP headers...

4252

httpobj.putheader('Accept', '*/*')

4253

httpobj.putheader('Connection', 'Keep-Alive')

4254

httpobj.putheader('Content-type', 'application/x-www-form-urlencoded')

4255

httpobj.putheader('Content-length', '%d' % len(qs))

4256

httpobj.endheaders()

4257

httpobj.send(qs)

4258

### find out what the server said in response...

4259

reply, msg, hdrs = httpobj.getreply()

4260

if reply != 200:

4261

sys.stdout.write(httpobj.getfile().read())

4262

</PRE>

4263

Note that in general for "url encoded posts" (the default) query strings must be "quoted" to, for example, change equals signs and spaces to an encoded form when they occur in name or value. Use urllib.quote to perform this quoting. For example to send name="Guy Steele, Jr.":

4264

<P>

4265

<PRE>

4266

>>> from urllib import quote

4267

>>> x = quote("Guy Steele, Jr.")

4268

>>> x

4269

'Guy%20Steele,%20Jr.'

4270

>>> query_string = "name="+x

4271

>>> query_string

4272

'name=Guy%20Steele,%20Jr.'

4273

</PRE>

4274

<P>

4275

4276

<A HREF="faqw.py?req=edit&file=faq04.061.htp">Edit this entry</A> /

4277

4278

4279

/ Last changed on Mon Jun 21 03:47:07 1999 by

4280

4281

<P>

4282

4283

<HR>

4284

<H2><A NAME="4.62">4.62. If my program crashes with a bsddb (or anydbm) database open, it gets corrupted. How come?</A></H2>

4285

Databases opened for write access with the bsddb module (and often by

4286

the anydbm module, since it will preferentially use bsddb) must

4287

explicitly be closed using the close method of the database. The

4288

underlying libdb package caches database contents which need to be

4289

converted to on-disk form and written, unlike regular open files which

4290

already have the on-disk bits in the kernel's write buffer, where they

4291

can just be dumped by the kernel with the program exits.

4292

<P>

4293

If you have initialized a new bsddb database but not written anything to

4294

it before the program crashes, you will often wind up with a zero-length

4295

file and encounter an exception the next time the file is opened.

4296

<P>

4297

4298

<A HREF="faqw.py?req=edit&file=faq04.062.htp">Edit this entry</A> /

4299

4300

4301

/ Last changed on Mon Jun 3 01:15:01 2002 by

4302

<A HREF="mailto:neal@metaslash.com">Neal Norwitz</A>

4303

<P>

4304

4305

<HR>

4306

<H2><A NAME="4.63">4.63. How do I make a Python script executable on Unix?</A></H2>

4307

You need to do two things: the script file's mode must be executable

4308

(include the 'x' bit), and the first line must begin with #!

4309

followed by the pathname for the Python interpreter.

4310

<P>

4311

The first is done by executing 'chmod +x scriptfile' or perhaps

4312

'chmod 755 scriptfile'.

4313

<P>

4314

The second can be done in a number of way. The most straightforward

4315

way is to write

4316

<P>

4317

<PRE>

4318

#!/usr/local/bin/python

4319

</PRE>

4320

as the very first line of your file - or whatever the pathname is

4321

where the python interpreter is installed on your platform.

4322

<P>

4323

If you would like the script to be independent of where the python

4324

interpreter lives, you can use the "env" program. On almost all

4325

platforms, the following will work, assuming the python interpreter

4326

is in a directory on the user's $PATH:

4327

<P>

4328

<PRE>

4329

#! /usr/bin/env python

4330

</PRE>

4331

Note -- *don't* do this for CGI scripts. The $PATH variable for

4332

CGI scripts is often very minimal, so you need to use the actual

4333

absolute pathname of the interpreter.

4334

<P>

4335

Occasionally, a user's environment is so full that the /usr/bin/env

4336

program fails; or there's no env program at all.

4337

In that case, you can try the following hack (due to Alex Rezinsky):

4338

<P>

4339

<PRE>

4340

#! /bin/sh

4341

""":"

4342

exec python $0 ${1+"$@"}

4343

"""

4344

</PRE>

4345

The disadvantage is that this defines the script's __doc__ string.

4346

However, you can fix that by adding

4347

<P>

4348

<PRE>

4349

__doc__ = """...Whatever..."""

4350

</PRE>

4351

<P>

4352

4353

<A HREF="faqw.py?req=edit&file=faq04.063.htp">Edit this entry</A> /

4354

4355

4356

/ Last changed on Mon Jan 15 09:19:16 2001 by

4357

<A HREF="mailto:neal@metaslash.com">Neal Norwitz</A>

4358

<P>

4359

4360

<HR>

4361

<H2><A NAME="4.64">4.64. How do you remove duplicates from a list?</A></H2>

4362

See the Python Cookbook for a long discussion of many cool ways:

4363

<P>

4364

<PRE>

4365

<A HREF="http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/52560">http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/52560</A>

4366

</PRE>

4367

Generally, if you don't mind reordering the List

4368

<P>

4369

<PRE>

4370

if List:

4371

List.sort()

4372

last = List[-1]

4373

for i in range(len(List)-2, -1, -1):

4374

if last==List[i]: del List[i]

4375

else: last=List[i]

4376

</PRE>

4377

If all elements of the list may be used as

4378

dictionary keys (ie, they are all hashable)

4379

this is often faster

4380

<P>

4381

<PRE>

4382

d = {}

4383

for x in List: d[x]=x

4384

List = d.values()

4385

</PRE>

4386

Also, for extremely large lists you might

4387

consider more optimal alternatives to the first one.

4388

The second one is pretty good whenever it can

4389

be used.

4390

<P>

4391

4392

<A HREF="faqw.py?req=edit&file=faq04.064.htp">Edit this entry</A> /

4393

4394

4395

/ Last changed on Fri May 24 21:56:33 2002 by

4396

<A HREF="mailto:tim.one@comcast.net">Tim Peters</A>

4397

<P>

4398

4399

<HR>

4400

<H2><A NAME="4.65">4.65. Are there any known year 2000 problems in Python?</A></H2>

4401

I am not aware of year 2000 deficiencies in Python 1.5. Python does

4402

very few date calculations and for what it does, it relies on the C

4403

library functions. Python generally represent times either as seconds

4404

since 1970 or as a tuple (year, month, day, ...) where the year is

4405

expressed with four digits, which makes Y2K bugs unlikely. So as long

4406

as your C library is okay, Python should be okay. Of course, I cannot

4407

vouch for <I>your</I> Python code!

4408

<P>

4409

Given the nature of freely available software, I have to add that this statement is not

4410

legally binding. The Python copyright notice contains the following

4411

disclaimer:

4412

<P>

4413

<PRE>

4414

STICHTING MATHEMATISCH CENTRUM AND CNRI DISCLAIM ALL WARRANTIES WITH

4415

REGARD TO THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF

4416

MERCHANTABILITY AND FITNESS, IN NO EVENT SHALL STICHTING MATHEMATISCH

4417

CENTRUM OR CNRI BE LIABLE FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL

4418

DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR

4419

PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER

4420

TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR

4421

PERFORMANCE OF THIS SOFTWARE.

4422

</PRE>

4423

The good news is that <I>if</I> you encounter a problem, you have full

4424

source available to track it down and fix it!

4425

<P>

4426

4427

<A HREF="faqw.py?req=edit&file=faq04.065.htp">Edit this entry</A> /

4428

4429

4430

/ Last changed on Fri Apr 10 14:59:31 1998 by

4431

4432

<P>

4433

4434

<HR>

4435

<H2><A NAME="4.66">4.66. I want a version of map that applies a method to a sequence of objects! Help!</A></H2>

4436

Get fancy!

4437

<P>

4438

<PRE>

4439

def method_map(objects, method, arguments):

4440

"""method_map([a,b], "flog", (1,2)) gives [a.flog(1,2), b.flog(1,2)]"""

4441

nobjects = len(objects)

4442

methods = map(getattr, objects, [method]*nobjects)

4443

return map(apply, methods, [arguments]*nobjects)

4444

</PRE>

4445

It's generally a good idea to get to know the mysteries of map and apply

4446

and getattr and the other dynamic features of Python.

4447

<P>

4448

4449

<A HREF="faqw.py?req=edit&file=faq04.066.htp">Edit this entry</A> /

4450

4451

4452

/ Last changed on Mon Jan 5 14:21:14 1998 by

4453

<A HREF="mailto:aaron_watters@msn.com">Aaron Watters</A>

4454

<P>

4455

4456

<HR>

4457

<H2><A NAME="4.67">4.67. How do I generate random numbers in Python?</A></H2>

4458

The standard library module "random" implements a random number

4459

generator. Usage is simple:

4460

<P>

4461

<PRE>

4462

import random

4463

</PRE>

4464

<PRE>

4465

random.random()

4466

</PRE>

4467

This returns a random floating point number in the range [0, 1).

4468

<P>

4469

There are also many other specialized generators in this module, such

4470

as

4471

<P>

4472

<PRE>

4473

randrange(a, b) chooses an integer in the range [a, b)

4474

uniform(a, b) chooses a floating point number in the range [a, b)

4475

normalvariate(mean, sdev) sample from normal (Gaussian) distribution

4476

</PRE>

4477

Some higher-level functions operate on sequences directly, such as

4478

<P>

4479

<PRE>

4480

choice(S) chooses random element from a given sequence

4481

shuffle(L) shuffles a list in-place, i.e. permutes it randomly

4482

</PRE>

4483

There's also a class, Random, which you can instantiate

4484

to create independent multiple random number generators.

4485

<P>

4486

All this is documented in the library reference manual. Note that

4487

the module "whrandom" is obsolete.

4488

<P>

4489

4490

<A HREF="faqw.py?req=edit&file=faq04.067.htp">Edit this entry</A> /

4491

4492

4493

/ Last changed on Mon Jun 3 01:16:51 2002 by

4494

<A HREF="mailto:neal@metaslash.com">Neal Norwitz</A>

4495

<P>

4496

4497

<HR>

4498

<H2><A NAME="4.68">4.68. How do I access the serial (RS232) port?</A></H2>

4499

There's a Windows serial communication module (for communication

4500

over RS 232 serial ports) at

4501

<P>

4502

<PRE>

4503

<A HREF="ftp://ftp.python.org/pub/python/contrib/sio-151.zip">ftp://ftp.python.org/pub/python/contrib/sio-151.zip</A>

4504

<A HREF="http://www.python.org/ftp/python/contrib/sio-151.zip">http://www.python.org/ftp/python/contrib/sio-151.zip</A>

4505

</PRE>

4506

For DOS, try Hans Nowak's Python-DX, which supports this, at:

4507

<P>

4508

<PRE>

4509

<A HREF="http://www.cuci.nl/~hnowak">http://www.cuci.nl/~hnowak</A>/

4510

</PRE>

4511

For Unix, see a usenet post by Mitch Chapman:

4512

<P>

4513

<PRE>

4514

<A HREF="http://groups.google.com/groups?selm=34A04430.CF9@ohioee.com">http://groups.google.com/groups?selm=34A04430.CF9@ohioee.com</A>

4515

</PRE>

4516

For Win32, POSIX(Linux, BSD, *), Jython, Chris':

4517

<P>

4518

<PRE>

4519

<A HREF="http://pyserial.sourceforge.net">http://pyserial.sourceforge.net</A>

4520

</PRE>

4521

<P>

4522

4523

<A HREF="faqw.py?req=edit&file=faq04.068.htp">Edit this entry</A> /

4524

4525

4526

/ Last changed on Tue Jul 2 21:11:07 2002 by

4527

<A HREF="mailto:cliechti@gmx.net">Chris Liechti</A>

4528

<P>

4529

4530

<HR>

4531

<H2><A NAME="4.69">4.69. Images on Tk-Buttons don't work in Py15?</A></H2>

4532

They <I>do</I> work, but you must keep your own <I>reference</I> to the image

4533

object now. More verbosely, you must make sure that, say, a global

4534

variable or a class attribute refers to the object.

4535

<P>

4536

Quoting Fredrik Lundh from the mailinglist:

4537

<P>

4538

<PRE>

4539

Well, the Tk button widget keeps a reference to the internal

4540

photoimage object, but Tkinter does not. So when the last

4541

Python reference goes away, Tkinter tells Tk to release the

4542

photoimage. But since the image is in use by a widget, Tk

4543

doesn't destroy it. Not completely. It just blanks the image,

4544

making it completely transparent...

4545

</PRE>

4546

<PRE>

4547

And yes, there was a bug in the keyword argument handling

4548

in 1.4 that kept an extra reference around in some cases. And

4549

when Guido fixed that bug in 1.5, he broke quite a few Tkinter

4550

programs...

4551

</PRE>

4552

<P>

4553

4554

<A HREF="faqw.py?req=edit&file=faq04.069.htp">Edit this entry</A> /

4555

4556

4557

/ Last changed on Tue Feb 3 11:31:03 1998 by

4558

<A HREF="mailto:cjr@euronet.nl">Case Roole</A>

4559

<P>

4560

4561

<HR>

4562

<H2><A NAME="4.70">4.70. Where is the math.py (socket.py, regex.py, etc.) source file?</A></H2>

4563

If you can't find a source file for a module it may be a builtin

4564

or dynamically loaded module implemented in C, C++ or other

4565

compiled language. In this case you may not have the source

4566

file or it may be something like mathmodule.c, somewhere in

4567

a C source directory (not on the Python Path).

4568

<P>

4569

Fredrik Lundh (<A HREF="mailto:fredrik@pythonware.com">fredrik@pythonware.com</A>) explains (on the python-list):

4570

<P>

4571

There are (at least) three kinds of modules in Python:

4572

1) modules written in Python (.py);

4573

2) modules written in C and dynamically loaded (.dll, .pyd, .so, .sl, etc);

4574

3) modules written in C and linked with the interpreter; to get a list

4575

of these, type:

4576

<P>

4577

<PRE>

4578

import sys

4579

print sys.builtin_module_names

4580

</PRE>

4581

<P>

4582

4583

<A HREF="faqw.py?req=edit&file=faq04.070.htp">Edit this entry</A> /

4584

4585

4586

/ Last changed on Tue Feb 3 13:55:33 1998 by

4587

<A HREF="mailto:aaron_watters@msn.com">Aaron Watters</A>

4588

<P>

4589

4590

<HR>

4591

<H2><A NAME="4.71">4.71. How do I send mail from a Python script?</A></H2>

4592

The standard library module smtplib does this.

4593

Here's a very simple interactive mail

4594

sender that uses it. This method will work on any host that

4595

supports an SMTP listener.

4596

<P>

4597

<PRE>

4598

import sys, smtplib

4599

</PRE>

4600

<PRE>

4601

fromaddr = raw_input("From: ")

4602

toaddrs = raw_input("To: ").split(',')

4603

print "Enter message, end with ^D:"

4604

msg = ''

4605

while 1:

4606

line = sys.stdin.readline()

4607

if not line:

4608

break

4609

msg = msg + line

4610

</PRE>

4611

<PRE>

4612

# The actual mail send

4613

server = smtplib.SMTP('localhost')

4614

server.sendmail(fromaddr, toaddrs, msg)

4615

server.quit()

4616

</PRE>

4617

If the local host doesn't have an SMTP listener, you need to find one. The simple method is to ask the user. Alternately, you can use the DNS system to find the mail gateway(s) responsible for the source address.

4618

<P>

4619

A Unix-only alternative uses sendmail. The location of the

4620

sendmail program varies between systems; sometimes it is

4621

/usr/lib/sendmail, sometime /usr/sbin/sendmail. The sendmail manual

4622

page will help you out. Here's some sample code:

4623

<P>

4624

<PRE>

4625

SENDMAIL = "/usr/sbin/sendmail" # sendmail location

4626

import os

4627

p = os.popen("%s -t -i" % SENDMAIL, "w")

4628

p.write("To: <A HREF="mailto:cary@ratatosk.org">cary@ratatosk.org</A>\n")

4629

p.write("Subject: test\n")

4630

p.write("\n") # blank line separating headers from body

4631

p.write("Some text\n")

4632

p.write("some more text\n")

4633

sts = p.close()

4634

if sts != 0:

4635

print "Sendmail exit status", sts

4636

</PRE>

4637

<P>

4638

4639

<A HREF="faqw.py?req=edit&file=faq04.071.htp">Edit this entry</A> /

4640

4641

4642

/ Last changed on Mon Jun 3 07:05:12 2002 by

4643

<A HREF="mailto:smurf@noris.de">Matthias Urlichs</A>

4644

<P>

4645

4646

<HR>

4647

<H2><A NAME="4.72">4.72. How do I avoid blocking in connect() of a socket?</A></H2>

4648

The select module is widely known to help with asynchronous

4649

I/O on sockets once they are connected. However, it is less

4650

than common knowledge how to avoid blocking on the initial

4651

connect() call. Jeremy Hylton has the following advice (slightly

4652

edited):

4653

<P>

4654

To prevent the TCP connect from blocking, you can set the socket to

4655

non-blocking mode. Then when you do the connect(), you will either

4656

connect immediately (unlikely) or get an exception that contains the

4657

errno. errno.EINPROGRESS indicates that the connection is in

4658

progress, but hasn't finished yet. Different OSes will return

4659

different errnos, so you're going to have to check. I can tell you

4660

that different versions of Solaris return different errno values.

4661

<P>

4662

In Python 1.5 and later, you can use connect_ex() to avoid

4663

creating an exception. It will just return the errno value.

4664

<P>

4665

To poll, you can call connect_ex() again later -- 0 or errno.EISCONN

4666

indicate that you're connected -- or you can pass this socket to

4667

select (checking to see if it is writeable).

4668

<P>

4669

4670

<A HREF="faqw.py?req=edit&file=faq04.072.htp">Edit this entry</A> /

4671

4672

4673

/ Last changed on Tue Feb 24 21:30:45 1998 by

4674

4675

<P>

4676

4677

<HR>

4678

<H2><A NAME="4.73">4.73. How do I specify hexadecimal and octal integers?</A></H2>

4679

To specify an octal digit, precede the octal value with a zero. For example,

4680

to set the variable "a" to the octal value "10" (8 in decimal), type:

4681

<P>

4682

<PRE>

4683

>>> a = 010

4684

</PRE>

4685

To verify that this works, you can type "a" and hit enter while in the

4686

interpreter, which will cause Python to spit out the current value of "a"

4687

in decimal:

4688

<P>

4689

<PRE>

4690

>>> a

4691

8

4692

</PRE>

4693

Hexadecimal is just as easy. Simply precede the hexadecimal number with a

4694

zero, and then a lower or uppercase "x". Hexadecimal digits can be specified

4695

in lower or uppercase. For example, in the Python interpreter:

4696

<P>

4697

<PRE>

4698

>>> a = 0xa5

4699

>>> a

4700

165

4701

>>> b = 0XB2

4702

>>> b

4703

178

4704

</PRE>

4705

<P>

4706

4707

<A HREF="faqw.py?req=edit&file=faq04.073.htp">Edit this entry</A> /

4708

4709

4710

/ Last changed on Tue Mar 3 12:53:16 1998 by

4711

4712

<P>

4713

4714

<HR>

4715

<H2><A NAME="4.74">4.74. How to get a single keypress at a time?</A></H2>

4716

For Windows, see question 8.2. Here is an answer for Unix (see also 4.94).

4717

<P>

4718

There are several solutions; some involve using curses, which is a

4719

pretty big thing to learn. Here's a solution without curses, due

4720

to Andrew Kuchling (adapted from code to do a PGP-style

4721

randomness pool):

4722

<P>

4723

<PRE>

4724

import termios, sys, os

4725

fd = sys.stdin.fileno()

4726

old = termios.tcgetattr(fd)

4727

new = termios.tcgetattr(fd)

4728

new[3] = new[3] & ~termios.ICANON & ~termios.ECHO

4729

new[6][termios.VMIN] = 1

4730

new[6][termios.VTIME] = 0

4731

termios.tcsetattr(fd, termios.TCSANOW, new)

4732

s = '' # We'll save the characters typed and add them to the pool.

4733

try:

4734

while 1:

4735

c = os.read(fd, 1)

4736

print "Got character", `c`

4737

s = s+c

4738

finally:

4739

termios.tcsetattr(fd, termios.TCSAFLUSH, old)

4740

</PRE>

4741

You need the termios module for any of this to work, and I've only

4742

tried it on Linux, though it should work elsewhere. It turns off

4743

stdin's echoing and disables canonical mode, and then reads a

4744

character at a time from stdin, noting the time after each keystroke.

4745

<P>

4746

4747

<A HREF="faqw.py?req=edit&file=faq04.074.htp">Edit this entry</A> /

4748

4749

4750

/ Last changed on Thu Oct 24 00:36:56 2002 by

4751

<A HREF="mailto:cliechti@gmx.net">chris</A>

4752

<P>

4753

4754

<HR>

4755

<H2><A NAME="4.75">4.75. How can I overload constructors (or methods) in Python?</A></H2>

4756

(This actually applies to all methods, but somehow the question

4757

usually comes up first in the context of constructors.)

4758

<P>

4759

Where in C++ you'd write

4760

<P>

4761

<PRE>

4762

class C {

4763

C() { cout << "No arguments\n"; }

4764

C(int i) { cout << "Argument is " << i << "\n"; }

4765

}

4766

</PRE>

4767

in Python you have to write a single constructor that catches all

4768

cases using default arguments. For example:

4769

<P>

4770

<PRE>

4771

class C:

4772

def __init__(self, i=None):

4773

if i is None:

4774

print "No arguments"

4775

else:

4776

print "Argument is", i

4777

</PRE>

4778

This is not entirely equivalent, but close enough in practice.

4779

<P>

4780

You could also try a variable-length argument list, e.g.

4781

<P>

4782

<PRE>

4783

def __init__(self, *args):

4784

....

4785

</PRE>

4786

The same approach works for all method definitions.

4787

<P>

4788

4789

<A HREF="faqw.py?req=edit&file=faq04.075.htp">Edit this entry</A> /

4790

4791

4792

/ Last changed on Mon Apr 20 11:55:55 1998 by

4793

4794

<P>

4795

4796

<HR>

4797

<H2><A NAME="4.76">4.76. How do I pass keyword arguments from one method to another?</A></H2>

4798

Use apply. For example:

4799

<P>

4800

<PRE>

4801

class Account:

4802

def __init__(self, **kw):

4803

self.accountType = kw.get('accountType')

4804

self.balance = kw.get('balance')

4805

</PRE>

4806

<PRE>

4807

class CheckingAccount(Account):

4808

def __init__(self, **kw):

4809

kw['accountType'] = 'checking'

4810

apply(Account.__init__, (self,), kw)

4811

</PRE>

4812

<PRE>

4813

myAccount = CheckingAccount(balance=100.00)

4814

</PRE>

4815

In Python 2.0 you can call it directly using the new ** syntax:

4816

<P>

4817

<PRE>

4818

class CheckingAccount(Account):

4819

def __init__(self, **kw):

4820

kw['accountType'] = 'checking'

4821

Account.__init__(self, **kw)

4822

</PRE>

4823

or more generally:

4824

<P>

4825

<PRE>

4826

>>> def f(x, *y, **z):

4827

... print x,y,z

4828

...

4829

>>> Y = [1,2,3]

4830

>>> Z = {'foo':3,'bar':None}

4831

>>> f('hello', *Y, **Z)

4832

hello (1, 2, 3) {'foo': 3, 'bar': None}

4833

</PRE>

4834

<P>

4835

4836

<A HREF="faqw.py?req=edit&file=faq04.076.htp">Edit this entry</A> /

4837

4838

4839

/ Last changed on Thu Dec 28 13:04:01 2000 by

4840

<A HREF="mailto:pbjorn@uswest.net">Bjorn Pettersen</A>

4841

<P>

4842

4843

<HR>

4844

<H2><A NAME="4.77">4.77. What module should I use to help with generating HTML?</A></H2>

4845

Check out HTMLgen written by Robin Friedrich. It's a class library

4846

of objects corresponding to all the HTML 3.2 markup tags. It's used

4847

when you are writing in Python and wish to synthesize HTML pages for

4848

generating a web or for CGI forms, etc.

4849

<P>

4850

It can be found in the FTP contrib area on python.org or on the

4851

Starship. Use the search engines there to locate the latest version.

4852

<P>

4853

It might also be useful to consider DocumentTemplate, which offers clear

4854

separation between Python code and HTML code. DocumentTemplate is part

4855

of the Bobo objects publishing system (http:/www.digicool.com/releases)

4856

but can be used independantly of course!

4857

<P>

4858

4859

<A HREF="faqw.py?req=edit&file=faq04.077.htp">Edit this entry</A> /

4860

4861

4862

/ Last changed on Fri Aug 28 09:54:58 1998 by

4863

4864

<P>

4865

4866

<HR>

4867

<H2><A NAME="4.78">4.78. How do I create documentation from doc strings?</A></H2>

4868

Use gendoc, by Daniel Larson. See

4869

<P>

4870

<A HREF="http://starship.python.net/crew/danilo">http://starship.python.net/crew/danilo</A>/

4871

<P>

4872

It can create HTML from the doc strings in your Python source code.

4873

<P>

4874

4875

<A HREF="faqw.py?req=edit&file=faq04.078.htp">Edit this entry</A> /

4876

4877

4878

/ Last changed on Mon Oct 7 17:15:51 2002 by

4879

<A HREF="mailto:phil@dspfactory.com">Phil Rittenhouse</A>

4880

<P>

4881

4882

<HR>

4883

<H2><A NAME="4.79">4.79. How do I read (or write) binary data?</A></H2>

4884

For complex data formats, it's best to use

4885

use the struct module. It's documented in the library reference.

4886

It allows you to take a string read from a file containing binary

4887

data (usually numbers) and convert it to Python objects; and vice

4888

versa.

4889

<P>

4890

For example, the following code reads two 2-byte integers

4891

and one 4-byte integer in big-endian format from a file:

4892

<P>

4893

<PRE>

4894

import struct

4895

</PRE>

4896

<PRE>

4897

f = open(filename, "rb") # Open in binary mode for portability

4898

s = f.read(8)

4899

x, y, z = struct.unpack(">hhl", s)

4900

</PRE>

4901

The '>' in the format string forces bin-endian data; the letter

4902

'h' reads one "short integer" (2 bytes), and 'l' reads one

4903

"long integer" (4 bytes) from the string.

4904

<P>

4905

For data that is more regular (e.g. a homogeneous list of ints or

4906

floats), you can also use the array module, also documented

4907

in the library reference.

4908

<P>

4909

4910

<A HREF="faqw.py?req=edit&file=faq04.079.htp">Edit this entry</A> /

4911

4912

4913

/ Last changed on Wed Oct 7 09:16:45 1998 by

4914

4915

<P>

4916

4917

<HR>

4918

<H2><A NAME="4.80">4.80. I can't get key bindings to work in Tkinter</A></H2>

4919

An oft-heard complaint is that event handlers bound to events

4920

with the bind() method don't get handled even when the appropriate

4921

key is pressed.

4922

<P>

4923

The most common cause is that the widget to which the binding applies

4924

doesn't have "keyboard focus". Check out the Tk documentation

4925

for the focus command. Usually a widget is given the keyboard

4926

focus by clicking in it (but not for labels; see the taketocus

4927

option).

4928

<P>

4929

4930

<A HREF="faqw.py?req=edit&file=faq04.080.htp">Edit this entry</A> /

4931

4932

4933

/ Last changed on Fri Jun 12 09:37:33 1998 by

4934

4935

<P>

4936

4937

<HR>

4938

<H2><A NAME="4.81">4.81. "import crypt" fails</A></H2>

4939

[Unix]

4940

<P>

4941

Starting with Python 1.5, the crypt module is disabled by default.

4942

In order to enable it, you must go into the Python source tree and

4943

edit the file Modules/Setup to enable it (remove a '#' sign in

4944

front of the line starting with '#crypt'). Then rebuild.

4945

You may also have to add the string '-lcrypt' to that same line.

4946

<P>

4947

4948

<A HREF="faqw.py?req=edit&file=faq04.081.htp">Edit this entry</A> /

4949

4950

4951

/ Last changed on Wed Aug 5 08:57:09 1998 by

4952

4953

<P>

4954

4955

<HR>

4956

<H2><A NAME="4.82">4.82. Are there coding standards or a style guide for Python programs?</A></H2>

4957

Yes, Guido has written the "Python Style Guide". See

4958

<A HREF="http://www.python.org/doc/essays/styleguide.html">http://www.python.org/doc/essays/styleguide.html</A>

4959

<P>

4960

4961

<A HREF="faqw.py?req=edit&file=faq04.082.htp">Edit this entry</A> /

4962

4963

4964

/ Last changed on Tue Sep 29 09:50:27 1998 by

4965

<A HREF="mailto:vanandel@ucar.edu">Joseph VanAndel</A>

4966

<P>

4967

4968

<HR>

4969

<H2><A NAME="4.83">4.83. How do I freeze Tkinter applications?</A></H2>

4970

Freeze is a tool to create stand-alone applications (see 4.28).

4971

<P>

4972

When freezing Tkinter applications, the applications will not be

4973

truly stand-alone, as the application will still need the tcl and

4974

tk libraries.

4975

<P>

4976

One solution is to ship the application with the tcl and tk libraries,

4977

and point to them at run-time using the TCL_LIBRARY and TK_LIBRARY

4978

environment variables.

4979

<P>

4980

To get truly stand-alone applications, the Tcl scripts that form

4981

the library have to be integrated into the application as well. One

4982

tool supporting that is SAM (stand-alone modules), which is part

4983

of the Tix distribution (<A HREF="http://tix.mne.com">http://tix.mne.com</A>). Build Tix with SAM

4984

enabled, perform the appropriate call to Tclsam_init etc inside

4985

Python's Modules/tkappinit.c, and link with libtclsam

4986

and libtksam (you might include the Tix libraries as well).

4987

<P>

4988

4989

<A HREF="faqw.py?req=edit&file=faq04.083.htp">Edit this entry</A> /

4990

4991

4992

/ Last changed on Wed Jan 20 17:35:01 1999 by

4993

<A HREF="mailto:loewis@informatik.hu-berlin.de">Martin v. L�wis</A>

4994

<P>

4995

4996

<HR>

4997

<H2><A NAME="4.84">4.84. How do I create static class data and static class methods?</A></H2>

4998

[Tim Peters, <A HREF="mailto:tim_one@email.msn.com">tim_one@email.msn.com</A>]

4999

<P>

5000

Static data (in the sense of C++ or Java) is easy; static methods (again in the sense of C++ or Java) are not supported directly.

5001

<P>

5002

STATIC DATA

5003

<P>

5004

For example,

5005

<P>

5006

<PRE>

5007

class C:

5008

count = 0 # number of times C.__init__ called

5009

</PRE>

5010

<PRE>

5011

def __init__(self):

5012

C.count = C.count + 1

5013

</PRE>

5014

<PRE>

5015

def getcount(self):

5016

return C.count # or return self.count

5017

</PRE>

5018

c.count also refers to C.count for any c such that isinstance(c, C) holds, unless overridden by c itself or by some class on the base-class search path from c.__class__ back to C.

5019

<P>

5020

Caution: within a method of C,

5021

<P>

5022

<PRE>

5023

self.count = 42

5024

</PRE>

5025

creates a new and unrelated instance vrbl named "count" in self's own dict. So rebinding of a class-static data name needs the

5026

<P>

5027

<PRE>

5028

C.count = 314

5029

</PRE>

5030

form whether inside a method or not.

5031

<P>

5032

<P>

5033

STATIC METHODS

5034

<P>

5035

Static methods (as opposed to static data) are unnatural in Python, because

5036

<P>

5037

<PRE>

5038

C.getcount

5039

</PRE>

5040

returns an unbound method object, which can't be invoked without supplying an instance of C as the first argument.

5041

<P>

5042

The intended way to get the effect of a static method is via a module-level function:

5043

<P>

5044

<PRE>

5045

def getcount():

5046

return C.count

5047

</PRE>

5048

If your code is structured so as to define one class (or tightly related class hierarchy) per module, this supplies the desired encapsulation.

5049

<P>

5050

Several tortured schemes for faking static methods can be found by searching DejaNews. Most people feel such cures are worse than the disease. Perhaps the least obnoxious is due to Pekka Pessi (mailto:<A HREF="mailto:ppessi@hut.fi">ppessi@hut.fi</A>):

5051

<P>

5052

<PRE>

5053

# helper class to disguise function objects

5054

class _static:

5055

def __init__(self, f):

5056

self.__call__ = f

5057

</PRE>

5058

<PRE>

5059

class C:

5060

count = 0

5061

</PRE>

5062

<PRE>

5063

def __init__(self):

5064

C.count = C.count + 1

5065

</PRE>

5066

<PRE>

5067

def getcount():

5068

return C.count

5069

getcount = _static(getcount)

5070

</PRE>

5071

<PRE>

5072

def sum(x, y):

5073

return x + y

5074

sum = _static(sum)

5075

</PRE>

5076

<PRE>

5077

C(); C()

5078

c = C()

5079

print C.getcount() # prints 3

5080

print c.getcount() # prints 3

5081

print C.sum(27, 15) # prints 42

5082

</PRE>

5083

<P>

5084

5085

<A HREF="faqw.py?req=edit&file=faq04.084.htp">Edit this entry</A> /

5086

5087

5088

/ Last changed on Thu Jan 21 21:35:38 1999 by

5089

<A HREF="mailto:tim_one@email.msn.com">Tim Peters</A>

5090

<P>

5091

5092

<HR>

5093

<H2><A NAME="4.85">4.85. __import__('x.y.z') returns <module 'x'>; how do I get z?</A></H2>

5094

Try

5095

<P>

5096

<PRE>

5097

__import__('x.y.z').y.z

5098

</PRE>

5099

For more realistic situations, you may have to do something like

5100

<P>

5101

<PRE>

5102

m = __import__(s)

5103

for i in string.split(s, ".")[1:]:

5104

m = getattr(m, i)

5105

</PRE>

5106

<P>

5107

5108

<A HREF="faqw.py?req=edit&file=faq04.085.htp">Edit this entry</A> /

5109

5110

5111

/ Last changed on Thu Jan 28 11:01:43 1999 by

5112

5113

<P>

5114

5115

<HR>

5116

<H2><A NAME="4.86">4.86. Basic thread wisdom</A></H2>

5117

Please note that there is no way to take advantage of

5118

multiprocessor hardware using the Python thread model. The interpreter

5119

uses a global interpreter lock (GIL),

5120

which does not allow multiple threads to be concurrently active.

5121

<P>

5122

If you write a simple test program like this:

5123

<P>

5124

<PRE>

5125

import thread

5126

def run(name, n):

5127

for i in range(n): print name, i

5128

for i in range(10):

5129

thread.start_new(run, (i, 100))

5130

</PRE>

5131

none of the threads seem to run! The reason is that as soon as

5132

the main thread exits, all threads are killed.

5133

<P>

5134

A simple fix is to add a sleep to the end of the program,

5135

sufficiently long for all threads to finish:

5136

<P>

5137

<PRE>

5138

import thread, time

5139

def run(name, n):

5140

for i in range(n): print name, i

5141

for i in range(10):

5142

thread.start_new(run, (i, 100))

5143

time.sleep(10) # <----------------------------!

5144

</PRE>

5145

But now (on many platforms) the threads don't run in parallel,

5146

but appear to run sequentially, one at a time! The reason is

5147

that the OS thread scheduler doesn't start a new thread until

5148

the previous thread is blocked.

5149

<P>

5150

A simple fix is to add a tiny sleep to the start of the run

5151

function:

5152

<P>

5153

<PRE>

5154

import thread, time

5155

def run(name, n):

5156

time.sleep(0.001) # <---------------------!

5157

for i in range(n): print name, i

5158

for i in range(10):

5159

thread.start_new(run, (i, 100))

5160

time.sleep(10)

5161

</PRE>

5162

Some more hints:

5163

<P>

5164

Instead of using a time.sleep() call at the end, it's

5165

better to use some kind of semaphore mechanism. One idea is to

5166

use a the Queue module to create a queue object, let each thread

5167

append a token to the queue when it finishes, and let the main

5168

thread read as many tokens from the queue as there are threads.

5169

<P>

5170

Use the threading module instead of the thread module. It's part

5171

of Python since version 1.5.1. It takes care of all these details,

5172

and has many other nice features too!

5173

<P>

5174

5175

<A HREF="faqw.py?req=edit&file=faq04.086.htp">Edit this entry</A> /

5176

5177

5178

/ Last changed on Fri Feb 7 16:21:55 2003 by

5179

5180

<P>

5181

5182

<HR>

5183

<H2><A NAME="4.87">4.87. Why doesn't closing sys.stdout (stdin, stderr) really close it?</A></H2>

5184

Python file objects are a high-level layer of abstraction on top of C streams, which in turn are a medium-level layer of abstraction on top of (among other things) low-level C file descriptors.

5185

<P>

5186

For most file objects f you create in Python via the builtin "open" function, f.close() marks the Python file object as being closed from Python's point of view, and also arranges to close the underlying C stream. This happens automatically too, in f's destructor, when f becomes garbage.

5187

<P>

5188

But stdin, stdout and stderr are treated specially by Python, because of the special status also given to them by C: doing

5189

<P>

5190

<PRE>

5191

sys.stdout.close() # ditto for stdin and stderr

5192

</PRE>

5193

marks the Python-level file object as being closed, but does <I>not</I> close the associated C stream (provided sys.stdout is still bound to its default value, which is the stream C also calls "stdout").

5194

<P>

5195

To close the underlying C stream for one of these three, you should first be sure that's what you really want to do (e.g., you may confuse the heck out of extension modules trying to do I/O). If it is, use os.close:

5196

<P>

5197

<PRE>

5198

os.close(0) # close C's stdin stream

5199

os.close(1) # close C's stdout stream

5200

os.close(2) # close C's stderr stream

5201

</PRE>

5202

<P>

5203

5204

<A HREF="faqw.py?req=edit&file=faq04.087.htp">Edit this entry</A> /

5205

5206

5207

/ Last changed on Sat Apr 17 02:22:35 1999 by

5208

<A HREF="mailto:tim_one@email.msn.com">Tim Peters</A>

5209

<P>

5210

5211

<HR>

5212

<H2><A NAME="4.88">4.88. What kinds of global value mutation are thread-safe?</A></H2>

5213

[adapted from c.l.py responses by Gordon McMillan & GvR]

5214

<P>

5215

A global interpreter lock (GIL) is used internally to ensure that only one thread runs in the Python VM at a time. In general, Python offers to switch among threads only between bytecode instructions (how frequently it offers to switch can be set via sys.setcheckinterval). Each bytecode instruction-- and all the C implementation code reached from it --is therefore atomic.

5216

<P>

5217

In theory, this means an exact accounting requires an exact understanding of the PVM bytecode implementation. In practice, it means that operations on shared vrbls of builtin data types (ints, lists, dicts, etc) that "look atomic" really are.

5218

<P>

5219

For example, these are atomic (L, L1, L2 are lists, D, D1, D2 are dicts, x, y

5220

are objects, i, j are ints):

5221

<P>

5222

<PRE>

5223

L.append(x)

5224

L1.extend(L2)

5225

x = L[i]

5226

x = L.pop()

5227

L1[i:j] = L2

5228

L.sort()

5229

x = y

5230

x.field = y

5231

D[x] = y

5232

D1.update(D2)

5233

D.keys()

5234

</PRE>

5235

These aren't:

5236

<P>

5237

<PRE>

5238

i = i+1

5239

L.append(L[-1])

5240

L[i] = L[j]

5241

D[x] = D[x] + 1

5242

</PRE>

5243

Note: operations that replace other objects may invoke those other objects' __del__ method when their reference count reaches zero, and that can affect things. This is especially true for the mass updates to dictionaries and lists. When in doubt, use a mutex!

5244

<P>

5245

5246

<A HREF="faqw.py?req=edit&file=faq04.088.htp">Edit this entry</A> /

5247

5248

5249

/ Last changed on Fri Feb 7 16:21:03 2003 by

5250

5251

<P>

5252

5253

<HR>

5254

<H2><A NAME="4.89">4.89. How do I modify a string in place?</A></H2>

5255

Strings are immutable (see question 6.2) so you cannot modify a string

5256

directly. If you need an object with this ability, try converting the

5257

string to a list or take a look at the array module.

5258

<P>

5259

<PRE>

5260

>>> s = "Hello, world"

5261

>>> a = list(s)

5262

>>> print a

5263

['H', 'e', 'l', 'l', 'o', ',', ' ', 'w', 'o', 'r', 'l', 'd']

5264

>>> a[7:] = list("there!")

5265

>>> import string

5266

>>> print string.join(a, '')

5267

'Hello, there!'

5268

</PRE>

5269

<PRE>

5270

>>> import array

5271

>>> a = array.array('c', s)

5272

>>> print a

5273

array('c', 'Hello, world')

5274

>>> a[0] = 'y' ; print a

5275

array('c', 'yello world')

5276

>>> a.tostring()

5277

'yello, world'

5278

</PRE>

5279

<P>

5280

5281

<A HREF="faqw.py?req=edit&file=faq04.089.htp">Edit this entry</A> /

5282

5283

5284

/ Last changed on Tue May 18 01:22:47 1999 by

5285

<A HREF="mailto:dalke@bioreason.com">Andrew Dalke</A>

5286

<P>

5287

5288

<HR>

5289

<H2><A NAME="4.90">4.90. How to pass on keyword/optional parameters/arguments</A></H2>

5290

Q: How can I pass on optional or keyword parameters from one function to another?

5291

<P>

5292

<PRE>

5293

def f1(a, *b, **c):

5294

...

5295

</PRE>

5296

A: In Python 2.0 and above:

5297

<P>

5298

<PRE>

5299

def f2(x, *y, **z):

5300

...

5301

z['width']='14.3c'

5302

...

5303

f1(x, *y, **z)

5304

</PRE>

5305

<PRE>

5306

Note: y can be any sequence (e.g., list or tuple) and z must be a dict.

5307

</PRE>

5308

<P>

5309

A: For versions prior to 2.0, use 'apply', like:

5310

<P>

5311

<PRE>

5312

def f2(x, *y, **z):

5313

...

5314

z['width']='14.3c'

5315

...

5316

apply(f1, (x,)+y, z)

5317

</PRE>

5318

<P>

5319

5320

<A HREF="faqw.py?req=edit&file=faq04.090.htp">Edit this entry</A> /

5321

5322

5323

/ Last changed on Mon Jun 3 07:20:56 2002 by

5324

<A HREF="mailto:smurf@noris.de">Matthias Urlichs</A>

5325

<P>

5326

5327

<HR>

5328

<H2><A NAME="4.91">4.91. How can I get a dictionary to display its keys in a consistent order?</A></H2>

5329

In general, dictionaries store their keys in an unpredictable order,

5330

so the display order of a dictionary's elements will be similarly

5331

unpredictable.

5332

(See

5333

<a href="http://www.python.org/doc/FAQ.html#6.12">Question 6.12</a>

5334

to understand why this is so.)

5335

<P>

5336

This can be frustrating if you want to save a printable version to a

5337

file, make some changes and then compare it with some other printed

5338

dictionary. If you have such needs you can subclass UserDict.UserDict

5339

to create a SortedDict class that prints itself in a predictable order.

5340

Here's one simpleminded implementation of such a class:

5341

<P>

5342

<PRE>

5343

import UserDict, string

5344

</PRE>

5345

<PRE>

5346

class SortedDict(UserDict.UserDict):

5347

def __repr__(self):

5348

result = []

5349

append = result.append

5350

keys = self.data.keys()

5351

keys.sort()

5352

for k in keys:

5353

append("%s: %s" % (`k`, `self.data[k]`))

5354

return "{%s}" % string.join(result, ", ")

5355

</PRE>

5356

<PRE>

5357

___str__ = __repr__

5358

</PRE>

5359

<P>

5360

This will work for many common situations you might encounter, though

5361

it's far from a perfect solution. (It won't have any effect on the

5362

pprint module and does not transparently handle values that are or

5363

contain dictionaries.

5364

<P>

5365

5366

<A HREF="faqw.py?req=edit&file=faq04.091.htp">Edit this entry</A> /

5367

5368

5369

/ Last changed on Thu Sep 16 17:31:06 1999 by

5370

<A HREF="mailto:skip@mojam.com">Skip Montanaro</A>

5371

<P>

5372

5373

<HR>

5374

<H2><A NAME="4.92">4.92. Is there a Python tutorial?</A></H2>

5375

Yes. See question 1.20 at

5376

<A HREF="http://www.python.org/doc/FAQ.html#1.20">http://www.python.org/doc/FAQ.html#1.20</A>

5377

<P>

5378

5379

<A HREF="faqw.py?req=edit&file=faq04.092.htp">Edit this entry</A> /

5380

5381

5382

/ Last changed on Sat Dec 4 16:04:00 1999 by

5383

5384

<P>

5385

5386

<HR>

5387

<H2><A NAME="4.93">4.93. Deleted</A></H2>

5388

See 4.28

5389

<P>

5390

5391

<A HREF="faqw.py?req=edit&file=faq04.093.htp">Edit this entry</A> /

5392

5393

5394

/ Last changed on Tue May 28 20:40:37 2002 by

5395

5396

<P>

5397

5398

<HR>

5399

<H2><A NAME="4.94">4.94. How do I get a single keypress without blocking?</A></H2>

5400

There are several solutions; some involve using curses, which is a

5401

pretty big thing to learn. Here's a solution without curses. (see also 4.74, for Windows, see question 8.2)

5402

<P>

5403

<PRE>

5404

import termios, fcntl, sys, os

5405

fd = sys.stdin.fileno()

5406

</PRE>

5407

<PRE>

5408

oldterm = termios.tcgetattr(fd)

5409

newattr = termios.tcgetattr(fd)

5410

newattr[3] = newattr[3] & ~termios.ICANON & ~termios.ECHO

5411

termios.tcsetattr(fd, termios.TCSANOW, newattr)

5412

</PRE>

5413

<PRE>

5414

oldflags = fcntl.fcntl(fd, fcntl.F_GETFL)

5415

fcntl.fcntl(fd, fcntl.F_SETFL, oldflags | os.O_NONBLOCK)

5416

</PRE>

5417

<PRE>

5418

try:

5419

while 1:

5420

try:

5421

c = sys.stdin.read(1)

5422

print "Got character", `c`

5423

except IOError: pass

5424

finally:

5425

termios.tcsetattr(fd, termios.TCSAFLUSH, oldterm)

5426

fcntl.fcntl(fd, fcntl.F_SETFL, oldflags)

5427

</PRE>

5428

<P>

5429

You need the termios and the fcntl module for any of this to work,

5430

and I've only tried it on Linux, though it should work elsewhere.

5431

<P>

5432

In this code, characters are read and printed one at a time.

5433

<P>

5434

termios.tcsetattr() turns off stdin's echoing and disables canonical

5435

mode. fcntl.fnctl() is used to obtain stdin's file descriptor flags

5436

and modify them for non-blocking mode. Since reading stdin when it is

5437

empty results in an IOError, this error is caught and ignored.

5438

<P>

5439

5440

<A HREF="faqw.py?req=edit&file=faq04.094.htp">Edit this entry</A> /

5441

5442

5443

/ Last changed on Thu Oct 24 00:39:06 2002 by

5444

<A HREF="mailto:cliechti@gmx.net">chris</A>

5445

<P>

5446

5447

<HR>

5448

<H2><A NAME="4.95">4.95. Is there an equivalent to Perl chomp()? (Remove trailing newline from string)</A></H2>

5449

There are two partial substitutes. If you want to remove all trailing

5450

whitespace, use the method string.rstrip(). Otherwise, if there is only

5451

one line in the string, use string.splitlines()[0].

5452

<P>

5453

<PRE>

5454

-----------------------------------------------------------------------

5455

</PRE>

5456

<PRE>

5457

rstrip() is too greedy, it strips all trailing white spaces.

5458

splitlines() takes ControlM as line boundary.

5459

Consider these strings as input:

5460

"python python \r\n"

5461

"python\rpython\r\n"

5462

"python python \r\r\r\n"

5463

The results from rstrip()/splitlines() are perhaps not what we want.

5464

</PRE>

5465

<PRE>

5466

It seems re can perform this task.

5467

</PRE>

5468

<P>

5469

<PRE>

5470

#!/usr/bin/python

5471

# requires python2

5472

</PRE>

5473

<PRE>

5474

import re, os, StringIO

5475

</PRE>

5476

<PRE>

5477

lines=StringIO.StringIO(

5478

"The Python Programming Language\r\n"

5479

"The Python Programming Language \r \r \r\r\n"

5480

"The\rProgramming\rLanguage\r\n"

5481

"The\rProgramming\rLanguage\r\r\r\r\n"

5482

"The\r\rProgramming\r\rLanguage\r\r\r\r\n"

5483

)

5484

</PRE>

5485

<PRE>

5486

ln=re.compile("(?:[\r]?\n|\r)$") # dos:\r\n, unix:\n, mac:\r, others: unknown

5487

# os.linesep does not work if someone ftps(in binary mode) a dos/mac text file

5488

# to your unix box

5489

#ln=re.compile(os.linesep + "$")

5490

</PRE>

5491

<PRE>

5492

while 1:

5493

s=lines.readline()

5494

if not s: break

5495

print "1.(%s)" % `s.rstrip()`

5496

print "2.(%s)" % `ln.sub( "", s, 1)`

5497

print "3.(%s)" % `s.splitlines()[0]`

5498

print "4.(%s)" % `s.splitlines()`

5499

print

5500

</PRE>

5501

<PRE>

5502

lines.close()

5503

</PRE>

5504

<P>

5505

5506

<A HREF="faqw.py?req=edit&file=faq04.095.htp">Edit this entry</A> /

5507

5508

5509

/ Last changed on Wed Aug 8 09:51:34 2001 by

5510

<A HREF="mailto:serpent@ms5.hinet.net">Crystal</A>

5511

<P>

5512

5513

<HR>

5514

<H2><A NAME="4.96">4.96. Why is join() a string method when I'm really joining the elements of a (list, tuple, sequence)?</A></H2>

5515

Strings became much more like other standard types starting in release 1.6, when methods were added which give the same functionality that has always been available using the functions of the string module. These new methods have been widely accepted, but the one which appears to make (some) programmers feel uncomfortable is:

5516

<P>

5517

<PRE>

5518

", ".join(['1', '2', '4', '8', '16'])

5519

</PRE>

5520

which gives the result

5521

<P>

5522

<PRE>

5523

"1, 2, 4, 8, 16"

5524

</PRE>

5525

There are two usual arguments against this usage.

5526

<P>

5527

The first runs along the lines of: "It looks really ugly using a method of a string literal (string constant)", to which the answer is that it might, but a string literal is just a fixed value. If the methods are to be allowed on names bound to strings there is no logical reason to make them unavailable on literals. Get over it!

5528

<P>

5529

The second objection is typically cast as: "I am really telling a sequence to join its members together with a string constant". Sadly, you aren't. For some reason there seems to be much less difficulty with having split() as a string method, since in that case it is easy to see that

5530

<P>

5531

<PRE>

5532

"1, 2, 4, 8, 16".split(", ")

5533

</PRE>

5534

is an instruction to a string literal to return the substrings delimited by the given separator (or, by default, arbitrary runs of white space). In this case a Unicode string returns a list of Unicode strings, an ASCII string returns a list of ASCII strings, and everyone is happy.

5535

<P>

5536

join() is a string method because in using it you are telling the separator string to iterate over an arbitrary sequence, forming string representations of each of the elements, and inserting itself between the elements' representations. This method can be used with any argument which obeys the rules for sequence objects, inluding any new classes you might define yourself.

5537

<P>

5538

Because this is a string method it can work for Unicode strings as well as plain ASCII strings. If join() were a method of the sequence types then the sequence types would have to decide which type of string to return depending on the type of the separator.

5539

<P>

5540

If none of these arguments persuade you, then for the moment you can continue to use the join() function from the string module, which allows you to write

5541

<P>

5542

<PRE>

5543

string.join(['1', '2', '4', '8', '16'], ", ")

5544

</PRE>

5545

You will just have to try and forget that the string module actually uses the syntax you are compaining about to implement the syntax you prefer!

5546

<P>

5547

5548

<A HREF="faqw.py?req=edit&file=faq04.096.htp">Edit this entry</A> /

5549

5550

5551

/ Last changed on Fri Aug 2 15:51:58 2002 by

5552

<A HREF="mailto:sholden@holdenweb.com">Steve Holden</A>

5553

<P>

5554

5555

<HR>

5556

<H2><A NAME="4.97">4.97. How can my code discover the name of an object?</A></H2>

5557

Generally speaking, it can't, because objects don't really have names. The assignment statement does not store the assigned value in the name but a reference to it. Essentially, assignment creates a binding of a name to a value. The same is true of <I>def</I> and <I>class</I> statements, but in that case the value is a callable. Consider the following code:

5558

<P>

5559

<PRE>

5560

class A:

5561

pass

5562

</PRE>

5563

<PRE>

5564

B = A

5565

</PRE>

5566

<PRE>

5567

a = B()

5568

b = a

5569

print b

5570

<__main__.A instance at 016D07CC>

5571

print a

5572

<__main__.A instance at 016D07CC>

5573

</PRE>

5574

<P>

5575

Arguably the class has a name: even though it is bound to two names and invoked through the name B the created instance is still reported as an instance of class A. However, it is impossible to say whether the instance's name is a or b, since both names are bound to the same value.

5576

<P>

5577

Generally speaking it should not be necessary for your code to "know the names" of particular values. Unless you are deliberately writing introspective programs, this is usually an indication that a change of approach might be beneficial.

5578

<P>

5579

5580

<A HREF="faqw.py?req=edit&file=faq04.097.htp">Edit this entry</A> /

5581

5582

5583

/ Last changed on Thu Mar 8 03:53:39 2001 by

5584

<A HREF="mailto:sholden@holdenweb.com">Steve Holden</A>

5585

<P>

5586

5587

<HR>

5588

<H2><A NAME="4.98">4.98. Why are floating point calculations so inaccurate?</A></H2>

5589

The development version of the Python Tutorial now contains an Appendix with more info:

5590

<PRE>

5591

<A HREF="http://www.python.org/doc/current/tut/node14.html">http://www.python.org/doc/current/tut/node14.html</A>

5592

</PRE>

5593

People are often very surprised by results like this:

5594

<P>

5595

<PRE>

5596

>>> 1.2-1.0

5597

0.199999999999999996

5598

</PRE>

5599

And think it is a bug in Python. It's not. It's a problem caused by

5600

the internal representation of a floating point number. A floating point

5601

number is stored as a fixed number of binary digits.

5602

<P>

5603

In decimal math, there are many numbers that can't be represented

5604

with a fixed number of decimal digits, i.e.

5605

1/3 = 0.3333333333.......

5606

<P>

5607

In the binary case, 1/2 = 0.1, 1/4 = 0.01, 1/8 = 0.001, etc. There are

5608

a lot of numbers that can't be represented. The digits are cut off at

5609

some point.

5610

<P>

5611

Since Python 1.6, a floating point's repr() function prints as many

5612

digits are necessary to make eval(repr(f)) == f true for any float f.

5613

The str() function prints the more sensible number that was probably

5614

intended:

5615

<P>

5616

<PRE>

5617

>>> 0.2

5618

0.20000000000000001

5619

>>> print 0.2

5620

0.2

5621

</PRE>

5622

Again, this has nothing to do with Python, but with the way the

5623

underlying C platform handles floating points, and ultimately with

5624

the inaccuracy you'll always have when writing down numbers of fixed

5625

number of digit strings.

5626

<P>

5627

One of the consequences of this is that it is dangerous to compare

5628

the result of some computation to a float with == !

5629

Tiny inaccuracies may mean that == fails.

5630

<P>

5631

Instead try something like this:

5632

<P>

5633

<PRE>

5634

epsilon = 0.0000000000001 # Tiny allowed error

5635

expected_result = 0.4

5636

</PRE>

5637

<PRE>

5638

if expected_result-epsilon <= computation() <= expected_result+epsilon:

5639

...

5640

</PRE>

5641

<P>

5642

5643

<A HREF="faqw.py?req=edit&file=faq04.098.htp">Edit this entry</A> /

5644

5645

5646

/ Last changed on Mon Apr 1 22:18:47 2002 by

5647

<A HREF="mailto:fdrake@acm.org">Fred Drake</A>

5648

<P>

5649

5650

<HR>

5651

<H2><A NAME="4.99">4.99. I tried to open Berkeley DB file, but bsddb produces bsddb.error: (22, 'Invalid argument'). Help! How can I restore my data?</A></H2>

5652

Don't panic! Your data are probably intact. The most frequent cause

5653

for the error is that you tried to open an earlier Berkeley DB file

5654

with a later version of the Berkeley DB library.

5655

<P>

5656

Many Linux systems now have all three versions of Berkeley DB

5657

available. If you are migrating from version 1 to a newer version use

5658

db_dump185 to dump a plain text version of the database.

5659

If you are migrating from version 2 to version 3 use db2_dump to create

5660

a plain text version of the database. In either case, use db_load to

5661

create a new native database for the latest version installed on your

5662

computer. If you have version 3 of Berkeley DB installed, you should

5663

be able to use db2_load to create a native version 2 database.

5664

<P>

5665

You should probably move away from Berkeley DB version 1 files because

5666

the hash file code contains known bugs that can corrupt your data.

5667

<P>

5668

5669

<A HREF="faqw.py?req=edit&file=faq04.099.htp">Edit this entry</A> /

5670

5671

5672

/ Last changed on Wed Aug 29 16:04:29 2001 by

5673

<A HREF="mailto:skip@pobox.com">Skip Montanaro</A>

5674

<P>

5675

5676

<HR>

5677

<H2><A NAME="4.100">4.100. What are the "best practices" for using import in a module?</A></H2>

5678

First, the standard modules are great. Use them! The standard Python library is large and varied. Using modules can save you time and effort and will reduce maintainenance cost of your code. (Other programs are dedicated to supporting and fixing bugs in the standard Python modules. Coworkers may also be familiar with themodules that you use, reducing the amount of time it takes them to understand your code.)

5679

<P>

5680

The rest of this answer is largely a matter of personal preference, but here's what some newsgroup posters said (thanks to all who responded)

5681

<P>

5682

In general, don't use

5683

<PRE>

5684

from modulename import *

5685

</PRE>

5686

Doing so clutters the importer's namespace. Some avoid this idiom even with the few modules that were designed to be imported in this manner. (Modules designed in this manner include Tkinter, thread, and wxPython.)

5687

<P>

5688

Import modules at the top of a file, one module per line. Doing so makes it clear what other modules your code requires and avoids questions of whether the module name is in scope. Using one import per line makes it easy to add and delete module imports.

5689

<P>

5690

Move imports into a local scope (such as at the top of a function definition) if there are a lot of imports, and you're trying to avoid the cost (lots of initialization time) of many imports. This technique is especially helpful if many of the imports are unnecessary depending on how the program executes. You may also want to move imports into a function if the modules are only ever used in that function. Note that loading a module the first time may be expensive (because of the one time initialization of the module) but that loading a module multiple times is virtually free (a couple of dictionary lookups). Even if the module name has gone out of scope, the module is probably available in sys.modules. Thus, there isn't really anything wrong with putting no imports at the module level (if they aren't needed) and putting all of the imports at the function level.

5691

<P>

5692

It is sometimes necessary to move imports to a function or class to avoid problems with circular imports. Gordon says:

5693

<PRE>

5694

Circular imports are fine where both modules use the "import <module>"

5695

form of import. They fail when the 2nd module wants to grab a name

5696

out of the first ("from module import name") and the import is at

5697

the top level. That's because names in the 1st are not yet available,

5698

(the first module is busy importing the 2nd).

5699

</PRE>

5700

In this case, if the 2nd module is only used in one function, then the import can easily be moved into that function. By the time the import is called, the first module will have finished initializing, and the second module can do its import.

5701

<P>

5702

It may also be necessary to move imports out of the top level of code

5703

if some of the modules are platform-specific. In that case, it may not even be possible to import all of the modules at the top of the file. In this case, importing the correct modules in the corresponding platform-specific code is a good option.

5704

<P>

5705

If only instances of a specific class uses a module, then it is reasonable to import the module in the class's __init__ method and then assign the module to an instance variable so that the module is always available (via that instance variable) during the life of the object. Note that to delay an import until the class is instantiated, the import must be inside a method. Putting the import inside the class but outside of any method still causes the import to occur when the module is initialized.

5706

<P>

5707

5708

<A HREF="faqw.py?req=edit&file=faq04.100.htp">Edit this entry</A> /

5709

5710

5711

/ Last changed on Sat Aug 4 04:44:47 2001 by

5712

5713

<P>

5714

5715

<HR>

5716

<H2><A NAME="4.101">4.101. Is there a tool to help find bugs or perform static analysis?</A></H2>

5717

Yes. PyChecker is a static analysis tool for finding bugs

5718

in Python source code as well as warning about code complexity

5719

and style.

5720

<P>

5721

You can get PyChecker from: <A HREF="http://pychecker.sf.net">http://pychecker.sf.net</A>.

5722

<P>

5723

5724

<A HREF="faqw.py?req=edit&file=faq04.101.htp">Edit this entry</A> /

5725

5726

5727

/ Last changed on Fri Aug 10 15:42:11 2001 by

5728

5729

<P>

5730

5731

<HR>

5732

<H2><A NAME="4.102">4.102. UnicodeError: ASCII [decoding,encoding] error: ordinal not in range(128)</A></H2>

5733

This error indicates that your Python installation can handle

5734

only 7-bit ASCII strings. There are a couple ways to fix or

5735

workaround the problem.

5736

<P>

5737

If your programs must handle data in arbitary character set encodings, the environment the application runs in will generally identify the encoding of the data it is handing you. You need to convert the input to Unicode data using that encoding. For instance, a program that handles email or web input will typically find character set encoding information in Content-Type headers. This can then be used to properly convert input data to Unicode. Assuming the string referred to by "value" is encoded as UTF-8:

5738

<P>

5739

<PRE>

5740

value = unicode(value, "utf-8")

5741

</PRE>

5742

will return a Unicode object. If the data is not correctly encoded as UTF-8, the above call will raise a UnicodeError.

5743

<P>

5744

If you only want strings coverted to Unicode which have non-ASCII data, you can try converting them first assuming an ASCII encoding, and then generate Unicode objects if that fails:

5745

<P>

5746

<PRE>

5747

try:

5748

x = unicode(value, "ascii")

5749

except UnicodeError:

5750

value = unicode(value, "utf-8")

5751

else:

5752

# value was valid ASCII data

5753

pass

5754

</PRE>

5755

<P>

5756

If you normally use a character set encoding other than US-ASCII and only need to handle data in that encoding, the simplest way to fix the problem may be simply to set the encoding in sitecustomize.py. The following code is just a modified version of the encoding setup code from site.py with the relevant lines uncommented.

5757

<P>

5758

<PRE>

5759

# Set the string encoding used by the Unicode implementation.

5760

# The default is 'ascii'

5761

encoding = "ascii" # <= CHANGE THIS if you wish

5762

</PRE>

5763

<PRE>

5764

# Enable to support locale aware default string encodings.

5765

import locale

5766

loc = locale.getdefaultlocale()

5767

if loc[1]:

5768

encoding = loc[1]

5769

if encoding != "ascii":

5770

import sys

5771

sys.setdefaultencoding(encoding)

5772

</PRE>

5773

<P>

5774

Also note that on Windows, there is an encoding known as "mbcs", which uses an encoding specific to your current locale. In many cases, and particularly when working with COM, this may be an appropriate default encoding to use.

5775

<P>

5776

5777

<A HREF="faqw.py?req=edit&file=faq04.102.htp">Edit this entry</A> /

5778

5779

5780

/ Last changed on Sat Apr 13 04:45:41 2002 by

5781

<A HREF="mailto:skip@pobox.com">Skip Montanaro</A>

5782

<P>

5783

5784

<HR>

5785

<H2><A NAME="4.103">4.103. Using strings to call functions/methods</A></H2>

5786

There are various techniques:

5787

<P>

5788

* Use a dictionary pre-loaded with strings and functions. The primary

5789

advantage of this technique is that the strings do not need to match the

5790

names of the functions. This is also the primary technique used to

5791

emulate a case construct:

5792

<P>

5793

<PRE>

5794

def a():

5795

pass

5796

</PRE>

5797

<PRE>

5798

def b():

5799

pass

5800

</PRE>

5801

<PRE>

5802

dispatch = {'go': a, 'stop': b} # Note lack of parens for funcs

5803

</PRE>

5804

<PRE>

5805

dispatch[get_input()]() # Note trailing parens to call function

5806

</PRE>

5807

* Use the built-in function getattr():

5808

<P>

5809

<PRE>

5810

import foo

5811

getattr(foo, 'bar')()

5812

</PRE>

5813

Note that getattr() works on any object, including classes, class

5814

instances, modules, and so on.

5815

<P>

5816

This is used in several places in the standard library, like

5817

this:

5818

<P>

5819

<PRE>

5820

class Foo:

5821

def do_foo(self):

5822

...

5823

</PRE>

5824

<PRE>

5825

def do_bar(self):

5826

...

5827

</PRE>

5828

<PRE>

5829

f = getattr(foo_instance, 'do_' + opname)

5830

f()

5831

</PRE>

5832

<P>

5833

* Use locals() or eval() to resolve the function name:

5834

<P>

5835

def myFunc():

5836

<PRE>

5837

print "hello"

5838

</PRE>

5839

fname = "myFunc"

5840

<P>

5841

f = locals()[fname]

5842

f()

5843

<P>

5844

f = eval(fname)

5845

f()

5846

<P>

5847

Note: Using eval() can be dangerous. If you don't have absolute control

5848

over the contents of the string, all sorts of things could happen...

5849

<P>

5850

5851

<A HREF="faqw.py?req=edit&file=faq04.103.htp">Edit this entry</A> /

5852

5853

5854

/ Last changed on Thu Mar 21 08:14:58 2002 by

5855

<A HREF="mailto:erno-pyfaq@erno.iki.fi">Erno Kuusela</A>

5856

<P>

5857

5858

<HR>

5859

<H2><A NAME="4.104">4.104. How fast are exceptions?</A></H2>

5860

A try/except block is extremely efficient. Actually executing an

5861

exception is expensive. In older versions of Python (prior to 2.0), it

5862

was common to code this idiom:

5863

<P>

5864

<PRE>

5865

try:

5866

value = dict[key]

5867

except KeyError:

5868

dict[key] = getvalue(key)

5869

value = dict[key]

5870

</PRE>

5871

This idiom only made sense when you expected the dict to have the key

5872

95% of the time or more; other times, you coded it like this:

5873

<P>

5874

<PRE>

5875

if dict.has_key(key):

5876

value = dict[key]

5877

else:

5878

dict[key] = getvalue(key)

5879

value = dict[key]

5880

</PRE>

5881

In Python 2.0 and higher, of course, you can code this as

5882

<P>

5883

<PRE>

5884

value = dict.setdefault(key, getvalue(key))

5885

</PRE>

5886

However this evaluates getvalue(key) always, regardless of whether it's needed or not. So if it's slow or has a side effect you should use one of the above variants.

5887

<P>

5888

5889

<A HREF="faqw.py?req=edit&file=faq04.104.htp">Edit this entry</A> /

5890

5891

5892

/ Last changed on Mon Dec 9 10:12:30 2002 by

5893

5894

<P>

5895

5896

<HR>

5897

<H2><A NAME="4.105">4.105. Sharing global variables across modules</A></H2>

5898

The canonical way to share information across modules within a single

5899

program is to create a special module (often called config or cfg).

5900

Just import the config module in all modules of your application; the

5901

module then becomes available as a global name. Because there is only

5902

one instance of each module, any changes made to the module object get

5903

reflected everywhere. For example:

5904

<P>

5905

config.py:

5906

<P>

5907

<PRE>

5908

pass

5909

</PRE>

5910

mod.py:

5911

<P>

5912

<PRE>

5913

import config

5914

config.x = 1

5915

</PRE>

5916

main.py:

5917

<P>

5918

<PRE>

5919

import config

5920

import mod

5921

print config.x

5922

</PRE>

5923

Note that using a module is also the basis for implementing the

5924

Singleton design pattern, for the same reason.

5925

<P>

5926

5927

<A HREF="faqw.py?req=edit&file=faq04.105.htp">Edit this entry</A> /

5928

5929

5930

/ Last changed on Tue Apr 23 23:07:19 2002 by

5931

5932

<P>

5933

5934

<HR>

5935

<H2><A NAME="4.106">4.106. Why is cPickle so slow?</A></H2>

5936

Use the binary option. We'd like to make that the default, but it would

5937

break backward compatibility:

5938

<P>

5939

<PRE>

5940

largeString = 'z' * (100 * 1024)

5941

myPickle = cPickle.dumps(largeString, 1)

5942

</PRE>

5943

<P>

5944

5945

<A HREF="faqw.py?req=edit&file=faq04.106.htp">Edit this entry</A> /

5946

5947

5948

/ Last changed on Thu Aug 22 19:54:25 2002 by

5949

5950

<P>

5951

5952

<HR>

5953

<H2><A NAME="4.107">4.107. When importing module XXX, why do I get "undefined symbol: PyUnicodeUCS2_..." ?</A></H2>

5954

You are using a version of Python that uses a 4-byte representation for

5955

Unicode characters, but the extension module you are importing (possibly

5956

indirectly) was compiled using a Python that uses a 2-byte representation

5957

for Unicode characters (the default).

5958

<P>

5959

If instead the name of the undefined symbol starts with PyUnicodeUCS4_,

5960

the problem is the same by the relationship is reversed: Python was

5961

built using 2-byte Unicode characters, and the extension module was

5962

compiled using a Python with 4-byte Unicode characters.

5963

<P>

5964

This can easily occur when using pre-built extension packages. RedHat

5965

Linux 7.x, in particular, provides a "python2" binary that is compiled

5966

with 4-byte Unicode. This only causes the link failure if the extension

5967

uses any of the PyUnicode_*() functions. It is also a problem if if an

5968

extension uses any of the Unicode-related format specifiers for

5969

Py_BuildValue (or similar) or parameter-specifications for

5970

PyArg_ParseTuple().

5971

<P>

5972

You can check the size of the Unicode character a Python interpreter is

5973

using by checking the value of sys.maxunicode:

5974

<P>

5975

<PRE>

5976

>>> import sys

5977

>>> if sys.maxunicode > 65535:

5978

... print 'UCS4 build'

5979

... else:

5980

... print 'UCS2 build'

5981

</PRE>

5982

The only way to solve this problem is to use extension modules compiled

5983

with a Python binary built using the same size for Unicode characters.

5984

<P>

5985

5986

<A HREF="faqw.py?req=edit&file=faq04.107.htp">Edit this entry</A> /

5987

5988

5989

/ Last changed on Tue Aug 27 15:00:17 2002 by

5990

<A HREF="mailto:fdrake@acm.org">Fred Drake</A>

5991

<P>

5992

5993

<HR>

5994

<H2><A NAME="4.108">4.108. How do I create a .pyc file?</A></H2>

5995

QUESTION:

5996

<P>

5997

I have a module and I wish to generate a .pyc file.

5998

How do I do it? Everything I read says that generation of a .pyc file is

5999

"automatic", but I'm not getting anywhere.

6000

<P>

6001

<P>

6002

ANSWER:

6003

<P>

6004

When a module is imported for the first time (or when the source is more

6005

recent than the current compiled file) a .pyc file containing the compiled code should be created in the

6006

same directory as the .py file.

6007

<P>

6008

One reason that a .pyc file may not be created is permissions problems with the directory. This can happen, for example, if you develop as one user but run as another, such as if you are testing with a web server.

6009

<P>

6010

However, in most cases, that's not the problem.

6011

<P>

6012

Creation of a .pyc file is "automatic" if you are importing a module and Python has the

6013

ability (permissions, free space, etc...) to write the compiled module

6014

back to the directory. But note that running Python on a top level script is not considered an

6015

import and so no .pyc will be created automatically. For example, if you have a top-level module abc.py that imports another module xyz.py, when you run abc, xyz.pyc will be created since xyz is imported, but no abc.pyc file will be created since abc isn't imported.

6016

<P>

6017

If you need to create abc.pyc -- that is, to create a .pyc file for a

6018

module that is not imported -- you can. (Look up

6019

the py_compile and compileall modules in the Library Reference.)

6020

<P>

6021

You can manually compile any module using the "py_compile" module. One

6022

way is to use the compile() function in that module interactively:

6023

<P>

6024

<PRE>

6025

>>> import py_compile

6026

>>> py_compile.compile('abc.py')

6027

</PRE>

6028

This will write the .pyc to the same location as abc.py (or you

6029

can override that with the optional parameter cfile).

6030

<P>

6031

You can also automatically compile all files in a directory or

6032

directories using the "compileall" module, which can also be run

6033

straight from the command line.

6034

<P>

6035

You can do it from the shell (or DOS) prompt by entering:

6036

<PRE>

6037

python compile.py abc.py

6038

</PRE>

6039

or

6040

<PRE>

6041

python compile.py *

6042

</PRE>

6043

Or you can write a script to do it on a list of filenames that you enter.

6044

<P>

6045

<PRE>

6046

import sys

6047

from py_compile import compile

6048

</PRE>

6049

<PRE>

6050

if len(sys.argv) <= 1:

6051

sys.exit(1)

6052

</PRE>

6053

<PRE>

6054

for file in sys.argv[1:]:

6055

compile(file)

6056

</PRE>

6057

ACKNOWLEDGMENTS:

6058

<P>

6059

Steve Holden, David Bolen, Rich Somerfield, Oleg Broytmann, Steve Ferg

6060

<P>

6061

6062

<A HREF="faqw.py?req=edit&file=faq04.108.htp">Edit this entry</A> /

6063

6064

6065

/ Last changed on Wed Feb 12 15:58:25 2003 by

6066

<A HREF="mailto:steve@ferg.org">Stephen Ferg</A>

6067

<P>

6068

6069

<HR>

6070

<H1>5. Extending Python</H1>

6071

6072

<HR>

6073

<H2><A NAME="5.1">5.1. Can I create my own functions in C?</A></H2>

6074

Yes, you can create built-in modules containing functions,

6075

variables, exceptions and even new types in C. This is explained in

6076

the document "Extending and Embedding the Python Interpreter" (<A HREF="http://www.python.org/doc/current/ext/ext.html">http://www.python.org/doc/current/ext/ext.html</A>). Also read the chapter

6077

on dynamic loading.

6078

<P>

6079

There's more information on this in each of the Python books:

6080

Programming Python, Internet Programming with Python, and Das Python-Buch

6081

(in German).

6082

<P>

6083

6084

<A HREF="faqw.py?req=edit&file=faq05.001.htp">Edit this entry</A> /

6085

6086

6087

/ Last changed on Mon Dec 10 05:18:57 2001 by

6088

<A HREF="mailto:fdrake@acm.org">Fred L. Drake, Jr.</A>

6089

<P>

6090

6091

<HR>

6092

<H2><A NAME="5.2">5.2. Can I create my own functions in C++?</A></H2>

6093

Yes, using the C-compatibility features found in C++. Basically

6094

you place extern "C" { ... } around the Python include files and put

6095

extern "C" before each function that is going to be called by the

6096

Python interpreter. Global or static C++ objects with constructors

6097

are probably not a good idea.

6098

<P>

6099

6100

<A HREF="faqw.py?req=edit&file=faq05.002.htp">Edit this entry</A> /

6101

6102

<P>

6103

6104

<HR>

6105

<H2><A NAME="5.3">5.3. How can I execute arbitrary Python statements from C?</A></H2>

6106

The highest-level function to do this is PyRun_SimpleString() which takes

6107

a single string argument which is executed in the context of module

6108

__main__ and returns 0 for success and -1 when an exception occurred

6109

(including SyntaxError). If you want more control, use PyRun_String();

6110

see the source for PyRun_SimpleString() in Python/pythonrun.c.

6111

<P>

6112

6113

<A HREF="faqw.py?req=edit&file=faq05.003.htp">Edit this entry</A> /

6114

6115

6116

/ Last changed on Fri May 23 20:08:14 1997 by

6117

6118

<P>

6119

6120

<HR>

6121

<H2><A NAME="5.4">5.4. How can I evaluate an arbitrary Python expression from C?</A></H2>

6122

Call the function PyRun_String() from the previous question with the

6123

start symbol eval_input (Py_eval_input starting with 1.5a1); it

6124

parses an expression, evaluates it and returns its value.

6125

<P>

6126

6127

<A HREF="faqw.py?req=edit&file=faq05.004.htp">Edit this entry</A> /

6128

6129

6130

/ Last changed on Wed May 21 22:23:18 1997 by

6131

<A HREF="mailto:david_ascher@brown.edu">David Ascher</A>

6132

<P>

6133

6134

<HR>

6135

<H2><A NAME="5.5">5.5. How do I extract C values from a Python object?</A></H2>

6136

That depends on the object's type. If it's a tuple,

6137

PyTupleSize(o) returns its length and PyTuple_GetItem(o, i)

6138

returns its i'th item; similar for lists with PyListSize(o)

6139

and PyList_GetItem(o, i). For strings, PyString_Size(o) returns

6140

its length and PyString_AsString(o) a pointer to its value

6141

(note that Python strings may contain null bytes so strlen()

6142

is not safe). To test which type an object is, first make sure

6143

it isn't NULL, and then use PyString_Check(o), PyTuple_Check(o),

6144

PyList_Check(o), etc.

6145

<P>

6146

There is also a high-level API to Python objects which is

6147

provided by the so-called 'abstract' interface -- read

6148

Include/abstract.h for further details. It allows for example

6149

interfacing with any kind of Python sequence (e.g. lists and tuples)

6150

using calls like PySequence_Length(), PySequence_GetItem(), etc.)

6151

as well as many other useful protocols.

6152

<P>

6153

6154

<A HREF="faqw.py?req=edit&file=faq05.005.htp">Edit this entry</A> /

6155

6156

6157

/ Last changed on Wed May 21 22:34:20 1997 by

6158

<A HREF="mailto:david_ascher@brown.edu">David Ascher</A>

6159

<P>

6160

6161

<HR>

6162

<H2><A NAME="5.6">5.6. How do I use Py_BuildValue() to create a tuple of arbitrary length?</A></H2>

6163

You can't. Use t = PyTuple_New(n) instead, and fill it with

6164

objects using PyTuple_SetItem(t, i, o) -- note that this "eats" a

6165

reference count of o. Similar for lists with PyList_New(n) and

6166

PyList_SetItem(l, i, o). Note that you <I>must</I> set all the tuple items to

6167

some value before you pass the tuple to Python code --

6168

PyTuple_New(n) initializes them to NULL, which isn't a valid Python

6169

value.

6170

<P>

6171

6172

<A HREF="faqw.py?req=edit&file=faq05.006.htp">Edit this entry</A> /

6173

6174

6175

/ Last changed on Thu Jul 31 18:15:29 1997 by

6176

<A HREF="mailto:guido@python.org">Guido van Rossum</A>

6177

<P>

6178

6179

<HR>

6180

<H2><A NAME="5.7">5.7. How do I call an object's method from C?</A></H2>

6181

The PyObject_CallMethod() function can be used to call an arbitrary

6182

method of an object. The parameters are the object, the name of the

6183

method to call, a format string like that used with Py_BuildValue(), and the argument values:

6184

<P>

6185

<PRE>

6186

PyObject *

6187

PyObject_CallMethod(PyObject *object, char *method_name,

6188

char *arg_format, ...);

6189

</PRE>

6190

This works for any object that has methods -- whether built-in or

6191

user-defined. You are responsible for eventually DECREF'ing the

6192

return value.

6193

<P>

6194

To call, e.g., a file object's "seek" method with arguments 10, 0

6195

(assuming the file object pointer is "f"):

6196

<P>

6197

<PRE>

6198

res = PyObject_CallMethod(f, "seek", "(ii)", 10, 0);

6199

if (res == NULL) {

6200

... an exception occurred ...

6201

}

6202

else {

6203

Py_DECREF(res);

6204

}

6205

</PRE>

6206

Note that since PyObject_CallObject() <I>always</I> wants a tuple for the

6207

argument list, to call a function without arguments, pass "()" for the

6208

format, and to call a function with one argument, surround the argument

6209

in parentheses, e.g. "(i)".

6210

<P>

6211

6212

<A HREF="faqw.py?req=edit&file=faq05.007.htp">Edit this entry</A> /

6213

6214

6215

/ Last changed on Thu Jun 6 16:15:46 2002 by

6216

<A HREF="mailto:neal@metaslash.com">Neal Norwitz</A>

6217

<P>

6218

6219

<HR>

6220

<H2><A NAME="5.8">5.8. How do I catch the output from PyErr_Print() (or anything that prints to stdout/stderr)?</A></H2>

6221

(Due to Mark Hammond):

6222

<P>

6223

In Python code, define an object that supports the "write()" method.

6224

Redirect sys.stdout and sys.stderr to this object.

6225

Call print_error, or just allow the standard traceback mechanism to

6226

work. Then, the output will go wherever your write() method sends it.

6227

<P>

6228

The easiest way to do this is to use the StringIO class in the standard

6229

library.

6230

<P>

6231

Sample code and use for catching stdout:

6232

<PRE>

6233

>>> class StdoutCatcher:

6234

... def __init__(self):

6235

... self.data = ''

6236

... def write(self, stuff):

6237

... self.data = self.data + stuff

6238

...

6239

>>> import sys

6240

>>> sys.stdout = StdoutCatcher()

6241

>>> print 'foo'

6242

>>> print 'hello world!'

6243

>>> sys.stderr.write(sys.stdout.data)

6244

foo

6245

hello world!

6246

</PRE>

6247

<P>

6248

6249

<A HREF="faqw.py?req=edit&file=faq05.008.htp">Edit this entry</A> /

6250

6251

6252

/ Last changed on Wed Dec 16 18:34:25 1998 by

6253

<A HREF="mailto:richard@bofh.asn.au">Richard Jones</A>

6254

<P>

6255

6256

<HR>

6257

<H2><A NAME="5.9">5.9. How do I access a module written in Python from C?</A></H2>

6258

You can get a pointer to the module object as follows:

6259

<P>

6260

<PRE>

6261

module = PyImport_ImportModule("<modulename>");

6262

</PRE>

6263

If the module hasn't been imported yet (i.e. it is not yet present in

6264

sys.modules), this initializes the module; otherwise it simply returns

6265

the value of sys.modules["<modulename>"]. Note that it doesn't enter

6266

the module into any namespace -- it only ensures it has been

6267

initialized and is stored in sys.modules.

6268

<P>

6269

You can then access the module's attributes (i.e. any name defined in

6270

the module) as follows:

6271

<P>

6272

<PRE>

6273

attr = PyObject_GetAttrString(module, "<attrname>");

6274

</PRE>

6275

Calling PyObject_SetAttrString(), to assign to variables in the module, also works.

6276

<P>

6277

6278

<A HREF="faqw.py?req=edit&file=faq05.009.htp">Edit this entry</A> /

6279

6280

6281

/ Last changed on Wed May 21 22:56:40 1997 by

6282

<A HREF="mailto:david_ascher@brown.edu">david ascher</A>

6283

<P>

6284

6285

<HR>

6286

<H2><A NAME="5.10">5.10. How do I interface to C++ objects from Python?</A></H2>

6287

Depending on your requirements, there are many approaches. To do

6288

this manually, begin by reading the "Extending and Embedding" document

6289

(Doc/ext.tex, see also <A HREF="http://www.python.org/doc">http://www.python.org/doc</A>/). Realize

6290

that for the Python run-time system, there isn't a whole lot of

6291

difference between C and C++ -- so the strategy to build a new Python

6292

type around a C structure (pointer) type will also work for C++

6293

objects.

6294

<P>

6295

A useful automated approach (which also works for C) is SWIG:

6296

<A HREF="http://www.swig.org">http://www.swig.org</A>/.

6297

<P>

6298

6299

<A HREF="faqw.py?req=edit&file=faq05.010.htp">Edit this entry</A> /

6300

6301

6302

/ Last changed on Fri Oct 15 05:14:01 1999 by

6303

<A HREF="mailto:sjoerd.mullender@oratrix.nl">Sjoerd Mullender</A>

6304

<P>

6305

6306

<HR>

6307

<H2><A NAME="5.11">5.11. mSQLmodule (or other old module) won't build with Python 1.5 (or later)</A></H2>

6308

Since python-1.4 "Python.h" will have the file includes needed in an

6309

extension module.

6310

Backward compatibility is dropped after version 1.4 and therefore

6311

mSQLmodule.c will not build as "allobjects.h" cannot be found.

6312

The following change in mSQLmodule.c is harmless when building it with

6313

1.4 and necessary when doing so for later python versions:

6314

<P>

6315

Remove lines:

6316

<P>

6317

<PRE>

6318

#include "allobjects.h"

6319

#include "modsupport.h"

6320

</PRE>

6321

And insert instead:

6322

<P>

6323

<PRE>

6324

#include "Python.h"

6325

</PRE>

6326

You may also need to add

6327

<P>

6328

<PRE>

6329

#include "rename2.h"

6330

</PRE>

6331

if the module uses "old names".

6332

<P>

6333

This may happen with other ancient python modules as well,

6334

and the same fix applies.

6335

<P>

6336

6337

<A HREF="faqw.py?req=edit&file=faq05.011.htp">Edit this entry</A> /

6338

6339

6340

/ Last changed on Sun Dec 21 02:03:35 1997 by

6341

6342

<P>

6343

6344

<HR>

6345

<H2><A NAME="5.12">5.12. I added a module using the Setup file and the make fails! Huh?</A></H2>

6346

Setup must end in a newline, if there is no newline there it gets

6347

very sad. Aside from this possibility, maybe you have other

6348

non-Python-specific linkage problems.

6349

<P>

6350

6351

<A HREF="faqw.py?req=edit&file=faq05.012.htp">Edit this entry</A> /

6352

6353

6354

/ Last changed on Tue Jun 24 15:54:01 1997 by

6355

<A HREF="mailto:aaron_watters@msn.com">aaron watters</A>

6356

<P>

6357

6358

<HR>

6359

<H2><A NAME="5.13">5.13. I want to compile a Python module on my Red Hat Linux system, but some files are missing.</A></H2>

6360

Red Hat's RPM for Python doesn't include the

6361

/usr/lib/python1.x/config/ directory, which contains various files required

6362

for compiling Python extensions.

6363

Install the python-devel RPM to get the necessary files.

6364

<P>

6365

6366

<A HREF="faqw.py?req=edit&file=faq05.013.htp">Edit this entry</A> /

6367

6368

6369

/ Last changed on Tue Jan 26 13:44:04 1999 by

6370

<A HREF="mailto:akuchling@acm.org">A.M. Kuchling</A>

6371

<P>

6372

6373

<HR>

6374

<H2><A NAME="5.14">5.14. What does "SystemError: _PyImport_FixupExtension: module yourmodule not loaded" mean?</A></H2>

6375

This means that you have created an extension module named "yourmodule", but your module init function does not initialize with that name.

6376

<P>

6377

Every module init function will have a line similar to:

6378

<P>

6379

<PRE>

6380

module = Py_InitModule("yourmodule", yourmodule_functions);

6381

</PRE>

6382

If the string passed to this function is not the same name as your extenion module, the SystemError will be raised.

6383

<P>

6384

6385

<A HREF="faqw.py?req=edit&file=faq05.014.htp">Edit this entry</A> /

6386

6387

6388

/ Last changed on Thu Mar 25 07:16:08 1999 by

6389

<A HREF="mailto:mhammond@skippinet.com.au">Mark Hammond</A>

6390

<P>

6391

6392

<HR>

6393

<H2><A NAME="5.15">5.15. How to tell "incomplete input" from "invalid input"?</A></H2>

6394

Sometimes you want to emulate the Python interactive interpreter's

6395

behavior, where it gives you a continuation prompt when the input

6396

is incomplete (e.g. you typed the start of an "if" statement

6397

or you didn't close your parentheses or triple string quotes),

6398

but it gives you a syntax error message immediately when the input

6399

is invalid.

6400

<P>

6401

In Python you can use the codeop module, which approximates the

6402

parser's behavior sufficiently. IDLE uses this, for example.

6403

<P>

6404

The easiest way to do it in C is to call PyRun_InteractiveLoop()

6405

(in a separate thread maybe) and let the Python interpreter handle

6406

the input for you. You can also set the PyOS_ReadlineFunctionPointer

6407

to point at your custom input function. See Modules/readline.c and

6408

Parser/myreadline.c for more hints.

6409

<P>

6410

However sometimes you have to run the embedded Python interpreter

6411

in the same thread as your rest application and you can't allow the

6412

PyRun_InteractiveLoop() to stop while waiting for user input.

6413

The one solution then is to call PyParser_ParseString()

6414

and test for e.error equal to E_EOF (then the input is incomplete).

6415

Sample code fragment, untested, inspired by code from Alex Farber:

6416

<P>

6417

<PRE>

6418

#include <Python.h>

6419

#include <node.h>

6420

#include <errcode.h>

6421

#include <grammar.h>

6422

#include <parsetok.h>

6423

#include <compile.h>

6424

</PRE>

6425

<PRE>

6426

int testcomplete(char *code)

6427

/* code should end in \n */

6428

/* return -1 for error, 0 for incomplete, 1 for complete */

6429

{

6430

node *n;

6431

perrdetail e;

6432

</PRE>

6433

<PRE>

6434

n = PyParser_ParseString(code, &_PyParser_Grammar,

6435

Py_file_input, &e);

6436

if (n == NULL) {

6437

if (e.error == E_EOF)

6438

return 0;

6439

return -1;

6440

}

6441

</PRE>

6442

<PRE>

6443

PyNode_Free(n);

6444

return 1;

6445

}

6446

</PRE>

6447

Another solution is trying to compile the received string with

6448

Py_CompileString(). If it compiles fine - try to execute the returned

6449

code object by calling PyEval_EvalCode(). Otherwise save the input for

6450

later. If the compilation fails, find out if it's an error or just

6451

more input is required - by extracting the message string from the

6452

exception tuple and comparing it to the "unexpected EOF while parsing".

6453

Here is a complete example using the GNU readline library (you may

6454

want to ignore SIGINT while calling readline()):

6455

<P>

6456

<PRE>

6457

#include <stdio.h>

6458

#include <readline.h>

6459

</PRE>

6460

<PRE>

6461

#include <Python.h>

6462

#include <object.h>

6463

#include <compile.h>

6464

#include <eval.h>

6465

</PRE>

6466

<PRE>

6467

int main (int argc, char* argv[])

6468

{

6469

int i, j, done = 0; /* lengths of line, code */

6470

char ps1[] = ">>> ";

6471

char ps2[] = "... ";

6472

char *prompt = ps1;

6473

char *msg, *line, *code = NULL;

6474

PyObject *src, *glb, *loc;

6475

PyObject *exc, *val, *trb, *obj, *dum;

6476

</PRE>

6477

<PRE>

6478

Py_Initialize ();

6479

loc = PyDict_New ();

6480

glb = PyDict_New ();

6481

PyDict_SetItemString (glb, "__builtins__", PyEval_GetBuiltins ());

6482

</PRE>

6483

<PRE>

6484

while (!done)

6485

{

6486

line = readline (prompt);

6487

</PRE>

6488

<PRE>

6489

if (NULL == line) /* CTRL-D pressed */

6490

{

6491

done = 1;

6492

}

6493

else

6494

{

6495

i = strlen (line);

6496

</PRE>

6497

<PRE>

6498

if (i > 0)

6499

add_history (line); /* save non-empty lines */

6500

</PRE>

6501

<PRE>

6502

if (NULL == code) /* nothing in code yet */

6503

j = 0;

6504

else

6505

j = strlen (code);

6506

</PRE>

6507

<PRE>

6508

code = realloc (code, i + j + 2);

6509

if (NULL == code) /* out of memory */

6510

exit (1);

6511

</PRE>

6512

<PRE>

6513

if (0 == j) /* code was empty, so */

6514

code[0] = '\0'; /* keep strncat happy */

6515

</PRE>

6516

<PRE>

6517

strncat (code, line, i); /* append line to code */

6518

code[i + j] = '\n'; /* append '\n' to code */

6519

code[i + j + 1] = '\0';

6520

</PRE>

6521

<PRE>

6522

src = Py_CompileString (code, "<stdin>", Py_single_input);

6523

</PRE>

6524

<PRE>

6525

if (NULL != src) /* compiled just fine - */

6526

{

6527

if (ps1 == prompt || /* ">>> " or */

6528

'\n' == code[i + j - 1]) /* "... " and double '\n' */

6529

{ /* so execute it */

6530

dum = PyEval_EvalCode ((PyCodeObject *)src, glb, loc);

6531

Py_XDECREF (dum);

6532

Py_XDECREF (src);

6533

free (code);

6534

code = NULL;

6535

if (PyErr_Occurred ())

6536

PyErr_Print ();

6537

prompt = ps1;

6538

}

6539

} /* syntax error or E_EOF? */

6540

else if (PyErr_ExceptionMatches (PyExc_SyntaxError))

6541

{

6542

PyErr_Fetch (&exc, &val, &trb); /* clears exception! */

6543

</PRE>

6544

<PRE>

6545

if (PyArg_ParseTuple (val, "sO", &msg, &obj) &&

6546

!strcmp (msg, "unexpected EOF while parsing")) /* E_EOF */

6547

{

6548

Py_XDECREF (exc);

6549

Py_XDECREF (val);

6550

Py_XDECREF (trb);

6551

prompt = ps2;

6552

}

6553

else /* some other syntax error */

6554

{

6555

PyErr_Restore (exc, val, trb);

6556

PyErr_Print ();

6557

free (code);

6558

code = NULL;

6559

prompt = ps1;

6560

}

6561

}

6562

else /* some non-syntax error */

6563

{

6564

PyErr_Print ();

6565

free (code);

6566

code = NULL;

6567

prompt = ps1;

6568

}

6569

</PRE>

6570

<PRE>

6571

free (line);

6572

}

6573

}

6574

</PRE>

6575

<PRE>

6576

Py_XDECREF(glb);

6577

Py_XDECREF(loc);

6578

Py_Finalize();

6579

exit(0);

6580

}

6581

</PRE>

6582

<P>

6583

6584

<A HREF="faqw.py?req=edit&file=faq05.015.htp">Edit this entry</A> /

6585

6586

6587

/ Last changed on Wed Mar 15 09:47:24 2000 by

6588

<A HREF="mailto:farber@cpan.org">Alex Farber</A>

6589

<P>

6590

6591

<HR>

6592

<H2><A NAME="5.16">5.16. How do I debug an extension?</A></H2>

6593

When using gdb with dynamically loaded extensions, you can't set a

6594

breakpoint in your extension until your extension is loaded.

6595

<P>

6596

In your .gdbinit file (or interactively), add the command

6597

<P>

6598

br _PyImport_LoadDynamicModule

6599

<P>

6600

<P>

6601

$ gdb /local/bin/python

6602

<P>

6603

gdb) run myscript.py

6604

<P>

6605

gdb) continue # repeat until your extension is loaded

6606

<P>

6607

gdb) finish # so that your extension is loaded

6608

<P>

6609

gdb) br myfunction.c:50

6610

<P>

6611

gdb) continue

6612

<P>

6613

6614

<A HREF="faqw.py?req=edit&file=faq05.016.htp">Edit this entry</A> /

6615

6616

6617

/ Last changed on Fri Oct 20 11:10:32 2000 by

6618

<A HREF="mailto:vanandel@ucar.edu">Joe VanAndel</A>

6619

<P>

6620

6621

<HR>

6622

<H2><A NAME="5.17">5.17. How do I find undefined Linux g++ symbols, __builtin_new or __pure_virtural</A></H2>

6623

To dynamically load g++ extension modules, you must recompile python, relink python using g++ (change LINKCC in the python Modules Makefile), and link your extension module using g++ (e.g., "g++ -shared -o mymodule.so mymodule.o").

6624

<P>

6625

6626

<A HREF="faqw.py?req=edit&file=faq05.017.htp">Edit this entry</A> /

6627

6628

6629

/ Last changed on Sun Jan 14 18:03:51 2001 by

6630

<A HREF="mailto:dbo@angryduck.com">douglas orr</A>

6631

<P>

6632

6633

<HR>

6634

<H2><A NAME="5.18">5.18. How do I define and create objects corresponding to built-in/extension types</A></H2>

6635

Usually you would like to be able to inherit from a Python type when

6636

you ask this question. The bottom line for Python 2.2 is: types and classes are miscible. You build instances by calling classes, and you can build subclasses to your heart's desire.

6637

<P>

6638

You need to be careful when instantiating immutable types like integers or strings. See <A HREF="http://www.amk.ca/python/2.2">http://www.amk.ca/python/2.2</A>/, section 2, for details.

6639

<P>

6640

Prior to version 2.2, Python (like Java) insisted that there are first-class and second-class objects (the former are types, the latter classes), and never the twain shall meet.

6641

<P>

6642

The library has, however, done a good job of providing class wrappers for the more commonly desired objects (see UserDict, UserList and UserString for examples), and more are always welcome if you happen to be in the mood to write code. These wrappers still exist in Python 2.2.

6643

<P>

6644

6645

<A HREF="faqw.py?req=edit&file=faq05.018.htp">Edit this entry</A> /

6646

6647

6648

/ Last changed on Mon Jun 10 15:14:07 2002 by

6649

<A HREF="mailto:smurf@noris.de">Matthias Urlichs</A>

6650

<P>

6651

6652

<HR>

6653

<H1>6. Python's design</H1>

6654

6655

<HR>

6656

<H2><A NAME="6.1">6.1. Why isn't there a switch or case statement in Python?</A></H2>

6657

You can do this easily enough with a sequence of

6658

if... elif... elif... else. There have been some proposals for switch

6659

statement syntax, but there is no consensus (yet) on whether and how

6660

to do range tests.

6661

<P>

6662

6663

<A HREF="faqw.py?req=edit&file=faq06.001.htp">Edit this entry</A> /

6664

6665

<P>

6666

6667

<HR>

6668

<H2><A NAME="6.2">6.2. Why does Python use indentation for grouping of statements?</A></H2>

6669

Basically I believe that using indentation for grouping is

6670

extremely elegant and contributes a lot to the clarity of the average

6671

Python program. Most people learn to love this feature after a while.

6672

Some arguments for it:

6673

<P>

6674

Since there are no begin/end brackets there cannot be a disagreement

6675

between grouping perceived by the parser and the human reader. I

6676

remember long ago seeing a C fragment like this:

6677

<P>

6678

<PRE>

6679

if (x <= y)

6680

x++;

6681

y--;

6682

z++;

6683

</PRE>

6684

and staring a long time at it wondering why y was being decremented

6685

even for x > y... (And I wasn't a C newbie then either.)

6686

<P>

6687

Since there are no begin/end brackets, Python is much less prone to

6688

coding-style conflicts. In C there are loads of different ways to

6689

place the braces (including the choice whether to place braces around

6690

single statements in certain cases, for consistency). If you're used

6691

to reading (and writing) code that uses one style, you will feel at

6692

least slightly uneasy when reading (or being required to write)

6693

another style.

6694

Many coding styles place begin/end brackets on a line by themself.

6695

This makes programs considerably longer and wastes valuable screen

6696

space, making it harder to get a good overview over a program.

6697

Ideally, a function should fit on one basic tty screen (say, 20

6698

lines). 20 lines of Python are worth a LOT more than 20 lines of C.

6699

This is not solely due to the lack of begin/end brackets (the lack of

6700

declarations also helps, and the powerful operations of course), but

6701

it certainly helps!

6702

<P>

6703

6704

<A HREF="faqw.py?req=edit&file=faq06.002.htp">Edit this entry</A> /

6705

6706

6707

/ Last changed on Wed May 21 16:00:15 1997 by

6708

6709

<P>

6710

6711

<HR>

6712

<H2><A NAME="6.3">6.3. Why are Python strings immutable?</A></H2>

6713

There are two advantages. One is performance: knowing that a

6714

string is immutable makes it easy to lay it out at construction time

6715

-- fixed and unchanging storage requirements. (This is also one of

6716

the reasons for the distinction between tuples and lists.) The

6717

other is that strings in Python are considered as "elemental" as

6718

numbers. No amount of activity will change the value 8 to anything

6719

else, and in Python, no amount of activity will change the string

6720

"eight" to anything else. (Adapted from Jim Roskind)

6721

<P>

6722

6723

<A HREF="faqw.py?req=edit&file=faq06.003.htp">Edit this entry</A> /

6724

6725

<P>

6726

6727

<HR>

6728

<H2><A NAME="6.4">6.4. Delete</A></H2>

6729

<P>

6730

<P>

6731

6732

<A HREF="faqw.py?req=edit&file=faq06.004.htp">Edit this entry</A> /

6733

6734

6735

/ Last changed on Tue Jan 2 03:05:25 2001 by

6736

<A HREF="mailto:moshez@zadka.site.co.il">Moshe Zadka</A>

6737

<P>

6738

6739

<HR>

6740

<H2><A NAME="6.5">6.5. Why does Python use methods for some functionality (e.g. list.index()) but functions for other (e.g. len(list))?</A></H2>

6741

The major reason is history. Functions were used for those

6742

operations that were generic for a group of types and which

6743

were intended to work even for objects that didn't have

6744

methods at all (e.g. numbers before type/class unification

6745

began, or tuples).

6746

<P>

6747

It is also convenient to have a function that can readily be applied

6748

to an amorphous collection of objects when you use the functional features of Python (map(), apply() et al).

6749

<P>

6750

In fact, implementing len(), max(), min() as a built-in function is

6751

actually less code than implementing them as methods for each type.

6752

One can quibble about individual cases but it's a part of Python,

6753

and it's too late to change such things fundamentally now. The

6754

functions have to remain to avoid massive code breakage.

6755

<P>

6756

Note that for string operations Python has moved from external functions

6757

(the string module) to methods. However, len() is still a function.

6758

<P>

6759

6760

<A HREF="faqw.py?req=edit&file=faq06.005.htp">Edit this entry</A> /

6761

6762

6763

/ Last changed on Thu May 30 14:08:58 2002 by

6764

<A HREF="mailto:sholden@holdenweb.com">Steve Holden</A>

6765

<P>

6766

6767

<HR>

6768

<H2><A NAME="6.6">6.6. Why can't I derive a class from built-in types (e.g. lists or files)?</A></H2>

6769

As of Python 2.2, you can derive from built-in types. For previous versions, the answer is:

6770

<P>

6771

This is caused by the relatively late addition of (user-defined)

6772

classes to the language -- the implementation framework doesn't easily

6773

allow it. See the answer to question 4.2 for a work-around. This

6774

<I>may</I> be fixed in the (distant) future.

6775

<P>

6776

6777

<A HREF="faqw.py?req=edit&file=faq06.006.htp">Edit this entry</A> /

6778

6779

6780

/ Last changed on Thu May 23 02:53:22 2002 by

6781

<A HREF="mailto:neal@metaslash.com">Neal Norwitz</A>

6782

<P>

6783

6784

<HR>

6785

<H2><A NAME="6.7">6.7. Why must 'self' be declared and used explicitly in method definitions and calls?</A></H2>

6786

So, is your current programming language C++ or Java? :-)

6787

When classes were added to Python, this was (again) the simplest way of

6788

implementing methods without too many changes to the interpreter. The

6789

idea was borrowed from Modula-3. It turns out to be very useful, for

6790

a variety of reasons.

6791

<P>

6792

First, it makes it more obvious that you are using a method or

6793

instance attribute instead of a local variable. Reading "self.x" or

6794

"self.meth()" makes it absolutely clear that an instance variable or

6795

method is used even if you don't know the class definition by heart.

6796

In C++, you can sort of tell by the lack of a local variable

6797

declaration (assuming globals are rare or easily recognizable) -- but

6798

in Python, there are no local variable declarations, so you'd have to

6799

look up the class definition to be sure.

6800

<P>

6801

Second, it means that no special syntax is necessary if you want to

6802

explicitly reference or call the method from a particular class. In

6803

C++, if you want to use a method from base class that is overridden in

6804

a derived class, you have to use the :: operator -- in Python you can

6805

write baseclass.methodname(self, <argument list>). This is

6806

particularly useful for __init__() methods, and in general in cases

6807

where a derived class method wants to extend the base class method of

6808

the same name and thus has to call the base class method somehow.

6809

<P>

6810

Lastly, for instance variables, it solves a syntactic problem with

6811

assignment: since local variables in Python are (by definition!) those

6812

variables to which a value assigned in a function body (and that

6813

aren't explicitly declared global), there has to be some way to tell

6814

the interpreter that an assignment was meant to assign to an instance

6815

variable instead of to a local variable, and it should preferably be

6816

syntactic (for efficiency reasons). C++ does this through

6817

declarations, but Python doesn't have declarations and it would be a

6818

pity having to introduce them just for this purpose. Using the

6819

explicit "self.var" solves this nicely. Similarly, for using instance

6820

variables, having to write "self.var" means that references to

6821

unqualified names inside a method don't have to search the instance's

6822

directories.

6823

<P>

6824

6825

<A HREF="faqw.py?req=edit&file=faq06.007.htp">Edit this entry</A> /

6826

6827

6828

/ Last changed on Fri Jan 12 08:01:50 2001 by

6829

<A HREF="mailto:sholden@holdenweb.com">Steve Holden</A>

6830

<P>

6831

6832

<HR>

6833

<H2><A NAME="6.8">6.8. Can't you emulate threads in the interpreter instead of relying on an OS-specific thread implementation?</A></H2>

6834

Answer 1: Unfortunately, the interpreter pushes at least one C stack

6835

frame for each Python stack frame. Also, extensions can call back into

6836

Python at almost random moments. Therefore a complete threads

6837

implementation requires thread support for C.

6838

<P>

6839

Answer 2: Fortunately, there is Stackless Python, which has a completely redesigned interpreter loop that avoids the C stack. It's still experimental but looks very promising. Although it is binary compatible with standard Python, it's still unclear whether Stackless will make it into the core -- maybe it's just too revolutionary. Stackless Python currently lives here: <A HREF="http://www.stackless.com">http://www.stackless.com</A>. A microthread implementation that uses it can be found here: <A HREF="http://world.std.com/~wware/uthread.html">http://world.std.com/~wware/uthread.html</A>.

6840

<P>

6841

6842

<A HREF="faqw.py?req=edit&file=faq06.008.htp">Edit this entry</A> /

6843

6844

6845

/ Last changed on Sat Apr 15 08:18:16 2000 by

6846

<A HREF="mailto:just@letterror.com">Just van Rossum</A>

6847

<P>

6848

6849

<HR>

6850

<H2><A NAME="6.9">6.9. Why can't lambda forms contain statements?</A></H2>

6851

Python lambda forms cannot contain statements because Python's

6852

syntactic framework can't handle statements nested inside expressions.

6853

<P>

6854

However, in Python, this is not a serious problem. Unlike lambda

6855

forms in other languages, where they add functionality, Python lambdas

6856

are only a shorthand notation if you're too lazy to define a function.

6857

<P>

6858

Functions are already first class objects in Python, and can be

6859

declared in a local scope. Therefore the only advantage of using a

6860

lambda form instead of a locally-defined function is that you don't need to invent a name for the function -- but that's just a local variable to which the function object (which is exactly the same type of object that a lambda form yields) is assigned!

6861

<P>

6862

6863

<A HREF="faqw.py?req=edit&file=faq06.009.htp">Edit this entry</A> /

6864

6865

6866

/ Last changed on Sun Jun 14 14:15:17 1998 by

6867

<A HREF="mailto:tim_one@email.msn.com">Tim Peters</A>

6868

<P>

6869

6870

<HR>

6871

<H2><A NAME="6.10">6.10. [deleted]</A></H2>

6872

[lambda vs non-nested scopes used to be here]

6873

<P>

6874

6875

<A HREF="faqw.py?req=edit&file=faq06.010.htp">Edit this entry</A> /

6876

6877

6878

/ Last changed on Thu Mar 21 05:20:56 2002 by

6879

<A HREF="mailto:erno-pyfaq@erno.iki.fi">Erno Kuusela</A>

6880

<P>

6881

6882

<HR>

6883

<H2><A NAME="6.11">6.11. [deleted]</A></H2>

6884

[recursive functions vs non-nested scopes used to be here]

6885

<P>

6886

6887

<A HREF="faqw.py?req=edit&file=faq06.011.htp">Edit this entry</A> /

6888

6889

6890

/ Last changed on Thu Mar 21 05:22:04 2002 by

6891

<A HREF="mailto:erno-pyfaq@erno.iki.fi">Erno Kuusela</A>

6892

<P>

6893

6894

<HR>

6895

<H2><A NAME="6.12">6.12. Why is there no more efficient way of iterating over a dictionary than first constructing the list of keys()?</A></H2>

6896

As of Python 2.2, you can now iterate over a dictionary directly,

6897

using the new implied dictionary iterator:

6898

<P>

6899

<PRE>

6900

for k in d: ...

6901

</PRE>

6902

There are also methods returning iterators over the values and items:

6903

<P>

6904

<PRE>

6905

for k in d.iterkeys(): # same as above

6906

for v in d.itervalues(): # iterate over values

6907

for k, v in d.iteritems(): # iterate over items

6908

</PRE>

6909

All these require that you do not modify the dictionary during the loop.

6910

<P>

6911

For previous Python versions, the following defense should do:

6912

<P>

6913

Have you tried it? I bet it's fast enough for your purposes! In

6914

most cases such a list takes only a few percent of the space occupied

6915

by the dictionary. Apart from the fixed header,

6916

the list needs only 4 bytes (the size of a pointer) per

6917

key. A dictionary uses 12 bytes per key plus between 30 and 70

6918

percent hash table overhead, plus the space for the keys and values.

6919

By necessity, all keys are distinct objects, and a string object (the most

6920

common key type) costs at least 20 bytes plus the length of the

6921

string. Add to that the values contained in the dictionary, and you

6922

see that 4 bytes more per item really isn't that much more memory...

6923

<P>

6924

A call to dict.keys() makes one fast scan over the dictionary

6925

(internally, the iteration function does exist) copying the pointers

6926

to the key objects into a pre-allocated list object of the right size.

6927

The iteration time isn't lost (since you'll have to iterate anyway --

6928

unless in the majority of cases your loop terminates very prematurely

6929

(which I doubt since you're getting the keys in random order).

6930

<P>

6931

I don't expose the dictionary iteration operation to Python

6932

programmers because the dictionary shouldn't be modified during the

6933

entire iteration -- if it is, there's a small chance that the

6934

dictionary is reorganized because the hash table becomes too full, and

6935

then the iteration may miss some items and see others twice. Exactly

6936

because this only occurs rarely, it would lead to hidden bugs in

6937

programs: it's easy never to have it happen during test runs if you

6938

only insert or delete a few items per iteration -- but your users will

6939

surely hit upon it sooner or later.

6940

<P>

6941

6942

<A HREF="faqw.py?req=edit&file=faq06.012.htp">Edit this entry</A> /

6943

6944

6945

/ Last changed on Fri May 24 21:24:08 2002 by

6946

6947

<P>

6948

6949

<HR>

6950

<H2><A NAME="6.13">6.13. Can Python be compiled to machine code, C or some other language?</A></H2>

6951

Not easily. Python's high level data types, dynamic typing of

6952

objects and run-time invocation of the interpreter (using eval() or

6953

exec) together mean that a "compiled" Python program would probably

6954

consist mostly of calls into the Python run-time system, even for

6955

seemingly simple operations like "x+1".

6956

<P>

6957

Several projects described in the Python newsgroup or at past

6958

Python conferences have shown that this approach is feasible,

6959

although the speedups reached so far are only modest (e.g. 2x).

6960

JPython uses the same strategy for compiling to Java bytecode.

6961

(Jim Hugunin has demonstrated that in combination with whole-program

6962

analysis, speedups of 1000x are feasible for small demo programs.

6963

See the website for the 1997 Python conference.)

6964

<P>

6965

Internally, Python source code is always translated into a "virtual

6966

machine code" or "byte code" representation before it is interpreted

6967

(by the "Python virtual machine" or "bytecode interpreter"). In order

6968

to avoid the overhead of parsing and translating modules that rarely

6969

change over and over again, this byte code is written on a file whose

6970

name ends in ".pyc" whenever a module is parsed (from a file whose

6971

name ends in ".py"). When the corresponding .py file is changed, it

6972

is parsed and translated again and the .pyc file is rewritten.

6973

<P>

6974

There is no performance difference once the .pyc file has been loaded

6975

(the bytecode read from the .pyc file is exactly the same as the bytecode

6976

created by direct translation). The only difference is that loading

6977

code from a .pyc file is faster than parsing and translating a .py

6978

file, so the presence of precompiled .pyc files will generally improve

6979

start-up time of Python scripts. If desired, the Lib/compileall.py

6980

module/script can be used to force creation of valid .pyc files for a

6981

given set of modules.

6982

<P>

6983

Note that the main script executed by Python, even if its filename

6984

ends in .py, is not compiled to a .pyc file. It is compiled to

6985

bytecode, but the bytecode is not saved to a file.

6986

<P>

6987

If you are looking for a way to translate Python programs in order to

6988

distribute them in binary form, without the need to distribute the

6989

interpreter and library as well, have a look at the freeze.py script

6990

in the Tools/freeze directory. This creates a single binary file

6991

incorporating your program, the Python interpreter, and those parts of

6992

the Python library that are needed by your program. Of course, the

6993

resulting binary will only run on the same type of platform as that

6994

used to create it.

6995

<P>

6996

Newsflash: there are now several programs that do this, to some extent.

6997

Look for Psyco, Pyrex, PyInline, Py2Cmod, and Weave.

6998

<P>

6999

7000

<A HREF="faqw.py?req=edit&file=faq06.013.htp">Edit this entry</A> /

7001

7002

7003

/ Last changed on Fri May 24 21:26:19 2002 by

7004

7005

<P>

7006

7007

<HR>

7008

<H2><A NAME="6.14">6.14. How does Python manage memory?</A></H2>

7009

The details of Python memory management depend on the implementation.

7010

The standard Python implementation (the C implementation) uses reference

7011

counting and another mechanism to collect reference cycles.

7012

<P>

7013

Jython relies on the Java runtime; so it uses

7014

the JVM's garbage collector. This difference can cause some subtle

7015

porting problems if your Python code depends on the behavior of

7016

the reference counting implementation.

7017

<P>

7018

The reference cycle collector was added in CPython 2.0. It

7019

periodically executes a cycle detection algorithm which looks for inaccessible cycles and deletes the objects involved. A new gc module provides functions to perform a garbage collection, obtain debugging statistics, and tuning the collector's parameters.

7020

<P>

7021

The detection of cycles can be disabled when Python is compiled, if you can't afford even a tiny speed penalty or suspect that the cycle collection is buggy, by specifying the "--without-cycle-gc" switch when running the configure script.

7022

<P>

7023

Sometimes objects get stuck in "tracebacks" temporarily and hence are not deallocated when you might expect. Clear the tracebacks via

7024

<P>

7025

<PRE>

7026

import sys

7027

sys.exc_traceback = sys.last_traceback = None

7028

</PRE>

7029

Tracebacks are used for reporting errors and implementing debuggers and related things. They contain a portion of the program state extracted during the handling of an exception (usually the most recent exception).

7030

<P>

7031

In the absence of circularities and modulo tracebacks, Python programs need not explicitly manage memory.

7032

<P>

7033

Why python doesn't use a more traditional garbage collection

7034

scheme? For one thing, unless this were

7035

added to C as a standard feature, it's a portability pain in the ass.

7036

And yes, I know about the Xerox library. It has bits of assembler

7037

code for <I>most</I> <I>common</I> platforms. Not for all. And although it is

7038

mostly transparent, it isn't completely transparent (when I once

7039

linked Python with it, it dumped core).

7040

<P>

7041

Traditional GC also becomes a problem when Python gets embedded into

7042

other applications. While in a stand-alone Python it may be fine to

7043

replace the standard malloc() and free() with versions provided by the

7044

GC library, an application embedding Python may want to have its <I>own</I>

7045

substitute for malloc() and free(), and may not want Python's. Right

7046

now, Python works with anything that implements malloc() and free()

7047

properly.

7048

<P>

7049

In Jython, the following code (which is

7050

fine in C Python) will probably run out of file descriptors long before

7051

it runs out of memory:

7052

<P>

7053

<PRE>

7054

for file in <very long list of files>:

7055

f = open(file)

7056

c = f.read(1)

7057

</PRE>

7058

Using the current reference counting and destructor scheme, each new

7059

assignment to f closes the previous file. Using GC, this is not

7060

guaranteed. Sure, you can think of ways to fix this. But it's not

7061

off-the-shelf technology. If you want to write code that will

7062

work with any Python implementation, you should explicitly close

7063

the file; this will work regardless of GC:

7064

<P>

7065

<PRE>

7066

for file in <very long list of files>:

7067

f = open(file)

7068

c = f.read(1)

7069

f.close()

7070

</PRE>

7071

<P>

7072

7073

<A HREF="faqw.py?req=edit&file=faq06.014.htp">Edit this entry</A> /

7074

7075

7076

/ Last changed on Thu Mar 21 05:35:38 2002 by

7077

<A HREF="mailto:erno-pyfaq@erno.iki.fi">Erno Kuusela</A>

7078

<P>

7079

7080

<HR>

7081

<H2><A NAME="6.15">6.15. Why are there separate tuple and list data types?</A></H2>

7082

This is done so that tuples can be immutable while lists are mutable.

7083

<P>

7084

Immutable tuples are useful in situations where you need to pass a few

7085

items to a function and don't want the function to modify the tuple;

7086

for example,

7087

<P>

7088

<PRE>

7089

point1 = (120, 140)

7090

point2 = (200, 300)

7091

record(point1, point2)

7092

draw(point1, point2)

7093

</PRE>

7094

You don't want to have to think about what would happen if record()

7095

changed the coordinates -- it can't, because the tuples are immutable.

7096

<P>

7097

On the other hand, when creating large lists dynamically, it is

7098

absolutely crucial that they are mutable -- adding elements to a tuple

7099

one by one requires using the concatenation operator, which makes it

7100

quadratic in time.

7101

<P>

7102

As a general guideline, use tuples like you would use structs in C or

7103

records in Pascal, use lists like (variable length) arrays.

7104

<P>

7105

7106

<A HREF="faqw.py?req=edit&file=faq06.015.htp">Edit this entry</A> /

7107

7108

7109

/ Last changed on Fri May 23 15:26:03 1997 by

7110

7111

<P>

7112

7113

<HR>

7114

<H2><A NAME="6.16">6.16. How are lists implemented?</A></H2>

7115

Despite what a Lisper might think, Python's lists are really

7116

variable-length arrays. The implementation uses a contiguous

7117

array of references to other objects, and keeps a pointer

7118

to this array (as well as its length) in a list head structure.

7119

<P>

7120

This makes indexing a list (a[i]) an operation whose cost is

7121

independent of the size of the list or the value of the index.

7122

<P>

7123

When items are appended or inserted, the array of references is resized.

7124

Some cleverness is applied to improve the performance of appending

7125

items repeatedly; when the array must be grown, some extra space

7126

is allocated so the next few times don't require an actual resize.

7127

<P>

7128

7129

<A HREF="faqw.py?req=edit&file=faq06.016.htp">Edit this entry</A> /

7130

7131

7132

/ Last changed on Fri May 23 15:32:24 1997 by

7133

7134

<P>

7135

7136

<HR>

7137

<H2><A NAME="6.17">6.17. How are dictionaries implemented?</A></H2>

7138

Python's dictionaries are implemented as resizable hash tables.

7139

<P>

7140

Compared to B-trees, this gives better performance for lookup

7141

(the most common operation by far) under most circumstances,

7142

and the implementation is simpler.

7143

<P>

7144

7145

<A HREF="faqw.py?req=edit&file=faq06.017.htp">Edit this entry</A> /

7146

7147

7148

/ Last changed on Fri May 23 23:51:14 1997 by

7149

<A HREF="mailto:Vladimir.Marangozov@inrialpes.fr">Vladimir Marangozov</A>

7150

<P>

7151

7152

<HR>

7153

<H2><A NAME="6.18">6.18. Why must dictionary keys be immutable?</A></H2>

7154

The hash table implementation of dictionaries uses a hash value

7155

calculated from the key value to find the key. If the key were

7156

a mutable object, its value could change, and thus its hash could

7157

change. But since whoever changes the key object can't tell that

7158

is incorporated in a dictionary, it can't move the entry around in

7159

the dictionary. Then, when you try to look up the same object

7160

in the dictionary, it won't be found, since its hash value is different;

7161

and if you try to look up the old value, it won't be found either,

7162

since the value of the object found in that hash bin differs.

7163

<P>

7164

If you think you need to have a dictionary indexed with a list,

7165

try to use a tuple instead. The function tuple(l) creates a tuple

7166

with the same entries as the list l.

7167

<P>

7168

Some unacceptable solutions that have been proposed:

7169

<P>

7170

- Hash lists by their address (object ID). This doesn't work because

7171

if you construct a new list with the same value it won't be found;

7172

e.g.,

7173

<P>

7174

<PRE>

7175

d = {[1,2]: '12'}

7176

print d[[1,2]]

7177

</PRE>

7178

will raise a KeyError exception because the id of the [1,2] used

7179

in the second line differs from that in the first line.

7180

In other words, dictionary keys should be compared using '==', not using 'is'.

7181

<P>

7182

- Make a copy when using a list as a key. This doesn't work because

7183

the list (being a mutable object) could contain a reference to itself,

7184

and then the copying code would run into an infinite loop.

7185

<P>

7186

- Allow lists as keys but tell the user not to modify them. This would

7187

allow a class of hard-to-track bugs in programs that I'd rather not see;

7188

it invalidates an important invariant of dictionaries (every value in

7189

d.keys() is usable as a key of the dictionary).

7190

<P>

7191

- Mark lists as read-only once they are used as a dictionary key.

7192

The problem is that it's not just the top-level object that could change

7193

its value; you could use a tuple containing a list as a key. Entering

7194

anything as a key into a dictionary would require marking all objects

7195

reachable from there as read-only -- and again, self-referential objects

7196

could cause an infinite loop again (and again and again).

7197

<P>

7198

There is a trick to get around this if you need to, but

7199

use it at your own risk: You

7200

can wrap a mutable structure inside a class instance which

7201

has both a __cmp__ and a __hash__ method.

7202

<P>

7203

<PRE>

7204

class listwrapper:

7205

def __init__(self, the_list):

7206

self.the_list = the_list

7207

def __cmp__(self, other):

7208

return self.the_list == other.the_list

7209

def __hash__(self):

7210

l = self.the_list

7211

result = 98767 - len(l)*555

7212

for i in range(len(l)):

7213

try:

7214

result = result + (hash(l[i]) % 9999999) * 1001 + i

7215

except:

7216

result = (result % 7777777) + i * 333

7217

return result

7218

</PRE>

7219

Note that the hash computation is complicated by the

7220

possibility that some members of the list may be unhashable

7221

and also by the possibility of arithmetic overflow.

7222

<P>

7223

You must make

7224

sure that the hash value for all such wrapper objects that reside in a

7225

dictionary (or other hash based structure), remain fixed while

7226

the object is in the dictionary (or other structure).

7227

<P>

7228

Furthermore it must always be the case that if

7229

o1 == o2 (ie o1.__cmp__(o2)==0) then hash(o1)==hash(o2)

7230

(ie, o1.__hash__() == o2.__hash__()), regardless of whether

7231

the object is in a dictionary or not.

7232

If you fail to meet these restrictions dictionaries and other

7233

hash based structures may misbehave!

7234

<P>

7235

In the case of listwrapper above whenever the wrapper

7236

object is in a dictionary the wrapped list must not change

7237

to avoid anomalies. Don't do this unless you are prepared

7238

to think hard about the requirements and the consequences

7239

of not meeting them correctly. You've been warned!

7240

<P>

7241

7242

<A HREF="faqw.py?req=edit&file=faq06.018.htp">Edit this entry</A> /

7243

7244

7245

/ Last changed on Thu Jul 10 10:08:40 1997 by

7246

<A HREF="mailto:aaron_watters@msn.com">aaron watters</A>

7247

<P>

7248

7249

<HR>

7250

<H2><A NAME="6.19">6.19. How the heck do you make an array in Python?</A></H2>

7251

["this", 1, "is", "an", "array"]

7252

<P>

7253

Lists are arrays in the C or Pascal sense of the word (see question

7254

6.16). The array module also provides methods for creating arrays

7255

of fixed types with compact representations (but they are slower to

7256

index than lists). Also note that the Numerics extensions and others

7257

define array-like structures with various characteristics as well.

7258

<P>

7259

To get Lisp-like lists, emulate cons cells

7260

<P>

7261

<PRE>

7262

lisp_list = ("like", ("this", ("example", None) ) )

7263

</PRE>

7264

using tuples (or lists, if you want mutability). Here the analogue

7265

of lisp car is lisp_list[0] and the analogue of cdr is lisp_list[1].

7266

Only do this if you're sure you really need to (it's usually a lot

7267

slower than using Python lists).

7268

<P>

7269

Think of Python lists as mutable heterogeneous arrays of

7270

Python objects (say that 10 times fast :) ).

7271

<P>

7272

7273

<A HREF="faqw.py?req=edit&file=faq06.019.htp">Edit this entry</A> /

7274

7275

7276

/ Last changed on Wed Aug 13 07:08:27 1997 by

7277

<A HREF="mailto:aaron_watters@msn.com">aaron watters</A>

7278

<P>

7279

7280

<HR>

7281

<H2><A NAME="6.20">6.20. Why doesn't list.sort() return the sorted list?</A></H2>

7282

In situations where performance matters, making a copy of the list

7283

just to sort it would be wasteful. Therefore, list.sort() sorts

7284

the list in place. In order to remind you of that fact, it does

7285

not return the sorted list. This way, you won't be fooled into

7286

accidentally overwriting a list when you need a sorted copy but also

7287

need to keep the unsorted version around.

7288

<P>

7289

As a result, here's the idiom to iterate over the keys of a dictionary

7290

in sorted order:

7291

<P>

7292

<PRE>

7293

keys = dict.keys()

7294

keys.sort()

7295

for key in keys:

7296

...do whatever with dict[key]...

7297

</PRE>

7298

<P>

7299

7300

<A HREF="faqw.py?req=edit&file=faq06.020.htp">Edit this entry</A> /

7301

7302

7303

/ Last changed on Thu Dec 2 17:01:52 1999 by

7304

<A HREF="mailto:fdrake@acm.org">Fred L. Drake, Jr.</A>

7305

<P>

7306

7307

<HR>

7308

<H2><A NAME="6.21">6.21. How do you specify and enforce an interface spec in Python?</A></H2>

7309

An interfaces specification for a module as provided

7310

by languages such as C++ and java describes the prototypes

7311

for the methods and functions of the module. Many feel

7312

that compile time enforcement of interface specifications

7313

help aid in the construction of large programs. Python

7314

does not support interface specifications directly, but many

7315

of their advantages can be obtained by an appropriate

7316

test discipline for components, which can often be very

7317

easily accomplished in Python. There is also a tool, PyChecker,

7318

which can be used to find problems due to subclassing.

7319

<P>

7320

A good test suite for a module can at

7321

once provide a regression test and serve as a module interface

7322

specification (even better since it also gives example usage). Look to

7323

many of the standard libraries which often have a "script

7324

interpretation" which provides a simple "self test." Even

7325

modules which use complex external interfaces can often

7326

be tested in isolation using trivial "stub" emulations of the

7327

external interface.

7328

<P>

7329

An appropriate testing discipline (if enforced) can help

7330

build large complex applications in Python as well as having interface

7331

specifications would do (or better). Of course Python allows you

7332

to get sloppy and not do it. Also you might want to design

7333

your code with an eye to make it easily tested.

7334

<P>

7335

7336

<A HREF="faqw.py?req=edit&file=faq06.021.htp">Edit this entry</A> /

7337

7338

7339

/ Last changed on Thu May 23 03:05:29 2002 by

7340

<A HREF="mailto:neal@metaslash.com">Neal Norwitz</A>

7341

<P>

7342

7343

<HR>

7344

<H2><A NAME="6.22">6.22. Why do all classes have the same type? Why do instances all have the same type?</A></H2>

7345

The Pythonic use of the word "type" is quite different from

7346

common usage in much of the rest of the programming language

7347

world. A "type" in Python is a description for an object's operations

7348

as implemented in C. All classes have the same operations

7349

implemented in C which sometimes "call back" to differing program

7350

fragments implemented in Python, and hence all classes have the

7351

same type. Similarly at the C level all class instances have the

7352

same C implementation, and hence all instances have the same

7353

type.

7354

<P>

7355

Remember that in Python usage "type" refers to a C implementation

7356

of an object. To distinguish among instances of different classes

7357

use Instance.__class__, and also look to 4.47. Sorry for the

7358

terminological confusion, but at this point in Python's development

7359

nothing can be done!

7360

<P>

7361

7362

<A HREF="faqw.py?req=edit&file=faq06.022.htp">Edit this entry</A> /

7363

7364

7365

/ Last changed on Tue Jul 1 12:35:47 1997 by

7366

<A HREF="mailto:aaron_watters@msn.com">aaron watters</A>

7367

<P>

7368

7369

<HR>

7370

<H2><A NAME="6.23">6.23. Why isn't all memory freed when Python exits?</A></H2>

7371

Objects referenced from Python module global name spaces are

7372

not always deallocated when Python exits.

7373

<P>

7374

This may happen if there are circular references (see question

7375

4.17). There are also certain bits of memory that are allocated

7376

by the C library that are impossible to free (e.g. a tool

7377

like Purify will complain about these).

7378

<P>

7379

But in general, Python 1.5 and beyond

7380

(in contrast with earlier versions) is quite agressive about

7381

cleaning up memory on exit.

7382

<P>

7383

If you want to force Python to delete certain things on deallocation

7384

use the sys.exitfunc hook to force those deletions. For example

7385

if you are debugging an extension module using a memory analysis

7386

tool and you wish to make Python deallocate almost everything

7387

you might use an exitfunc like this one:

7388

<P>

7389

<PRE>

7390

import sys

7391

</PRE>

7392

<PRE>

7393

def my_exitfunc():

7394

print "cleaning up"

7395

import sys

7396

# do order dependant deletions here

7397

...

7398

# now delete everything else in arbitrary order

7399

for x in sys.modules.values():

7400

d = x.__dict__

7401

for name in d.keys():

7402

del d[name]

7403

</PRE>

7404

<PRE>

7405

sys.exitfunc = my_exitfunc

7406

</PRE>

7407

Other exitfuncs can be less drastic, of course.

7408

<P>

7409

(In fact, this one just does what Python now already does itself;

7410

but the example of using sys.exitfunc to force cleanups is still

7411

useful.)

7412

<P>

7413

7414

<A HREF="faqw.py?req=edit&file=faq06.023.htp">Edit this entry</A> /

7415

7416

7417

/ Last changed on Tue Sep 29 09:46:26 1998 by

7418

7419

<P>

7420

7421

<HR>

7422

<H2><A NAME="6.24">6.24. Why no class methods or mutable class variables?</A></H2>

7423

The notation

7424

<P>

7425

<PRE>

7426

instance.attribute(arg1, arg2)

7427

</PRE>

7428

usually translates to the equivalent of

7429

<P>

7430

<PRE>

7431

Class.attribute(instance, arg1, arg2)

7432

</PRE>

7433

where Class is a (super)class of instance. Similarly

7434

<P>

7435

<PRE>

7436

instance.attribute = value

7437

</PRE>

7438

sets an attribute of an instance (overriding any attribute of a class

7439

that instance inherits).

7440

<P>

7441

Sometimes programmers want to have

7442

different behaviours -- they want a method which does not bind

7443

to the instance and a class attribute which changes in place.

7444

Python does not preclude these behaviours, but you have to

7445

adopt a convention to implement them. One way to accomplish

7446

this is to use "list wrappers" and global functions.

7447

<P>

7448

<PRE>

7449

def C_hello():

7450

print "hello"

7451

</PRE>

7452

<PRE>

7453

class C:

7454

hello = [C_hello]

7455

counter = [0]

7456

</PRE>

7457

<PRE>

7458

I = C()

7459

</PRE>

7460

Here I.hello[0]() acts very much like a "class method" and

7461

I.counter[0] = 2 alters C.counter (and doesn't override it).

7462

If you don't understand why you'd ever want to do this, that's

7463

because you are pure of mind, and you probably never will

7464

want to do it! This is dangerous trickery, not recommended

7465

when avoidable. (Inspired by Tim Peter's discussion.)

7466

<P>

7467

In Python 2.2, you can do this using the new built-in operations

7468

classmethod and staticmethod.

7469

See <A HREF="http://www.python.org/2.2/descrintro.html#staticmethods">http://www.python.org/2.2/descrintro.html#staticmethods</A>

7470

<P>

7471

7472

<A HREF="faqw.py?req=edit&file=faq06.024.htp">Edit this entry</A> /

7473

7474

7475

/ Last changed on Tue Sep 11 15:59:37 2001 by

7476

7477

<P>

7478

7479

<HR>

7480

<H2><A NAME="6.25">6.25. Why are default values sometimes shared between objects?</A></H2>

7481

It is often expected that a function CALL creates new objects for default

7482

values. This is not what happens. Default values are created when the

7483

function is DEFINED, that is, there is only one such object that all

7484

functions refer to. If that object is changed, subsequent calls to the

7485

function will refer to this changed object. By definition, immutable objects

7486

(like numbers, strings, tuples, None) are safe from change. Changes to mutable

7487

objects (like dictionaries, lists, class instances) is what causes the

7488

confusion.

7489

<P>

7490

Because of this feature it is good programming practice not to use mutable

7491

objects as default values, but to introduce them in the function.

7492

Don't write:

7493

<P>

7494

<PRE>

7495

def foo(dict={}): # XXX shared reference to one dict for all calls

7496

...

7497

</PRE>

7498

but:

7499

<PRE>

7500

def foo(dict=None):

7501

if dict is None:

7502

dict = {} # create a new dict for local namespace

7503

</PRE>

7504

See page 182 of "Internet Programming with Python" for one discussion

7505

of this feature. Or see the top of page 144 or bottom of page 277 in

7506

"Programming Python" for another discussion.

7507

<P>

7508

7509

<A HREF="faqw.py?req=edit&file=faq06.025.htp">Edit this entry</A> /

7510

7511

7512

/ Last changed on Sat Aug 16 07:03:35 1997 by

7513

<A HREF="mailto:cjr@bound.xs4all.nl">Case Roole</A>

7514

<P>

7515

7516

<HR>

7517

7518

Actually, you can use exceptions to provide a "structured goto"

7519

that even works across function calls. Many feel that exceptions

7520

can conveniently emulate all reasonable uses of the "go" or "goto"

7521

constructs of C, Fortran, and other languages. For example:

7522

<P>

7523

<PRE>

7524

class label: pass # declare a label

7525

try:

7526

...

7527

if (condition): raise label() # goto label

7528

...

7529

except label: # where to goto

7530

pass

7531

...

7532

</PRE>

7533

This doesn't allow you to jump into the middle of a loop, but

7534

that's usually considered an abuse of goto anyway. Use sparingly.

7535

<P>

7536

7537

<A HREF="faqw.py?req=edit&file=faq06.026.htp">Edit this entry</A> /

7538

7539

7540

/ Last changed on Wed Sep 10 07:16:44 1997 by

7541

<A HREF="mailto:aaron_watters@msn.com">aaron watters</A>

7542

<P>

7543

7544

<HR>

7545

<H2><A NAME="6.27">6.27. How do you make a higher order function in Python?</A></H2>

7546

You have two choices: you can use default arguments and override

7547

them or you can use "callable objects." For example suppose you

7548

wanted to define linear(a,b) which returns a function f where f(x)

7549

computes the value a*x+b. Using default arguments:

7550

<P>

7551

<PRE>

7552

def linear(a,b):

7553

def result(x, a=a, b=b):

7554

return a*x + b

7555

return result

7556

</PRE>

7557

Or using callable objects:

7558

<P>

7559

<PRE>

7560

class linear:

7561

def __init__(self, a, b):

7562

self.a, self.b = a,b

7563

def __call__(self, x):

7564

return self.a * x + self.b

7565

</PRE>

7566

In both cases:

7567

<P>

7568

<PRE>

7569

taxes = linear(0.3,2)

7570

</PRE>

7571

gives a callable object where taxes(10e6) == 0.3 * 10e6 + 2.

7572

<P>

7573

The defaults strategy has the disadvantage that the default arguments

7574

could be accidentally or maliciously overridden. The callable objects

7575

approach has the disadvantage that it is a bit slower and a bit

7576

longer. Note however that a collection of callables can share

7577

their signature via inheritance. EG

7578

<P>

7579

<PRE>

7580

class exponential(linear):

7581

# __init__ inherited

7582

def __call__(self, x):

7583

return self.a * (x ** self.b)

7584

</PRE>

7585

On comp.lang.python, <A HREF="mailto:zenin@bawdycaste.org">zenin@bawdycaste.org</A> points out that

7586

an object can encapsulate state for several methods in order

7587

to emulate the "closure" concept from functional programming

7588

languages, for example:

7589

<P>

7590

<PRE>

7591

class counter:

7592

value = 0

7593

def set(self, x): self.value = x

7594

def up(self): self.value=self.value+1

7595

def down(self): self.value=self.value-1

7596

</PRE>

7597

<PRE>

7598

count = counter()

7599

inc, dec, reset = count.up, count.down, count.set

7600

</PRE>

7601

Here inc, dec and reset act like "functions which share the

7602

same closure containing the variable count.value" (if you

7603

like that way of thinking).

7604

<P>

7605

7606

<A HREF="faqw.py?req=edit&file=faq06.027.htp">Edit this entry</A> /

7607

7608

7609

/ Last changed on Fri Sep 25 08:38:35 1998 by

7610

<A HREF="mailto:arw@pythonpros.com">Aaron Watters</A>

7611

<P>

7612

7613

<HR>

7614

<H2><A NAME="6.28">6.28. Why do I get a SyntaxError for a 'continue' inside a 'try'?</A></H2>

7615

This is an implementation limitation,

7616

caused by the extremely simple-minded

7617

way Python generates bytecode. The try block pushes something on the

7618

"block stack" which the continue would have to pop off again. The

7619

current code generator doesn't have the data structures around so that

7620

'continue' can generate the right code.

7621

<P>

7622

Note that JPython doesn't have this restriction!

7623

<P>

7624

7625

<A HREF="faqw.py?req=edit&file=faq06.028.htp">Edit this entry</A> /

7626

7627

7628

/ Last changed on Fri May 22 15:01:07 1998 by

7629

7630

<P>

7631

7632

<HR>

7633

<H2><A NAME="6.29">6.29. Why can't raw strings (r-strings) end with a backslash?</A></H2>

7634

More precisely, they can't end with an odd number of backslashes:

7635

the unpaired backslash at the end escapes the closing quote character,

7636

leaving an unterminated string.

7637

<P>

7638

Raw strings were designed to ease creating input for processors (chiefly

7639

regular expression engines) that want to do their own backslash escape processing. Such processors consider an unmatched trailing backslash to be an error anyway, so raw strings disallow that. In return, they allow you to pass on the string quote character by escaping it with a backslash. These rules work well when r-strings are used for their intended purpose.

7640

<P>

7641

If you're trying to build Windows pathnames, note that all Windows system calls accept forward slashes too:

7642

<P>

7643

<PRE>

7644

f = open("/mydir/file.txt") # works fine!

7645

</PRE>

7646

If you're trying to build a pathname for a DOS command, try e.g. one of

7647

<P>

7648

<PRE>

7649

dir = r"\this\is\my\dos\dir" "\\"

7650

dir = r"\this\is\my\dos\dir\ "[:-1]

7651

dir = "\\this\\is\\my\\dos\\dir\\"

7652

</PRE>

7653

<P>

7654

7655

<A HREF="faqw.py?req=edit&file=faq06.029.htp">Edit this entry</A> /

7656

7657

7658

/ Last changed on Mon Jul 13 20:50:20 1998 by

7659

<A HREF="mailto:tim_one@email.msn.com">Tim Peters</A>

7660

<P>

7661

7662

<HR>

7663

<H2><A NAME="6.30">6.30. Why can't I use an assignment in an expression?</A></H2>

7664

Many people used to C or Perl complain that they want to be able to

7665

use e.g. this C idiom:

7666

<P>

7667

<PRE>

7668

while (line = readline(f)) {

7669

...do something with line...

7670

}

7671

</PRE>

7672

where in Python you're forced to write this:

7673

<P>

7674

<PRE>

7675

while 1:

7676

line = f.readline()

7677

if not line:

7678

break

7679

...do something with line...

7680

</PRE>

7681

This issue comes up in the Python newsgroup with alarming frequency

7682

-- search Deja News for past messages about assignment expression.

7683

The reason for not allowing assignment in Python expressions

7684

is a common, hard-to-find bug in those other languages,

7685

caused by this construct:

7686

<P>

7687

<PRE>

7688

if (x = 0) {

7689

...error handling...

7690

}

7691

else {

7692

...code that only works for nonzero x...

7693

}

7694

</PRE>

7695

Many alternatives have been proposed. Most are hacks that save some

7696

typing but use arbitrary or cryptic syntax or keywords,

7697

and fail the simple criterion that I use for language change proposals:

7698

it should intuitively suggest the proper meaning to a human reader

7699

who has not yet been introduced with the construct.

7700

<P>

7701

The earliest time something can be done about this will be with

7702

Python 2.0 -- if it is decided that it is worth fixing.

7703

An interesting phenomenon is that most experienced Python programmers

7704

recognize the "while 1" idiom and don't seem to be missing the

7705

assignment in expression construct much; it's only the newcomers

7706

who express a strong desire to add this to the language.

7707

<P>

7708

One fairly elegant solution would be to introduce a new operator

7709

for assignment in expressions spelled ":=" -- this avoids the "="

7710

instead of "==" problem. It would have the same precedence

7711

as comparison operators but the parser would flag combination with

7712

other comparisons (without disambiguating parentheses) as an error.

7713

<P>

7714

Finally -- there's an alternative way of spelling this that seems

7715

attractive but is generally less robust than the "while 1" solution:

7716

<P>

7717

<PRE>

7718

line = f.readline()

7719

while line:

7720

...do something with line...

7721

line = f.readline()

7722

</PRE>

7723

The problem with this is that if you change your mind about exactly

7724

how you get the next line (e.g. you want to change it into

7725

sys.stdin.readline()) you have to remember to change two places

7726

in your program -- the second one hidden at the bottom of the loop.

7727

<P>

7728

7729

<A HREF="faqw.py?req=edit&file=faq06.030.htp">Edit this entry</A> /

7730

7731

7732

/ Last changed on Tue May 18 00:57:41 1999 by

7733

<A HREF="mailto:dalke@bioreason.com">Andrew Dalke</A>

7734

<P>

7735

7736

<HR>

7737

<H2><A NAME="6.31">6.31. Why doesn't Python have a "with" statement like some other languages?</A></H2>

7738

Basically, because such a construct would be terribly ambiguous. Thanks to Carlos Ribeiro for the following remarks:

7739

<P>

7740

Some languages, such as Object Pascal, Delphi, and C++, use static types. So it is possible to know, in an unambiguous way, what member is being assigned in a "with" clause. This is the main point - the compiler <I>always</I> knows the scope of every variable at compile time.

7741

<P>

7742

Python uses dynamic types. It is impossible to know in advance which

7743

attribute will be referenced at runtime. Member attributes may be added or removed from objects on the fly. This would make it impossible to know, from a simple reading, what attribute is being referenced - a local one, a global one, or a member attribute.

7744

<P>

7745

For instance, take the following snippet (it is incomplete btw, just to

7746

give you the idea):

7747

<P>

7748

<PRE>

7749

def with_is_broken(a):

7750

with a:

7751

print x

7752

</PRE>

7753

The snippet assumes that "a" must have a member attribute called "x".

7754

However, there is nothing in Python that guarantees that. What should

7755

happen if "a" is, let us say, an integer? And if I have a global variable named "x", will it end up being used inside the with block? As you see, the dynamic nature of Python makes such choices much harder.

7756

<P>

7757

The primary benefit of "with" and similar language features (reduction of code volume) can, however, easily be achieved in Python by assignment. Instead of:

7758

<P>

7759

<PRE>

7760

function(args).dict[index][index].a = 21

7761

function(args).dict[index][index].b = 42

7762

function(args).dict[index][index].c = 63

7763

</PRE>

7764

would become:

7765

<P>

7766

<PRE>

7767

ref = function(args).dict[index][index]

7768

ref.a = 21

7769

ref.b = 42

7770

ref.c = 63

7771

</PRE>

7772

This also has the happy side-effect of increasing execution speed, since name bindings are resolved at run-time in Python, and the second method only needs to perform the resolution once. If the referenced object does not have a, b and c attributes, of course, the end result is still a run-time exception.

7773

<P>

7774

7775

<A HREF="faqw.py?req=edit&file=faq06.031.htp">Edit this entry</A> /

7776

7777

7778

/ Last changed on Fri Jan 11 14:32:58 2002 by

7779

<A HREF="mailto:sholden@holdenweb.com">Steve Holden</A>

7780

<P>

7781

7782

<HR>

7783

<H2><A NAME="6.32">6.32. Why are colons required for if/while/def/class?</A></H2>

7784

The colon is required primarily to enhance readability (one of the

7785

results of the experimental ABC language). Consider this:

7786

<P>

7787

<PRE>

7788

if a==b

7789

print a

7790

</PRE>

7791

versus

7792

<P>

7793

<PRE>

7794

if a==b:

7795

print a

7796

</PRE>

7797

Notice how the second one is slightly easier to read. Notice further how

7798

a colon sets off the example in the second line of this FAQ answer; it's

7799

a standard usage in English. Finally, the colon makes it easier for

7800

editors with syntax highlighting.

7801

<P>

7802

7803

<A HREF="faqw.py?req=edit&file=faq06.032.htp">Edit this entry</A> /

7804

7805

7806

/ Last changed on Mon Jun 3 07:22:57 2002 by

7807

<A HREF="mailto:smurf@noris.de">Matthias Urlichs</A>

7808

<P>

7809

7810

<HR>

7811

<H2><A NAME="6.33">6.33. Can't we get rid of the Global Interpreter Lock?</A></H2>

7812

The Global Interpreter Lock (GIL) is often seen as a hindrance to

7813

Python's deployment on high-end multiprocessor server machines,

7814

because a multi-threaded Python program effectively only uses

7815

one CPU, due to the insistence that (almost) all Python code

7816

can only run while the GIL is held.

7817

<P>

7818

Back in the days of Python 1.5, Greg Stein actually implemented

7819

a comprehensive patch set ("free threading")

7820

that removed the GIL, replacing it with

7821

fine-grained locking. Unfortunately, even on Windows (where locks

7822

are very efficient) this ran ordinary Python code about twice as

7823

slow as the interpreter using the GIL. On Linux the performance

7824

loss was even worse (pthread locks aren't as efficient).

7825

<P>

7826

Since then, the idea of getting rid of the GIL has occasionally

7827

come up but nobody has found a way to deal with the expected slowdown;

7828

Greg's free threading patch set has not been kept up-to-date for

7829

later Python versions.

7830

<P>

7831

This doesn't mean that you can't make good use of Python on

7832

multi-CPU machines! You just have to be creative with dividing

7833

the work up between multiple <I>processes</I> rather than multiple

7834

<I>threads</I>.

7835

<P>

7836

<P>

7837

It has been suggested that the GIL should be a per-interpreter-state

7838

lock rather than truly global; interpreters then wouldn't be able

7839

to share objects. Unfortunately, this isn't likely to happen either.

7840

<P>

7841

It would be a tremendous amount of work, because many object

7842

implementations currently have global state. E.g. small ints and

7843

small strings are cached; these caches would have to be moved to the

7844

interpreter state. Other object types have their own free list; these

7845

free lists would have to be moved to the interpreter state. And so

7846

on.

7847

<P>

7848

And I doubt that it can even be done in finite time, because the same

7849

problem exists for 3rd party extensions. It is likely that 3rd party

7850

extensions are being written at a faster rate than you can convert

7851

them to store all their global state in the interpreter state.

7852

<P>

7853

And finally, once you have multiple interpreters not sharing any

7854

state, what have you gained over running each interpreter

7855

in a separate process?

7856

<P>

7857

7858

<A HREF="faqw.py?req=edit&file=faq06.033.htp">Edit this entry</A> /

7859

7860

7861

/ Last changed on Fri Feb 7 16:34:01 2003 by

7862

7863

<P>

7864

7865

<HR>

7866

<H1>7. Using Python on non-UNIX platforms</H1>

7867

7868

<HR>

7869

<H2><A NAME="7.1">7.1. Is there a Mac version of Python?</A></H2>

7870

Yes, it is maintained by Jack Jansen. See Jack's MacPython Page:

7871

<P>

7872

<PRE>

7873

<A HREF="http://www.cwi.nl/~jack/macpython.html">http://www.cwi.nl/~jack/macpython.html</A>

7874

</PRE>

7875

<P>

7876

7877

<A HREF="faqw.py?req=edit&file=faq07.001.htp">Edit this entry</A> /

7878

7879

7880

/ Last changed on Fri May 4 09:33:42 2001 by

7881

7882

<P>

7883

7884

<HR>

7885

<H2><A NAME="7.2">7.2. Are there DOS and Windows versions of Python?</A></H2>

7886

Yes. The core windows binaries are available from <A HREF="http://www.python.org/windows">http://www.python.org/windows</A>/. There is a plethora of Windows extensions available, including a large number of not-always-compatible GUI toolkits. The core binaries include the standard Tkinter GUI extension.

7887

<P>

7888

Most windows extensions can be found (or referenced) at <A HREF="http://www.python.org/windows">http://www.python.org/windows</A>/

7889

<P>

7890

Windows 3.1/DOS support seems to have dropped off recently. You may need to settle for an old version of Python one these platforms. One such port is WPY

7891

<P>

7892

WPY: Ports to DOS, Windows 3.1(1), Windows 95, Windows NT and OS/2.

7893

Also contains a GUI package that offers portability between Windows

7894

(not DOS) and Unix, and native look and feel on both.

7895

<A HREF="ftp://ftp.python.org/pub/python/wpy">ftp://ftp.python.org/pub/python/wpy</A>/.

7896

<P>

7897

7898

<A HREF="faqw.py?req=edit&file=faq07.002.htp">Edit this entry</A> /

7899

7900

7901

/ Last changed on Tue Jun 2 20:21:57 1998 by

7902

<A HREF="mailto:MHammond@skippinet.com.au">Mark Hammond</A>

7903

<P>

7904

7905

<HR>

7906

<H2><A NAME="7.3">7.3. Is there an OS/2 version of Python?</A></H2>

7907

Yes, see <A HREF="http://www.python.org/download/download_os2.html">http://www.python.org/download/download_os2.html</A>.

7908

<P>

7909

7910

<A HREF="faqw.py?req=edit&file=faq07.003.htp">Edit this entry</A> /

7911

7912

7913

/ Last changed on Tue Sep 7 11:33:16 1999 by

7914

7915

<P>

7916

7917

<HR>

7918

<H2><A NAME="7.4">7.4. Is there a VMS version of Python?</A></H2>

7919

Jean-Fran�ois Pi�ronne has ported 2.1.3 to OpenVMS. It can be found at

7920

<<A HREF="http://vmspython.dyndns.org">http://vmspython.dyndns.org</A>/>.

7921

<P>

7922

7923

<A HREF="faqw.py?req=edit&file=faq07.004.htp">Edit this entry</A> /

7924

7925

7926

/ Last changed on Thu Sep 19 15:40:38 2002 by

7927

<A HREF="mailto:skip@pobox.com">Skip Montanaro</A>

7928

<P>

7929

7930

<HR>

7931

<H2><A NAME="7.5">7.5. What about IBM mainframes, or other non-UNIX platforms?</A></H2>

7932

I haven't heard about these, except I remember hearing about an

7933

OS/9 port and a port to Vxworks (both operating systems for embedded

7934

systems). If you're interested in any of this, go directly to the

7935

newsgroup and ask there, you may find exactly what you need. For

7936

example, a port to MPE/iX 5.0 on HP3000 computers was just announced,

7937

see <A HREF="http://www.allegro.com/software">http://www.allegro.com/software</A>/.

7938

<P>

7939

On the IBM mainframe side, for Z/OS there's a port of python 1.4 that goes with their open-unix package, formely OpenEdition MVS, (<A HREF="http://www-1.ibm.com/servers/eserver/zseries/zos/unix/python.html">http://www-1.ibm.com/servers/eserver/zseries/zos/unix/python.html</A>). On a side note, there's also a java vm ported - so, in theory, jython could run too.

7940

<P>

7941

7942

<A HREF="faqw.py?req=edit&file=faq07.005.htp">Edit this entry</A> /

7943

7944

7945

/ Last changed on Mon Nov 18 03:18:39 2002 by

7946

<A HREF="mailto:bjessen@uol.com.br">Bruno Jessen</A>

7947

<P>

7948

7949

<HR>

7950

<H2><A NAME="7.6">7.6. Where are the source or Makefiles for the non-UNIX versions?</A></H2>

7951

The standard sources can (almost) be used. Additional sources can

7952

be found in the platform-specific subdirectories of the distribution.

7953

<P>

7954

7955

<A HREF="faqw.py?req=edit&file=faq07.006.htp">Edit this entry</A> /

7956

7957

<P>

7958

7959

<HR>

7960

<H2><A NAME="7.7">7.7. What is the status and support for the non-UNIX versions?</A></H2>

7961

I don't have access to most of these platforms, so in general I am

7962

dependent on material submitted by volunteers. However I strive to

7963

integrate all changes needed to get it to compile on a particular

7964

platform back into the standard sources, so porting of the next

7965

version to the various non-UNIX platforms should be easy.

7966

(Note that Linux is classified as a UNIX platform here. :-)

7967

<P>

7968

Some specific platforms:

7969

<P>

7970

Windows: all versions (95, 98, ME, NT, 2000, XP) are supported,

7971

all python.org releases come with a Windows installer.

7972

<P>

7973

MacOS: Jack Jansen does an admirable job of keeping the Mac version

7974

up to date (both MacOS X and older versions);

7975

see <A HREF="http://www.cwi.nl/~jack/macpython.html">http://www.cwi.nl/~jack/macpython.html</A>

7976

<P>

7977

For all supported platforms, see <A HREF="http://www.python.org/download">http://www.python.org/download</A>/

7978

(follow the link to "Other platforms" for less common platforms)

7979

<P>

7980

7981

<A HREF="faqw.py?req=edit&file=faq07.007.htp">Edit this entry</A> /

7982

7983

7984

/ Last changed on Fri May 24 21:34:24 2002 by

7985

7986

<P>

7987

7988

<HR>

7989

<H2><A NAME="7.8">7.8. I have a PC version but it appears to be only a binary. Where's the library?</A></H2>

7990

If you are running any version of Windows, then you have the wrong distribution. The FAQ lists current Windows versions. Notably, Pythonwin and wpy provide fully functional installations.

7991

<P>

7992

But if you are sure you have the only distribution with a hope of working on

7993

your system, then...

7994

<P>

7995

You still need to copy the files from the distribution directory

7996

"python/Lib" to your system. If you don't have the full distribution,

7997

you can get the file lib<version>.tar.gz from most ftp sites carrying

7998

Python; this is a subset of the distribution containing just those

7999

files, e.g. <A HREF="ftp://ftp.python.org/pub/python/src/lib1.4.tar.gz">ftp://ftp.python.org/pub/python/src/lib1.4.tar.gz</A>.

8000

<P>

8001

Once you have installed the library, you need to point sys.path to it.

8002

Assuming the library is in C:\misc\python\lib, the following commands

8003

will point your Python interpreter to it (note the doubled backslashes

8004

-- you can also use single forward slashes instead):

8005

<P>

8006

<PRE>

8007

>>> import sys

8008

>>> sys.path.insert(0, 'C:\\misc\\python\\lib')

8009

>>>

8010

</PRE>

8011

For a more permanent effect, set the environment variable PYTHONPATH,

8012

as follows (talking to a DOS prompt):

8013

<P>

8014

<PRE>

8015

C> SET PYTHONPATH=C:\misc\python\lib

8016

</PRE>

8017

<P>

8018

8019

<A HREF="faqw.py?req=edit&file=faq07.008.htp">Edit this entry</A> /

8020

8021

8022

/ Last changed on Fri May 23 16:28:27 1997 by

8023

<A HREF="mailto:klm@python.org">Ken Manheimer</A>

8024

<P>

8025

8026

<HR>

8027

<H2><A NAME="7.9">7.9. Where's the documentation for the Mac or PC version?</A></H2>

8028

The documentation for the Unix version also applies to the Mac and

8029

PC versions. Where applicable, differences are indicated in the text.

8030

<P>

8031

8032

<A HREF="faqw.py?req=edit&file=faq07.009.htp">Edit this entry</A> /

8033

8034

<P>

8035

8036

<HR>

8037

<H2><A NAME="7.10">7.10. How do I create a Python program file on the Mac or PC?</A></H2>

8038

Use an external editor. On the Mac, BBEdit seems to be a popular

8039

no-frills text editor. I work like this: start the interpreter; edit

8040

a module file using BBedit; import and test it in the interpreter;

8041

edit again in BBedit; then use the built-in function reload() to

8042

re-read the imported module; etc. In the 1.4 distribution

8043

you will find a BBEdit extension that makes life a little easier:

8044

it can tell the interpreter to execute the current window.

8045

See :Mac:Tools:BBPy:README.

8046

<P>

8047

Regarding the same question for the PC, Kurt Wm. Hemr writes: "While

8048

anyone with a pulse could certainly figure out how to do the same on

8049

MS-Windows, I would recommend the NotGNU Emacs clone for MS-Windows.

8050

Not only can you easily resave and "reload()" from Python after making

8051

changes, but since WinNot auto-copies to the clipboard any text you

8052

select, you can simply select the entire procedure (function) which

8053

you changed in WinNot, switch to QWPython, and shift-ins to reenter

8054

the changed program unit."

8055

<P>

8056

If you're using Windows95 or Windows NT, you should also know about

8057

PythonWin, which provides a GUI framework, with an mouse-driven

8058

editor, an object browser, and a GUI-based debugger. See

8059

<PRE>

8060

<A HREF="http://www.python.org/ftp/python/pythonwin">http://www.python.org/ftp/python/pythonwin</A>/

8061

</PRE>

8062

for details.

8063

<P>

8064

8065

<A HREF="faqw.py?req=edit&file=faq07.010.htp">Edit this entry</A> /

8066

8067

8068

/ Last changed on Sun May 25 10:04:25 1997 by

8069

8070

<P>

8071

8072

<HR>

8073

<H2><A NAME="7.11">7.11. How can I use Tkinter on Windows 95/NT?</A></H2>

8074

Starting from Python 1.5, it's very easy -- just download and install

8075

Python and Tcl/Tk and you're in business. See

8076

<P>

8077

<PRE>

8078

<A HREF="http://www.python.org/download/download_windows.html">http://www.python.org/download/download_windows.html</A>

8079

</PRE>

8080

One warning: don't attempt to use Tkinter from PythonWin

8081

(Mark Hammond's IDE). Use it from the command line interface

8082

(python.exe) or the windowless interpreter (pythonw.exe).

8083

<P>

8084

8085

<A HREF="faqw.py?req=edit&file=faq07.011.htp">Edit this entry</A> /

8086

8087

8088

/ Last changed on Fri Jun 12 09:32:48 1998 by

8089

8090

<P>

8091

8092

<HR>

8093

<H2><A NAME="7.12">7.12. cgi.py (or other CGI programming) doesn't work sometimes on NT or win95!</A></H2>

8094

Be sure you have the latest python.exe, that you are using

8095

python.exe rather than a GUI version of python and that you

8096

have configured the server to execute

8097

<P>

8098

<PRE>

8099

"...\python.exe -u ..."

8100

</PRE>

8101

for the cgi execution. The -u (unbuffered) option on NT and

8102

win95 prevents the interpreter from altering newlines in the

8103

standard input and output. Without it post/multipart requests

8104

will seem to have the wrong length and binary (eg, GIF)

8105

responses may get garbled (resulting in, eg, a "broken image").

8106

<P>

8107

8108

<A HREF="faqw.py?req=edit&file=faq07.012.htp">Edit this entry</A> /

8109

8110

8111

/ Last changed on Wed Jul 30 10:48:02 1997 by

8112

<A HREF="mailto:aaron_watters@msn.com">aaron watters</A>

8113

<P>

8114

8115

<HR>

8116

<H2><A NAME="7.13">7.13. Why doesn't os.popen() work in PythonWin on NT?</A></H2>

8117

The reason that os.popen() doesn't work from within PythonWin is due to a bug in Microsoft's C Runtime Library (CRT). The CRT assumes you have a Win32 console attached to the process.

8118

<P>

8119

You should use the win32pipe module's popen() instead which doesn't depend on having an attached Win32 console.

8120

<P>

8121

Example:

8122

<PRE>

8123

import win32pipe

8124

f = win32pipe.popen('dir /c c:\\')

8125

print f.readlines()

8126

f.close()

8127

</PRE>

8128

<P>

8129

8130

<A HREF="faqw.py?req=edit&file=faq07.013.htp">Edit this entry</A> /

8131

8132

8133

/ Last changed on Thu Jul 31 15:34:09 1997 by

8134

8135

<P>

8136

8137

<HR>

8138

<H2><A NAME="7.14">7.14. How do I use different functionality on different platforms with the same program?</A></H2>

8139

Remember that Python is extremely dynamic and that you

8140

can use this dynamism to configure a program at run-time to

8141

use available functionality on different platforms. For example

8142

you can test the sys.platform and import different modules based

8143

on its value.

8144

<P>

8145

<PRE>

8146

import sys

8147

if sys.platform == "win32":

8148

import win32pipe

8149

popen = win32pipe.popen

8150

else:

8151

import os

8152

popen = os.popen

8153

</PRE>

8154

(See FAQ 7.13 for an explanation of why you might want to

8155

do something like this.) Also you can try to import a module

8156

and use a fallback if the import fails:

8157

<P>

8158

<PRE>

8159

try:

8160

import really_fast_implementation

8161

choice = really_fast_implementation

8162

except ImportError:

8163

import slower_implementation

8164

choice = slower_implementation

8165

</PRE>

8166

<P>

8167

8168

<A HREF="faqw.py?req=edit&file=faq07.014.htp">Edit this entry</A> /

8169

8170

8171

/ Last changed on Wed Aug 13 07:39:06 1997 by

8172

<A HREF="mailto:aaron_watters@msn.com">aaron watters</A>

8173

<P>

8174

8175

<HR>

8176

<H2><A NAME="7.15">7.15. Is there an Amiga version of Python?</A></H2>

8177

Yes. See the AmigaPython homepage at <A HREF="http://www.bigfoot.com/~irmen/python.html">http://www.bigfoot.com/~irmen/python.html</A>.

8178

<P>

8179

8180

<A HREF="faqw.py?req=edit&file=faq07.015.htp">Edit this entry</A> /

8181

8182

8183

/ Last changed on Mon Dec 14 06:53:32 1998 by

8184

<A HREF="mailto:irmen@bigfoot.com">Irmen de Jong</A>

8185

<P>

8186

8187

<HR>

8188

<H2><A NAME="7.16">7.16. Why doesn't os.popen()/win32pipe.popen() work on Win9x?</A></H2>

8189

There is a bug in Win9x that prevents os.popen/win32pipe.popen* from working. The good news is there is a way to work around this problem.

8190

The Microsoft Knowledge Base article that you need to lookup is: Q150956. You will find links to the knowledge base at:

8191

<A HREF="http://www.microsoft.com/kb">http://www.microsoft.com/kb</A>.

8192

<P>

8193

8194

<A HREF="faqw.py?req=edit&file=faq07.016.htp">Edit this entry</A> /

8195

8196

8197

/ Last changed on Fri Jun 25 10:45:38 1999 by

8198

8199

<P>

8200

8201

<HR>

8202

<H1>8. Python on Windows</H1>

8203

8204

<HR>

8205

<H2><A NAME="8.1">8.1. Using Python for CGI on Microsoft Windows</A></H2>

8206

** Setting up the Microsoft IIS Server/Peer Server

8207

<P>

8208

On the Microsoft IIS

8209

server or on the Win95 MS Personal Web Server

8210

you set up python in the same way that you

8211

would set up any other scripting engine.

8212

<P>

8213

Run regedt32 and go to:

8214

<P>

8215

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\W3SVC\Parameters\ScriptMap

8216

<P>

8217

and enter the following line (making any specific changes that your system may need)

8218

<P>

8219

.py :REG_SZ: c:\<path to python>\python.exe -u %s %s

8220

<P>

8221

This line will allow you to call your script with a simple reference like:

8222

http://yourserver/scripts/yourscript.py

8223

provided "scripts" is an "executable" directory for your server (which

8224

it usually is by default).

8225

The "-u" flag specifies unbuffered and binary mode for stdin - needed when working with binary data

8226

<P>

8227

In addition, it is recommended by people who would know that using ".py" may

8228

not be a good idea for the file extensions when used in this context

8229

(you might want to reserve *.py for support modules and use *.cgi or *.cgp

8230

for "main program" scripts).

8231

However, that issue is beyond this Windows FAQ entry.

8232

<P>

8233

<P>

8234

** Apache configuration

8235

<P>

8236

In the Apache configuration file httpd.conf, add the following line at

8237

the end of the file:

8238

<P>

8239

ScriptInterpreterSource Registry

8240

<P>

8241

Then, give your Python CGI-scripts the extension .py and put them in the cgi-bin directory.

8242

<P>

8243

<P>

8244

** Netscape Servers:

8245

Information on this topic exists at:

8246

<A HREF="http://home.netscape.com/comprod/server_central/support/fasttrack_man/programs.htm#1010870">http://home.netscape.com/comprod/server_central/support/fasttrack_man/programs.htm#1010870</A>

8247

<P>

8248

8249

<A HREF="faqw.py?req=edit&file=faq08.001.htp">Edit this entry</A> /

8250

8251

8252

/ Last changed on Wed Mar 27 12:25:54 2002 by

8253

<A HREF="mailto:gerhard@bigfoot.de">Gerhard H�ring</A>

8254

<P>

8255

8256

<HR>

8257

<H2><A NAME="8.2">8.2. How to check for a keypress without blocking?</A></H2>

8258

Use the msvcrt module. This is a standard Windows-specific extensions

8259

in Python 1.5 and beyond. It defines a function kbhit() which checks

8260

whether a keyboard hit is present; also getch() which gets one

8261

character without echo. Plus a few other goodies.

8262

<P>

8263

(Search for "keypress" to find an answer for Unix as well.)

8264

<P>

8265

8266

<A HREF="faqw.py?req=edit&file=faq08.002.htp">Edit this entry</A> /

8267

8268

8269

/ Last changed on Mon Mar 30 16:21:46 1998 by

8270

8271

<P>

8272

8273

<HR>

8274

<H2><A NAME="8.3">8.3. $PYTHONPATH</A></H2>

8275

In MS-DOS derived environments, a unix variable such as $PYTHONPATH is

8276

set as PYTHONPATH, without the dollar sign. PYTHONPATH is useful for

8277

specifying the location of library files.

8278

<P>

8279

8280

<A HREF="faqw.py?req=edit&file=faq08.003.htp">Edit this entry</A> /

8281

8282

8283

/ Last changed on Thu Jun 11 00:41:26 1998 by

8284

8285

<P>

8286

8287

<HR>

8288

<H2><A NAME="8.4">8.4. dedent syntax errors</A></H2>

8289

The FAQ does not recommend using tabs, and Guido's Python Style Guide recommends 4 spaces for distributed Python code; this is also the Emacs python-mode default; see

8290

<P>

8291

<PRE>

8292

<A HREF="http://www.python.org/doc/essays/styleguide.html">http://www.python.org/doc/essays/styleguide.html</A>

8293

</PRE>

8294

Under any editor mixing tabs and spaces is a bad idea. MSVC is no different in this respect, and is easily configured to use spaces: Take Tools -> Options -> Tabs, and for file type "Default" set "Tab size" and "Indent size" to 4, and select the "Insert spaces" radio button.

8295

<P>

8296

If you suspect mixed tabs and spaces are causing problems in leading whitespace, run Python with the -t switch or, run Tools/Scripts/tabnanny.py to check a directory tree in batch mode.

8297

<P>

8298

8299

<A HREF="faqw.py?req=edit&file=faq08.004.htp">Edit this entry</A> /

8300

8301

8302

/ Last changed on Mon Feb 12 15:04:14 2001 by

8303

<A HREF="mailto:sholden@holdenweb.com">Steve Holden</A>

8304

<P>

8305

8306

<HR>

8307

<H2><A NAME="8.5">8.5. How do I emulate os.kill() in Windows?</A></H2>

8308

Use win32api:

8309

<P>

8310

<PRE>

8311

def kill(pid):

8312

"""kill function for Win32"""

8313

import win32api

8314

handle = win32api.OpenProcess(1, 0, pid)

8315

return (0 != win32api.TerminateProcess(handle, 0))

8316

</PRE>

8317

<P>

8318

8319

<A HREF="faqw.py?req=edit&file=faq08.005.htp">Edit this entry</A> /

8320

8321

8322

/ Last changed on Sat Aug 8 18:55:06 1998 by

8323

<A HREF="mailto:jeffbauer@bigfoot.com">Jeff Bauer</A>

8324

<P>

8325

8326

<HR>

8327

<H2><A NAME="8.6">8.6. Why does os.path.isdir() fail on NT shared directories?</A></H2>

8328

The solution appears to be always append the "\\" on

8329

the end of shared drives.

8330

<P>

8331

<PRE>

8332

>>> import os

8333

>>> os.path.isdir( '\\\\rorschach\\public')

8334

0

8335

>>> os.path.isdir( '\\\\rorschach\\public\\')

8336

1

8337

</PRE>

8338

[Blake Winton responds:]

8339

I've had the same problem doing "Start >> Run" and then a

8340

directory on a shared drive. If I use "\\rorschach\public",

8341

it will fail, but if I use "\\rorschach\public\", it will

8342

work. For that matter, os.stat() does the same thing (well,

8343

it gives an error for "\\\\rorschach\\public", but you get

8344

the idea)...

8345

<P>

8346

I've got a theory about why this happens, but it's only

8347

a theory. NT knows the difference between shared directories,

8348

and regular directories. "\\rorschach\public" isn't a

8349

directory, it's _really_ an IPC abstraction. This is sort

8350

of lended credence to by the fact that when you're mapping

8351

a network drive, you can't map "\\rorschach\public\utils",

8352

but only "\\rorschach\public".

8353

<P>

8354

[Clarification by <A HREF="mailto:funkster@midwinter.com">funkster@midwinter.com</A>]

8355

It's not actually a Python

8356

question, as Python is working just fine; it's clearing up something

8357

a bit muddled about Windows networked drives.

8358

<P>

8359

It helps to think of share points as being like drive letters.

8360

Example:

8361

<PRE>

8362

k: is not a directory

8363

k:\ is a directory

8364

k:\media is a directory

8365

k:\media\ is not a directory

8366

</PRE>

8367

The same rules apply if you substitute "k:" with "\\conky\foo":

8368

<PRE>

8369

\\conky\foo is not a directory

8370

\\conky\foo\ is a directory

8371

\\conky\foo\media is a directory

8372

\\conky\foo\media\ is not a directory

8373

</PRE>

8374

<P>

8375

8376

<A HREF="faqw.py?req=edit&file=faq08.006.htp">Edit this entry</A> /

8377

8378

8379

/ Last changed on Sun Jan 31 08:44:48 1999 by

8380

8381

<P>

8382

8383

<HR>

8384

<H2><A NAME="8.7">8.7. PyRun_SimpleFile() crashes on Windows but not on Unix</A></H2>

8385

I've seen a number of reports of PyRun_SimpleFile() failing

8386

in a Windows port of an application embedding Python that worked

8387

fine on Unix. PyRun_SimpleString() works fine on both platforms.

8388

<P>

8389

I think this happens because the application was compiled with a

8390

different set of compiler flags than Python15.DLL. It seems that some

8391

compiler flags affect the standard I/O library in such a way that

8392

using different flags makes calls fail. You need to set it for

8393

the non-debug multi-threaded DLL (/MD on the command line, or can be set via MSVC under Project Settings->C++/Code Generation then the "Use rum-time library" dropdown.)

8394

<P>

8395

Also note that you can not mix-and-match Debug and Release versions. If you wish to use the Debug Multithreaded DLL, then your module _must_ have an "_d" appended to the base name.

8396

<P>

8397

8398

<A HREF="faqw.py?req=edit&file=faq08.007.htp">Edit this entry</A> /

8399

8400

8401

/ Last changed on Wed Nov 17 17:37:07 1999 by

8402

<A HREF="mailto:mhammond@skippinet.com.au">Mark Hammond</A>

8403

<P>

8404

8405

<HR>

8406

<H2><A NAME="8.8">8.8. Import of _tkinter fails on Windows 95/98</A></H2>

8407

Sometimes, the import of _tkinter fails on Windows 95 or 98,

8408

complaining with a message like the following:

8409

<P>

8410

<PRE>

8411

ImportError: DLL load failed: One of the library files needed

8412

to run this application cannot be found.

8413

</PRE>

8414

It could be that you haven't installed Tcl/Tk, but if you did

8415

install Tcl/Tk, and the Wish application works correctly,

8416

the problem may be that its installer didn't

8417

manage to edit the autoexec.bat file correctly. It tries to add a

8418

statement that changes the PATH environment variable to include

8419

the Tcl/Tk 'bin' subdirectory, but sometimes this edit doesn't

8420

quite work. Opening it with notepad usually reveals what the

8421

problem is.

8422

<P>

8423

(One additional hint, noted by David Szafranski: you can't use

8424

long filenames here; e.g. use C:\PROGRA~1\Tcl\bin instead of

8425

C:\Program Files\Tcl\bin.)

8426

<P>

8427

8428

<A HREF="faqw.py?req=edit&file=faq08.008.htp">Edit this entry</A> /

8429

8430

8431

/ Last changed on Wed Dec 2 22:32:41 1998 by

8432

8433

<P>

8434

8435

<HR>

8436

<H2><A NAME="8.9">8.9. Can't extract the downloaded documentation on Windows</A></H2>

8437

Sometimes, when you download the documentation package to a Windows

8438

machine using a web browser, the file extension of the saved file

8439

ends up being .EXE. This is a mistake; the extension should be .TGZ.

8440

<P>

8441

Simply rename the downloaded file to have the .TGZ extension, and

8442

WinZip will be able to handle it. (If your copy of WinZip doesn't,

8443

get a newer one from <A HREF="http://www.winzip.com">http://www.winzip.com</A>.)

8444

<P>

8445

8446

<A HREF="faqw.py?req=edit&file=faq08.009.htp">Edit this entry</A> /

8447

8448

8449

/ Last changed on Sat Nov 21 13:41:35 1998 by

8450

8451

<P>

8452

8453

<HR>

8454

<H2><A NAME="8.10">8.10. Can't get Py_RunSimpleFile() to work.</A></H2>

8455

This is very sensitive to the compiler vendor, version and (perhaps)

8456

even options. If the FILE* structure in your embedding program isn't

8457

the same as is assumed by the Python interpreter it won't work.

8458

<P>

8459

The Python 1.5.* DLLs (python15.dll) are all compiled

8460

with MS VC++ 5.0 and with multithreading-DLL options (/MD, I think).

8461

<P>

8462

If you can't change compilers or flags, try using Py_RunSimpleString().

8463

A trick to get it to run an arbitrary file is to construct a call to

8464

execfile() with the name of your file as argument.

8465

<P>

8466

8467

<A HREF="faqw.py?req=edit&file=faq08.010.htp">Edit this entry</A> /

8468

8469

8470

/ Last changed on Wed Jan 13 10:58:14 1999 by

8471

8472

<P>

8473

8474

<HR>

8475

<H2><A NAME="8.11">8.11. Where is Freeze for Windows?</A></H2>

8476

("Freeze" is a program that allows you to ship a Python program

8477

as a single stand-alone executable file. It is <I>not</I> a compiler,

8478

your programs don't run any faster, but they are more easily

8479

distributable (to platforms with the same OS and CPU). Read the

8480

README file of the freeze program for more disclaimers.)

8481

<P>

8482

You can use freeze on Windows, but you must download the source

8483

tree (see <A HREF="http://www.python.org/download/download_source.html">http://www.python.org/download/download_source.html</A>).

8484

This is recommended for Python 1.5.2 (and betas thereof) only;

8485

older versions don't quite work.

8486

<P>

8487

You need the Microsoft VC++ 5.0 compiler (maybe it works with

8488

6.0 too). You probably need to build Python -- the project files

8489

are all in the PCbuild directory.

8490

<P>

8491

The freeze program is in the Tools\freeze subdirectory of the source

8492

tree.

8493

<P>

8494

8495

<A HREF="faqw.py?req=edit&file=faq08.011.htp">Edit this entry</A> /

8496

8497

8498

/ Last changed on Wed Feb 17 18:47:24 1999 by

8499

8500

<P>

8501

8502

<HR>

8503

8504

Yes, .pyd files are dll's. But there are a few differences. If you

8505

have a DLL named foo.pyd, then it must have a function initfoo(). You

8506

can then write Python "import foo", and Python will search for foo.pyd

8507

(as well as foo.py, foo.pyc) and if it finds it, will attempt to call

8508

initfoo() to initialize it. You do not link your .exe with foo.lib,

8509

as that would cause Windows to require the DLL to be present.

8510

<P>

8511

Note that the search path for foo.pyd is PYTHONPATH, not the same as

8512

the path that Windows uses to search for foo.dll. Also, foo.pyd need

8513

not be present to run your program, whereas if you linked your program

8514

with a dll, the dll is required. Of course, foo.pyd is required if

8515

you want to say "import foo". In a dll, linkage is declared in the

8516

source code with __declspec(dllexport). In a .pyd, linkage is defined

8517

in a list of available functions.

8518

<P>

8519

8520

<A HREF="faqw.py?req=edit&file=faq08.012.htp">Edit this entry</A> /

8521

8522

8523

/ Last changed on Tue Nov 23 02:40:08 1999 by

8524

<A HREF="mailto:jquinn+psa@cs.oberlin.edu">Jameson Quinn</A>

8525

<P>

8526

8527

<HR>

8528

<H2><A NAME="8.13">8.13. Missing cw3215mt.dll (or missing cw3215.dll)</A></H2>

8529

Sometimes, when using Tkinter on Windows, you get an error that

8530

cw3215mt.dll or cw3215.dll is missing.

8531

<P>

8532

Cause: you have an old Tcl/Tk DLL built with cygwin in your path

8533

(probably C:\Windows). You must use the Tcl/Tk DLLs from the

8534

standard Tcl/Tk installation (Python 1.5.2 comes with one).

8535

<P>

8536

8537

<A HREF="faqw.py?req=edit&file=faq08.013.htp">Edit this entry</A> /

8538

8539

8540

/ Last changed on Fri Jun 11 00:54:13 1999 by

8541

8542

<P>

8543

8544

<HR>

8545

<H2><A NAME="8.14">8.14. How to make python scripts executable:</A></H2>

8546

[Blake Coverett]

8547

<P>

8548

Win2K:

8549

<P>

8550

The standard installer already associates the .py extension with a file type

8551

(Python.File) and gives that file type an open command that runs the

8552

interpreter (D:\Program Files\Python\python.exe "%1" %*). This is enough to

8553

make scripts executable from the command prompt as 'foo.py'. If you'd

8554

rather be able to execute the script by simple typing 'foo' with no

8555

extension you need to add .py to the PATHEXT environment variable.

8556

<P>

8557

WinNT:

8558

<P>

8559

The steps taken by the installed as described above allow you do run a

8560

script with 'foo.py', but a long time bug in the NT command processor

8561

prevents you from redirecting the input or output of any script executed in

8562

this way. This is often important.

8563

<P>

8564

An appropriate incantation for making a Python script executable under WinNT

8565

is to give the file an extension of .cmd and add the following as the first

8566

line:

8567

<P>

8568

<PRE>

8569

@setlocal enableextensions & python -x %~f0 %* & goto :EOF

8570

</PRE>

8571

Win9x:

8572

<P>

8573

[Due to Bruce Eckel]

8574

<P>

8575

<PRE>

8576

@echo off

8577

rem = """

8578

rem run python on this bat file. Needs the full path where

8579

rem you keep your python files. The -x causes python to skip

8580

rem the first line of the file:

8581

python -x c:\aaa\Python\\"%0".bat %1 %2 %3 %4 %5 %6 %7 %8 %9

8582

goto endofpython

8583

rem """

8584

</PRE>

8585

<PRE>

8586

# The python program goes here:

8587

</PRE>

8588

<PRE>

8589

print "hello, Python"

8590

</PRE>

8591

<PRE>

8592

# For the end of the batch file:

8593

rem = """

8594

:endofpython

8595

rem """

8596

</PRE>

8597

<P>

8598

8599

<A HREF="faqw.py?req=edit&file=faq08.014.htp">Edit this entry</A> /

8600

8601

8602

/ Last changed on Tue Nov 30 10:25:17 1999 by

8603

8604

<P>

8605

8606

<HR>

8607

<H2><A NAME="8.15">8.15. Warning about CTL3D32 version from installer</A></H2>

8608

The Python installer issues a warning like this:

8609

<P>

8610

<PRE>

8611

This version uses CTL3D32.DLL whitch is not the correct version.

8612

This version is used for windows NT applications only.

8613

</PRE>

8614

[Tim Peters]

8615

This is a Microsoft DLL, and a notorious

8616

source of problems. The msg means what it says: you have the wrong version

8617

of this DLL for your operating system. The Python installation did not

8618

cause this -- something else you installed previous to this overwrote the

8619

DLL that came with your OS (probably older shareware of some sort, but

8620

there's no way to tell now). If you search for "CTL3D32" using any search

8621

engine (AltaVista, for example), you'll find hundreds and hundreds of web

8622

pages complaining about the same problem with all sorts of installation

8623

programs. They'll point you to ways to get the correct version reinstalled

8624

on your system (since Python doesn't cause this, we can't fix it).

8625

<P>

8626

David A Burton has written a little program to fix this. Go to

8627

<A HREF="http://www.burtonsys.com/download.html">http://www.burtonsys.com/download.html</A> and click on "ctl3dfix.zip"

8628

<P>

8629

8630

<A HREF="faqw.py?req=edit&file=faq08.015.htp">Edit this entry</A> /

8631

8632

8633

/ Last changed on Thu Oct 26 15:42:00 2000 by

8634

8635

<P>

8636

8637

<HR>

8638

<H2><A NAME="8.16">8.16. How can I embed Python into a Windows application?</A></H2>

8639

Edward K. Ream <<A HREF="mailto:edream@tds.net">edream@tds.net</A>> writes

8640

<P>

8641

When '##' appears in a file name below, it is an abbreviated version number. For example, for Python 2.1.1, ## will be replaced by 21.

8642

<P>

8643

Embedding the Python interpreter in a Windows app can be summarized as

8644

follows:

8645

<P>

8646

1. Do _not_ build Python into your .exe file directly. On Windows,

8647

Python must be a DLL to handle importing modules that are themselves

8648

DLL's. (This is the first key undocumented fact.) Instead, link to

8649

python##.dll; it is typically installed in c:\Windows\System.

8650

<P>

8651

You can link to Python statically or dynamically. Linking statically

8652

means linking against python##.lib The drawback is that your app won't

8653

run if python##.dll does not exist on your system.

8654

<P>

8655

General note: python##.lib is the so-called "import lib" corresponding

8656

to python.dll. It merely defines symbols for the linker.

8657

<P>

8658

Borland note: convert python##.lib to OMF format using Coff2Omf.exe

8659

first.

8660

<P>

8661

Linking dynamically greatly simplifies link options; everything happens

8662

at run time. Your code must load python##.dll using the Windows

8663

LoadLibraryEx() routine. The code must also use access routines and

8664

data in python##.dll (that is, Python's C API's) using pointers

8665

obtained by the Windows GetProcAddress() routine. Macros can make

8666

using these pointers transparent to any C code that calls routines in

8667

Python's C API.

8668

<P>

8669

2. If you use SWIG, it is easy to create a Python "extension module"

8670

that will make the app's data and methods available to Python. SWIG

8671

will handle just about all the grungy details for you. The result is C

8672

code that you link _into your .exe file_ (!) You do _not_ have to

8673

create a DLL file, and this also simplifies linking.

8674

<P>

8675

3. SWIG will create an init function (a C function) whose name depends

8676

on the name of the extension module. For example, if the name of the

8677

module is leo, the init function will be called initleo(). If you use

8678

SWIG shadow classes, as you should, the init function will be called

8679

initleoc(). This initializes a mostly hidden helper class used by the

8680

shadow class.

8681

<P>

8682

The reason you can link the C code in step 2 into your .exe file is that

8683

calling the initialization function is equivalent to importing the

8684

module into Python! (This is the second key undocumented fact.)

8685

<P>

8686

4. In short, you can use the following code to initialize the Python

8687

interpreter with your extension module.

8688

<P>

8689

<PRE>

8690

#include "python.h"

8691

...

8692

Py_Initialize(); // Initialize Python.

8693

initmyAppc(); // Initialize (import) the helper class.

8694

PyRun_SimpleString("import myApp") ; // Import the shadow class.

8695

</PRE>

8696

5. There are two problems with Python's C API which will become apparent

8697

if you use a compiler other than MSVC, the compiler used to build

8698

python##.dll.

8699

<P>

8700

Problem 1: The so-called "Very High Level" functions that take FILE *

8701

arguments will not work in a multi-compiler environment; each compiler's

8702

notion of a struct FILE will be different. From an implementation

8703

standpoint these are very _low_ level functions.

8704

<P>

8705

Problem 2: SWIG generates the following code when generating wrappers to

8706

void functions:

8707

<P>

8708

<PRE>

8709

Py_INCREF(Py_None);

8710

_resultobj = Py_None;

8711

return _resultobj;

8712

</PRE>

8713

Alas, Py_None is a macro that expands to a reference to a complex data

8714

structure called _Py_NoneStruct inside python##.dll. Again, this code

8715

will fail in a mult-compiler environment. Replace such code by:

8716

<P>

8717

<PRE>

8718

return Py_BuildValue("");

8719

</PRE>

8720

It may be possible to use SWIG's %typemap command to make the change

8721

automatically, though I have not been able to get this to work (I'm a

8722

complete SWIG newbie).

8723

<P>

8724

6. Using a Python shell script to put up a Python interpreter window

8725

from inside your Windows app is not a good idea; the resulting window

8726

will be independent of your app's windowing system. Rather, you (or the

8727

wxPythonWindow class) should create a "native" interpreter window. It

8728

is easy to connect that window to the Python interpreter. You can

8729

redirect Python's i/o to _any_ object that supports read and write, so

8730

all you need is a Python object (defined in your extension module) that

8731

contains read() and write() methods.

8732

<P>

8733

8734

<A HREF="faqw.py?req=edit&file=faq08.016.htp">Edit this entry</A> /

8735

8736

8737

/ Last changed on Thu Jan 31 16:29:34 2002 by

8738

<A HREF="mailto:vkryukov@ufg.com">Victor Kryukov</A>

8739

<P>

8740

8741

<HR>

8742

<H2><A NAME="8.17">8.17. Setting up IIS 5 to use Python for CGI</A></H2>

8743

In order to set up Internet Information Services 5 to use Python for CGI processing, please see the following links:

8744

<P>

8745

<A HREF="http://www.e-coli.net/pyiis_server.html">http://www.e-coli.net/pyiis_server.html</A> (for Win2k Server)

8746

<A HREF="http://www.e-coli.net/pyiis.html">http://www.e-coli.net/pyiis.html</A> (for Win2k pro)

8747

<P>

8748

8749

<A HREF="faqw.py?req=edit&file=faq08.017.htp">Edit this entry</A> /

8750

8751

8752

/ Last changed on Fri Mar 22 22:05:51 2002 by

8753

<A HREF="mailto:dsavitsk@e-coli.net">douglas savitsky</A>

8754

<P>

8755

8756

<HR>

8757

<H2><A NAME="8.18">8.18. How do I run a Python program under Windows?</A></H2>

8758

This is not necessarily quite the straightforward question it appears

8759

to be. If you are already familiar with running programs from the

8760

Windows command line then everything will seem really easy and

8761

obvious. If your computer experience is limited then you might need a

8762

little more guidance. Also there are differences between Windows 95,

8763

98, NT, ME, 2000 and XP which can add to the confusion. You might

8764

think of this as "why I pay software support charges" if you have a

8765

helpful and friendly administrator to help you set things up without

8766

having to understand all this yourself. If so, then great! Show them

8767

this page and it should be a done deal.

8768

<P>

8769

Unless you use some sort of integrated development environment (such

8770

as PythonWin or IDLE, to name only two in a growing family) then you

8771

will end up <I>typing</I> Windows commands into what is variously referred

8772

to as a "DOS window" or "Command prompt window". Usually you can

8773

create such a window from your Start menu (under Windows 2000 I use

8774

"Start | Programs | Accessories | Command Prompt"). You should be

8775

able to recognize when you have started such a window because you will

8776

see a Windows "command prompt", which usually looks like this:

8777

<P>

8778

<PRE>

8779

C:\>

8780

</PRE>

8781

The letter may be different, and there might be other things after it,

8782

so you might just as easily see something like:

8783

<P>

8784

<PRE>

8785

D:\Steve\Projects\Python>

8786

</PRE>

8787

depending on how your computer has been set up and what else you have

8788

recently done with it. Once you have started such a window, you are

8789

well on the way to running Python programs.

8790

<P>

8791

You need to realize that your Python scripts have to be processed by

8792

another program, usually called the "Python interpreter". The

8793

interpreter reads your script, "compiles" it into "Python bytecodes"

8794

(which are instructions for an imaginary computer known as the "Python

8795

Virtual Machine") and then executes the bytecodes to run your

8796

program. So, how do you arrange for the interpreter to handle your

8797

Python?

8798

<P>

8799

First, you need to make sure that your command window recognises the

8800

word "python" as an instruction to start the interpreter. If you have

8801

opened a command window, you should try entering the command:

8802

<P>

8803

<PRE>

8804

python

8805

</PRE>

8806

and hitting return. If you then see something like:

8807

<P>

8808

<PRE>

8809

Python 2.2 (#28, Dec 21 2001, 12:21:22) [MSC 32 bit (Intel)] on win32

8810

Type "help", "copyright", "credits" or "license" for more information.

8811

>>>

8812

</PRE>

8813

then this part of the job has been correctly managed during Python's

8814

installation process, and you have started the interpreter in

8815

"interactive mode". That means you can enter Python statements or

8816

expressions interactively and have them executed or evaluated while

8817

you wait. This is one of Python's strongest features, but it takes a

8818

little getting used to. Check it by entering a few expressions of your

8819

choice and seeing the results...

8820

<P>

8821

<PRE>

8822

>>> print "Hello"

8823

Hello

8824

>>> "Hello" * 3

8825

HelloHelloHello

8826

</PRE>

8827

When you want to end your interactive Python session, enter a

8828

terminator (hold the Ctrl key down while you enter a Z, then hit the

8829

"Enter" key) to get back to your Windows command prompt. You may also

8830

find that you have a Start-menu entry such as "Start | Programs |

8831

Python 2.2 | Python (command line)" that results in you seeing the

8832

">>>" prompt in a new window. If so, the window will disappear after

8833

you enter the terminator -- Windows runs a single "python" command in

8834

the window, which terminates when you terminate the interpreter.

8835

<P>

8836

If the "python" command, instead of displaying the interpreter prompt ">>>", gives you a message like

8837

<P>

8838

<PRE>

8839

'python' is not recognized as an internal or external command,

8840

operable program or batch file.

8841

</PRE>

8842

or

8843

<P>

8844

<PRE>

8845

Bad command or filename

8846

</PRE>

8847

then you need to make sure that your computer knows where to find the

8848

Python interpreter. To do this you will have to modify a setting

8849

called the PATH, which is a just list of directories where Windows

8850

will look for programs. Rather than just enter the right command every

8851

time you create a command window, you should arrange for Python's

8852

installation directory to be added to the PATH of every command window

8853

as it starts. If you installed Python fairly recently then the command

8854

<P>

8855

<PRE>

8856

dir C:\py*

8857

</PRE>

8858

will probably tell you where it is installed. Alternatively, perhaps

8859

you made a note. Otherwise you will be reduced to a search of your

8860

whole disk ... break out the Windows explorer and use "Tools | Find"

8861

or hit the "Search" button and look for "python.exe". Suppose you

8862

discover that Python is installed in the C:\Python22 directory (the

8863

default at the time of writing) then you should make sure that

8864

entering the command

8865

<P>

8866

<PRE>

8867

c:\Python22\python

8868

</PRE>

8869

starts up the interpreter as above (and don't forget you'll need a

8870

"CTRL-Z" and an "Enter" to get out of it). Once you have verified the

8871

directory, you need to add it to the start-up routines your computer

8872

goes through. For older versions of Windows the easiest way to do

8873

this is to edit the C:\AUTOEXEC.BAT file. You would want to add a line

8874

like the following to AUTOEXEC.BAT:

8875

<P>

8876

<PRE>

8877

PATH C:\Python22;%PATH%

8878

</PRE>

8879

For Windows NT, 2000 and (I assume) XP, you will need to add a string

8880

such as

8881

<P>

8882

<PRE>

8883

;C:\Python22

8884

</PRE>

8885

to the current setting for the PATH environment variable, which you

8886

will find in the properties window of "My Computer" under the

8887

"Advanced" tab. Note that if you have sufficient privilege you might

8888

get a choice of installing the settings either for the Current User or

8889

for System. The latter is preferred if you want everybody to be able

8890

to run Python on the machine.

8891

<P>

8892

If you aren't confident doing any of these manipulations yourself, ask

8893

for help! At this stage you may or may not want to reboot your system

8894

to make absolutely sure the new setting has "taken" (don't you love

8895

the way Windows gives you these freqeuent coffee breaks). You probably

8896

won't need to for Windows NT, XP or 2000. You can also avoid it in

8897

earlier versions by editing the file C:\WINDOWS\COMMAND\CMDINIT.BAT

8898

instead of AUTOEXEC.BAT.

8899

<P>

8900

You should now be able to start a new command window, enter

8901

<P>

8902

<PRE>

8903

python

8904

</PRE>

8905

at the "C:>" (or whatever) prompt, and see the ">>>" prompt that

8906

indicates the Python interpreter is reading interactive commands.

8907

<P>

8908

Let's suppose you have a program called "pytest.py" in directory

8909

"C:\Steve\Projects\Python". A session to run that program might look

8910

like this:

8911

<P>

8912

<PRE>

8913

C:\> cd \Steve\Projects\Python

8914

C:\Steve\Projects\Python> python pytest.py

8915

</PRE>

8916

Because you added a file name to the command to start the interpreter,

8917

when it starts up it reads the Python script in the named file,

8918

compiles it, executes it, and terminates (so you see another "C:\>"

8919

prompt). You might also have entered

8920

<P>

8921

<PRE>

8922

C:\> python \Steve\Projects\Python\pytest.py

8923

</PRE>

8924

if you hadn't wanted to change your current directory.

8925

<P>

8926

Under NT, 2000 and XP you may well find that the installation process

8927

has also arranged that the command

8928

<P>

8929

<PRE>

8930

pytest.py

8931

</PRE>

8932

(or, if the file isn't in the current directory)

8933

<P>

8934

<PRE>

8935

C:\Steve\Projects\Python\pytest.py

8936

</PRE>

8937

will automatically recognize the ".py" extension and run the Python

8938

interpreter on the named file. Using this feature is fine, but <I>some</I>

8939

versions of Windows have bugs which mean that this form isn't exactly

8940

equivalent to using the interpreter explicitly, so be careful. Easier

8941

to remember, for now, that

8942

<P>

8943

<PRE>

8944

python C:\Steve\Projects\Python\pytest.py

8945

</PRE>

8946

works pretty close to the same, and redirection will work (more)

8947

reliably.

8948

<P>

8949

The important things to remember are:

8950

<P>

8951

1. Start Python from the Start Menu, or make sure the PATH is set

8952

correctly so Windows can find the Python interpreter.

8953

<P>

8954

<PRE>

8955

python

8956

</PRE>

8957

should give you a '>>>" prompt from the Python interpreter. Don't

8958

forget the CTRL-Z and ENTER to terminate the interpreter (and, if you

8959

started the window from the Start Menu, make the window disappear).

8960

<P>

8961

2. Once this works, you run programs with commands:

8962

<P>

8963

<PRE>

8964

python {program-file}

8965

</PRE>

8966

3. When you know the commands to use you can build Windows shortcuts

8967

to run the Python interpreter on any of your scripts, naming

8968

particular working directories, and adding them to your menus, but

8969

that's another lessFAQ. Take a look at

8970

<P>

8971

<PRE>

8972

python --help

8973

</PRE>

8974

if your needs are complex.

8975

<P>

8976

4. Interactive mode (where you see the ">>>" prompt) is best used

8977

<I>not</I> for running programs, which are better executed as in steps 2

8978

and 3, but for checking that individual statements and expressions do

8979

what you think they will, and for developing code by experiment.

8980

<P>

8981

8982

<A HREF="faqw.py?req=edit&file=faq08.018.htp">Edit this entry</A> /

8983

8984

8985

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8986

8987

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8988

8989

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8990

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8991

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8992

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