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Idioms and Anti-Idioms in Python
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This document is placed in the public domain.
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This document can be considered a companion to the tutorial. It shows how to use
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Python, and even more importantly, how *not* to use Python.
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Language Constructs You Should Not Use
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======================================
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While Python has relatively few gotchas compared to other languages, it still
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has some constructs which are only useful in corner cases, or are plain
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Inside Function Definitions
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^^^^^^^^^^^^^^^^^^^^^^^^^^^
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``from module import *`` is *invalid* inside function definitions. While many
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versions of Python do not check for the invalidity, it does not make it more
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valid, no more then having a smart lawyer makes a man innocent. Do not use it
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like that ever. Even in versions where it was accepted, it made the function
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execution slower, because the compiler could not be certain which names are
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local and which are global. In Python 2.1 this construct causes warnings, and
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sometimes even errors.
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While it is valid to use ``from module import *`` at module level it is usually
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a bad idea. For one, this loses an important property Python otherwise has ---
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you can know where each toplevel name is defined by a simple "search" function
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in your favourite editor. You also open yourself to trouble in the future, if
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some module grows additional functions or classes.
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One of the most awful question asked on the newsgroup is why this code::
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does not work. Of course, it works just fine (assuming you have a file called
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"www".) But it does not work if somewhere in the module, the statement ``from os
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import *`` is present. The :mod:`os` module has a function called :func:`open`
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which returns an integer. While it is very useful, shadowing builtins is one of
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its least useful properties.
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Remember, you can never know for sure what names a module exports, so either
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take what you need --- ``from module import name1, name2``, or keep them in the
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module and access on a per-need basis --- ``import module;print module.name``.
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There are situations in which ``from module import *`` is just fine:
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* The interactive prompt. For example, ``from math import *`` makes Python an
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amazing scientific calculator.
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* When extending a module in C with a module in Python.
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* When the module advertises itself as ``from import *`` safe.
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Unadorned :keyword:`exec`, :func:`execfile` and friends
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-------------------------------------------------------
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The word "unadorned" refers to the use without an explicit dictionary, in which
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case those constructs evaluate code in the *current* environment. This is
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dangerous for the same reasons ``from import *`` is dangerous --- it might step
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over variables you are counting on and mess up things for the rest of your code.
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Simply do not do that.
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>>> for name in sys.argv[1:]:
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>>> exec "%s=1" % name
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>>> def func(s, **kw):
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>>> for var, val in kw.items():
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>>> exec "s.%s=val" % var # invalid!
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>>> execfile("handler.py")
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>>> for name in sys.argv[1:]:
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>>> def func(s, **kw):
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>>> for var, val in kw.items():
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>>> setattr(s, var, val)
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>>> execfile("handle.py", d, d)
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>>> handle = d['handle']
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from module import name1, name2
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-------------------------------
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This is a "don't" which is much weaker then the previous "don't"s but is still
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something you should not do if you don't have good reasons to do that. The
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reason it is usually bad idea is because you suddenly have an object which lives
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in two separate namespaces. When the binding in one namespace changes, the
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binding in the other will not, so there will be a discrepancy between them. This
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happens when, for example, one module is reloaded, or changes the definition of
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a function at runtime.
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a = 2 # danger: foo.a != a
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Python has the ``except:`` clause, which catches all exceptions. Since *every*
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error in Python raises an exception, this makes many programming errors look
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like runtime problems, and hinders the debugging process.
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The following code shows a great example::
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foo = opne("file") # misspelled "open"
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sys.exit("could not open file!")
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The second line triggers a :exc:`NameError` which is caught by the except
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clause. The program will exit, and you will have no idea that this has nothing
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to do with the readability of ``"file"``.
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The example above is better written ::
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foo = opne("file") # will be changed to "open" as soon as we run it
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sys.exit("could not open file")
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There are some situations in which the ``except:`` clause is useful: for
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example, in a framework when running callbacks, it is good not to let any
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callback disturb the framework.
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Exceptions are a useful feature of Python. You should learn to raise them
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whenever something unexpected occurs, and catch them only where you can do
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something about them.
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The following is a very popular anti-idiom ::
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def get_status(file):
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if not os.path.exists(file):
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print "file not found"
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return open(file).readline()
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Consider the case the file gets deleted between the time the call to
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:func:`os.path.exists` is made and the time :func:`open` is called. That means
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the last line will throw an :exc:`IOError`. The same would happen if *file*
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exists but has no read permission. Since testing this on a normal machine on
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existing and non-existing files make it seem bugless, that means in testing the
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results will seem fine, and the code will get shipped. Then an unhandled
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:exc:`IOError` escapes to the user, who has to watch the ugly traceback.
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Here is a better way to do it. ::
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def get_status(file):
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return open(file).readline()
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except (IOError, OSError):
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print "file not found"
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In this version, \*either\* the file gets opened and the line is read (so it
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works even on flaky NFS or SMB connections), or the message is printed and the
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Still, :func:`get_status` makes too many assumptions --- that it will only be
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used in a short running script, and not, say, in a long running server. Sure,
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the caller could do something like ::
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status = get_status(log)
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So, try to make as few ``except`` clauses in your code --- those will usually be
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a catch-all in the :func:`main`, or inside calls which should always succeed.
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So, the best version is probably ::
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def get_status(file):
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return open(file).readline()
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The caller can deal with the exception if it wants (for example, if it tries
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several files in a loop), or just let the exception filter upwards to *its*
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The last version is not very good either --- due to implementation details, the
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file would not be closed when an exception is raised until the handler finishes,
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and perhaps not at all in non-C implementations (e.g., Jython). ::
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def get_status(file):
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Every so often, people seem to be writing stuff in the Python library again,
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usually poorly. While the occasional module has a poor interface, it is usually
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much better to use the rich standard library and data types that come with
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Python then inventing your own.
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A useful module very few people know about is :mod:`os.path`. It always has the
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correct path arithmetic for your operating system, and will usually be much
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better then whatever you come up with yourself.
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return os.path.join(dir, file)
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More useful functions in :mod:`os.path`: :func:`basename`, :func:`dirname` and
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There are also many useful builtin functions people seem not to be aware of for
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some reason: :func:`min` and :func:`max` can find the minimum/maximum of any
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sequence with comparable semantics, for example, yet many people write their own
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:func:`max`/:func:`min`. Another highly useful function is :func:`reduce`. A
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classical use of :func:`reduce` is something like ::
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nums = map(float, sys.argv[1:])
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print reduce(operator.add, nums)/len(nums)
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This cute little script prints the average of all numbers given on the command
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line. The :func:`reduce` adds up all the numbers, and the rest is just some
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pre- and postprocessing.
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On the same note, note that :func:`float`, :func:`int` and :func:`long` all
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accept arguments of type string, and so are suited to parsing --- assuming you
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are ready to deal with the :exc:`ValueError` they raise.
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Using Backslash to Continue Statements
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======================================
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Since Python treats a newline as a statement terminator, and since statements
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are often more then is comfortable to put in one line, many people do::
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if foo.bar()['first'][0] == baz.quux(1, 2)[5:9] and \
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calculate_number(10, 20) != forbulate(500, 360):
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You should realize that this is dangerous: a stray space after the ``\`` would
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make this line wrong, and stray spaces are notoriously hard to see in editors.
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In this case, at least it would be a syntax error, but if the code was::
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value = foo.bar()['first'][0]*baz.quux(1, 2)[5:9] \
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+ calculate_number(10, 20)*forbulate(500, 360)
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then it would just be subtly wrong.
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It is usually much better to use the implicit continuation inside parenthesis:
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This version is bulletproof::
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value = (foo.bar()['first'][0]*baz.quux(1, 2)[5:9]
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+ calculate_number(10, 20)*forbulate(500, 360))