1
:mod:`sys` --- System-specific parameters and functions
2
=======================================================
5
:synopsis: Access system-specific parameters and functions.
8
This module provides access to some variables used or maintained by the
9
interpreter and to functions that interact strongly with the interpreter. It is
15
The list of command line arguments passed to a Python script. ``argv[0]`` is the
16
script name (it is operating system dependent whether this is a full pathname or
17
not). If the command was executed using the :option:`-c` command line option to
18
the interpreter, ``argv[0]`` is set to the string ``'-c'``. If no script name
19
was passed to the Python interpreter, ``argv[0]`` is the empty string.
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To loop over the standard input, or the list of files given on the
22
command line, see the :mod:`fileinput` module.
27
An indicator of the native byte order. This will have the value ``'big'`` on
28
big-endian (most-significant byte first) platforms, and ``'little'`` on
29
little-endian (least-significant byte first) platforms.
34
.. data:: builtin_module_names
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A tuple of strings giving the names of all modules that are compiled into this
37
Python interpreter. (This information is not available in any other way ---
38
``modules.keys()`` only lists the imported modules.)
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.. function:: call_tracing(func, args)
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Call ``func(*args)``, while tracing is enabled. The tracing state is saved,
44
and restored afterwards. This is intended to be called from a debugger from
45
a checkpoint, to recursively debug some other code.
50
A string containing the copyright pertaining to the Python interpreter.
53
.. function:: _clear_type_cache()
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Clear the internal type cache. The type cache is used to speed up attribute
56
and method lookups. Use the function *only* to drop unnecessary references
57
during reference leak debugging.
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This function should be used for internal and specialized purposes only.
64
.. function:: _current_frames()
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Return a dictionary mapping each thread's identifier to the topmost stack frame
67
currently active in that thread at the time the function is called. Note that
68
functions in the :mod:`traceback` module can build the call stack given such a
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This is most useful for debugging deadlock: this function does not require the
72
deadlocked threads' cooperation, and such threads' call stacks are frozen for as
73
long as they remain deadlocked. The frame returned for a non-deadlocked thread
74
may bear no relationship to that thread's current activity by the time calling
75
code examines the frame.
77
This function should be used for internal and specialized purposes only.
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Integer specifying the handle of the Python DLL. Availability: Windows.
87
.. function:: displayhook(value)
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If *value* is not ``None``, this function prints it to ``sys.stdout``, and saves
90
it in ``__builtin__._``.
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``sys.displayhook`` is called on the result of evaluating an :term:`expression`
93
entered in an interactive Python session. The display of these values can be
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customized by assigning another one-argument function to ``sys.displayhook``.
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.. data:: dont_write_bytecode
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If this is true, Python won't try to write ``.pyc`` or ``.pyo`` files on the
100
import of source modules. This value is initially set to ``True`` or
101
``False`` depending on the :option:`-B` command line option and the
102
:envvar:`PYTHONDONTWRITEBYTECODE` environment variable, but you can set it
103
yourself to control bytecode file generation.
105
.. versionadded:: 2.6
108
.. function:: excepthook(type, value, traceback)
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This function prints out a given traceback and exception to ``sys.stderr``.
112
When an exception is raised and uncaught, the interpreter calls
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``sys.excepthook`` with three arguments, the exception class, exception
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instance, and a traceback object. In an interactive session this happens just
115
before control is returned to the prompt; in a Python program this happens just
116
before the program exits. The handling of such top-level exceptions can be
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customized by assigning another three-argument function to ``sys.excepthook``.
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.. data:: __displayhook__
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These objects contain the original values of ``displayhook`` and ``excepthook``
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at the start of the program. They are saved so that ``displayhook`` and
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``excepthook`` can be restored in case they happen to get replaced with broken
129
.. function:: exc_info()
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This function returns a tuple of three values that give information about the
132
exception that is currently being handled. The information returned is specific
133
both to the current thread and to the current stack frame. If the current stack
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frame is not handling an exception, the information is taken from the calling
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stack frame, or its caller, and so on until a stack frame is found that is
136
handling an exception. Here, "handling an exception" is defined as "executing
137
or having executed an except clause." For any stack frame, only information
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about the most recently handled exception is accessible.
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.. index:: object: traceback
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If no exception is being handled anywhere on the stack, a tuple containing three
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``None`` values is returned. Otherwise, the values returned are ``(type, value,
144
traceback)``. Their meaning is: *type* gets the exception type of the exception
145
being handled (a class object); *value* gets the exception parameter (its
146
:dfn:`associated value` or the second argument to :keyword:`raise`, which is
147
always a class instance if the exception type is a class object); *traceback*
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gets a traceback object (see the Reference Manual) which encapsulates the call
149
stack at the point where the exception originally occurred.
151
If :func:`exc_clear` is called, this function will return three ``None`` values
152
until either another exception is raised in the current thread or the execution
153
stack returns to a frame where another exception is being handled.
157
Assigning the *traceback* return value to a local variable in a function that is
158
handling an exception will cause a circular reference. This will prevent
159
anything referenced by a local variable in the same function or by the traceback
160
from being garbage collected. Since most functions don't need access to the
161
traceback, the best solution is to use something like ``exctype, value =
162
sys.exc_info()[:2]`` to extract only the exception type and value. If you do
163
need the traceback, make sure to delete it after use (best done with a
164
:keyword:`try` ... :keyword:`finally` statement) or to call :func:`exc_info` in
165
a function that does not itself handle an exception.
169
Beginning with Python 2.2, such cycles are automatically reclaimed when garbage
170
collection is enabled and they become unreachable, but it remains more efficient
171
to avoid creating cycles.
174
.. function:: exc_clear()
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This function clears all information relating to the current or last exception
177
that occurred in the current thread. After calling this function,
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:func:`exc_info` will return three ``None`` values until another exception is
179
raised in the current thread or the execution stack returns to a frame where
180
another exception is being handled.
182
This function is only needed in only a few obscure situations. These include
183
logging and error handling systems that report information on the last or
184
current exception. This function can also be used to try to free resources and
185
trigger object finalization, though no guarantee is made as to what objects will
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.. versionadded:: 2.3
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Use :func:`exc_info` instead.
198
Since they are global variables, they are not specific to the current thread, so
199
their use is not safe in a multi-threaded program. When no exception is being
200
handled, ``exc_type`` is set to ``None`` and the other two are undefined.
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.. data:: exec_prefix
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A string giving the site-specific directory prefix where the platform-dependent
206
Python files are installed; by default, this is also ``'/usr/local'``. This can
207
be set at build time with the ``--exec-prefix`` argument to the
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:program:`configure` script. Specifically, all configuration files (e.g. the
209
:file:`pyconfig.h` header file) are installed in the directory
210
:file:`{exec_prefix}/lib/python{X.Y}/config`, and shared library modules are
211
installed in :file:`{exec_prefix}/lib/python{X.Y}/lib-dynload`, where *X.Y*
212
is the version number of Python, for example ``2.7``.
217
A string giving the absolute path of the executable binary for the Python
218
interpreter, on systems where this makes sense. If Python is unable to retrieve
219
the real path to its executable, :data:`sys.executable` will be an empty string
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.. function:: exit([arg])
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Exit from Python. This is implemented by raising the :exc:`SystemExit`
226
exception, so cleanup actions specified by finally clauses of :keyword:`try`
227
statements are honored, and it is possible to intercept the exit attempt at
230
The optional argument *arg* can be an integer giving the exit status
231
(defaulting to zero), or another type of object. If it is an integer, zero
232
is considered "successful termination" and any nonzero value is considered
233
"abnormal termination" by shells and the like. Most systems require it to be
234
in the range 0-127, and produce undefined results otherwise. Some systems
235
have a convention for assigning specific meanings to specific exit codes, but
236
these are generally underdeveloped; Unix programs generally use 2 for command
237
line syntax errors and 1 for all other kind of errors. If another type of
238
object is passed, ``None`` is equivalent to passing zero, and any other
239
object is printed to :data:`stderr` and results in an exit code of 1. In
240
particular, ``sys.exit("some error message")`` is a quick way to exit a
241
program when an error occurs.
243
Since :func:`exit` ultimately "only" raises an exception, it will only exit
244
the process when called from the main thread, and the exception is not
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This value is not actually defined by the module, but can be set by the user (or
251
by a program) to specify a clean-up action at program exit. When set, it should
252
be a parameterless function. This function will be called when the interpreter
253
exits. Only one function may be installed in this way; to allow multiple
254
functions which will be called at termination, use the :mod:`atexit` module.
258
The exit function is not called when the program is killed by a signal, when a
259
Python fatal internal error is detected, or when ``os._exit()`` is called.
262
Use :mod:`atexit` instead.
267
The struct sequence *flags* exposes the status of command line flags. The
268
attributes are read only.
270
============================= ===================================
272
============================= ===================================
273
:const:`debug` :option:`-d`
274
:const:`py3k_warning` :option:`-3`
275
:const:`division_warning` :option:`-Q`
276
:const:`division_new` :option:`-Qnew <-Q>`
277
:const:`inspect` :option:`-i`
278
:const:`interactive` :option:`-i`
279
:const:`optimize` :option:`-O` or :option:`-OO`
280
:const:`dont_write_bytecode` :option:`-B`
281
:const:`no_user_site` :option:`-s`
282
:const:`no_site` :option:`-S`
283
:const:`ignore_environment` :option:`-E`
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:const:`tabcheck` :option:`-t` or :option:`-tt <-t>`
285
:const:`verbose` :option:`-v`
286
:const:`unicode` :option:`-U`
287
:const:`bytes_warning` :option:`-b`
288
:const:`hash_randomization` :option:`-R`
289
============================= ===================================
291
.. versionadded:: 2.6
293
.. versionadded:: 2.7.3
294
The ``hash_randomization`` attribute.
298
A structseq holding information about the float type. It contains low level
299
information about the precision and internal representation. The values
300
correspond to the various floating-point constants defined in the standard
301
header file :file:`float.h` for the 'C' programming language; see section
302
5.2.4.2.2 of the 1999 ISO/IEC C standard [C99]_, 'Characteristics of
303
floating types', for details.
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.. tabularcolumns:: |l|l|L|
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+---------------------+----------------+--------------------------------------------------+
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| attribute | float.h macro | explanation |
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+=====================+================+==================================================+
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| :const:`epsilon` | DBL_EPSILON | difference between 1 and the least value greater |
311
| | | than 1 that is representable as a float |
312
+---------------------+----------------+--------------------------------------------------+
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| :const:`dig` | DBL_DIG | maximum number of decimal digits that can be |
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| | | faithfully represented in a float; see below |
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+---------------------+----------------+--------------------------------------------------+
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| :const:`mant_dig` | DBL_MANT_DIG | float precision: the number of base-``radix`` |
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| | | digits in the significand of a float |
318
+---------------------+----------------+--------------------------------------------------+
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| :const:`max` | DBL_MAX | maximum representable finite float |
320
+---------------------+----------------+--------------------------------------------------+
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| :const:`max_exp` | DBL_MAX_EXP | maximum integer e such that ``radix**(e-1)`` is |
322
| | | a representable finite float |
323
+---------------------+----------------+--------------------------------------------------+
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| :const:`max_10_exp` | DBL_MAX_10_EXP | maximum integer e such that ``10**e`` is in the |
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| | | range of representable finite floats |
326
+---------------------+----------------+--------------------------------------------------+
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| :const:`min` | DBL_MIN | minimum positive normalized float |
328
+---------------------+----------------+--------------------------------------------------+
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| :const:`min_exp` | DBL_MIN_EXP | minimum integer e such that ``radix**(e-1)`` is |
330
| | | a normalized float |
331
+---------------------+----------------+--------------------------------------------------+
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| :const:`min_10_exp` | DBL_MIN_10_EXP | minimum integer e such that ``10**e`` is a |
333
| | | normalized float |
334
+---------------------+----------------+--------------------------------------------------+
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| :const:`radix` | FLT_RADIX | radix of exponent representation |
336
+---------------------+----------------+--------------------------------------------------+
337
| :const:`rounds` | FLT_ROUNDS | integer constant representing the rounding mode |
338
| | | used for arithmetic operations. This reflects |
339
| | | the value of the system FLT_ROUNDS macro at |
340
| | | interpreter startup time. See section 5.2.4.2.2 |
341
| | | of the C99 standard for an explanation of the |
342
| | | possible values and their meanings. |
343
+---------------------+----------------+--------------------------------------------------+
345
The attribute :attr:`sys.float_info.dig` needs further explanation. If
346
``s`` is any string representing a decimal number with at most
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:attr:`sys.float_info.dig` significant digits, then converting ``s`` to a
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float and back again will recover a string representing the same decimal
352
>>> sys.float_info.dig
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>>> s = '3.14159265358979' # decimal string with 15 significant digits
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>>> format(float(s), '.15g') # convert to float and back -> same value
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But for strings with more than :attr:`sys.float_info.dig` significant digits,
359
this isn't always true::
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>>> s = '9876543211234567' # 16 significant digits is too many!
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>>> format(float(s), '.16g') # conversion changes value
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.. versionadded:: 2.6
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.. data:: float_repr_style
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A string indicating how the :func:`repr` function behaves for
370
floats. If the string has value ``'short'`` then for a finite
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float ``x``, ``repr(x)`` aims to produce a short string with the
372
property that ``float(repr(x)) == x``. This is the usual behaviour
373
in Python 2.7 and later. Otherwise, ``float_repr_style`` has value
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``'legacy'`` and ``repr(x)`` behaves in the same way as it did in
375
versions of Python prior to 2.7.
377
.. versionadded:: 2.7
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.. function:: getcheckinterval()
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Return the interpreter's "check interval"; see :func:`setcheckinterval`.
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.. versionadded:: 2.3
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.. function:: getdefaultencoding()
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Return the name of the current default string encoding used by the Unicode
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.. versionadded:: 2.0
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.. function:: getdlopenflags()
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Return the current value of the flags that are used for :c:func:`dlopen` calls.
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The flag constants are defined in the :mod:`dl` and :mod:`DLFCN` modules.
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.. versionadded:: 2.2
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.. function:: getfilesystemencoding()
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Return the name of the encoding used to convert Unicode filenames into system
407
file names, or ``None`` if the system default encoding is used. The result value
408
depends on the operating system:
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* On Mac OS X, the encoding is ``'utf-8'``.
412
* On Unix, the encoding is the user's preference according to the result of
413
nl_langinfo(CODESET), or ``None`` if the ``nl_langinfo(CODESET)``
416
* On Windows NT+, file names are Unicode natively, so no conversion is
417
performed. :func:`getfilesystemencoding` still returns ``'mbcs'``, as
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this is the encoding that applications should use when they explicitly
419
want to convert Unicode strings to byte strings that are equivalent when
422
* On Windows 9x, the encoding is ``'mbcs'``.
424
.. versionadded:: 2.3
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.. function:: getrefcount(object)
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Return the reference count of the *object*. The count returned is generally one
430
higher than you might expect, because it includes the (temporary) reference as
431
an argument to :func:`getrefcount`.
434
.. function:: getrecursionlimit()
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Return the current value of the recursion limit, the maximum depth of the Python
437
interpreter stack. This limit prevents infinite recursion from causing an
438
overflow of the C stack and crashing Python. It can be set by
439
:func:`setrecursionlimit`.
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.. function:: getsizeof(object[, default])
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Return the size of an object in bytes. The object can be any type of
445
object. All built-in objects will return correct results, but this
446
does not have to hold true for third-party extensions as it is implementation
449
If given, *default* will be returned if the object does not provide means to
450
retrieve the size. Otherwise a :exc:`TypeError` will be raised.
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:func:`getsizeof` calls the object's ``__sizeof__`` method and adds an
453
additional garbage collector overhead if the object is managed by the garbage
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.. versionadded:: 2.6
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.. function:: _getframe([depth])
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Return a frame object from the call stack. If optional integer *depth* is
462
given, return the frame object that many calls below the top of the stack. If
463
that is deeper than the call stack, :exc:`ValueError` is raised. The default
464
for *depth* is zero, returning the frame at the top of the call stack.
468
This function should be used for internal and specialized purposes only.
469
It is not guaranteed to exist in all implementations of Python.
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.. function:: getprofile()
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single: profile function
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Get the profiler function as set by :func:`setprofile`.
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.. versionadded:: 2.6
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.. function:: gettrace()
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single: trace function
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Get the trace function as set by :func:`settrace`.
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The :func:`gettrace` function is intended only for implementing debuggers,
494
profilers, coverage tools and the like. Its behavior is part of the
495
implementation platform, rather than part of the language definition, and
496
thus may not be available in all Python implementations.
498
.. versionadded:: 2.6
501
.. function:: getwindowsversion()
503
Return a named tuple describing the Windows version
504
currently running. The named elements are *major*, *minor*,
505
*build*, *platform*, *service_pack*, *service_pack_minor*,
506
*service_pack_major*, *suite_mask*, and *product_type*.
507
*service_pack* contains a string while all other values are
508
integers. The components can also be accessed by name, so
509
``sys.getwindowsversion()[0]`` is equivalent to
510
``sys.getwindowsversion().major``. For compatibility with prior
511
versions, only the first 5 elements are retrievable by indexing.
513
*platform* may be one of the following values:
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+-----------------------------------------+-------------------------+
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| Constant | Platform |
517
+=========================================+=========================+
518
| :const:`0 (VER_PLATFORM_WIN32s)` | Win32s on Windows 3.1 |
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+-----------------------------------------+-------------------------+
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| :const:`1 (VER_PLATFORM_WIN32_WINDOWS)` | Windows 95/98/ME |
521
+-----------------------------------------+-------------------------+
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| :const:`2 (VER_PLATFORM_WIN32_NT)` | Windows NT/2000/XP/x64 |
523
+-----------------------------------------+-------------------------+
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| :const:`3 (VER_PLATFORM_WIN32_CE)` | Windows CE |
525
+-----------------------------------------+-------------------------+
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*product_type* may be one of the following values:
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+---------------------------------------+---------------------------------+
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| Constant | Meaning |
531
+=======================================+=================================+
532
| :const:`1 (VER_NT_WORKSTATION)` | The system is a workstation. |
533
+---------------------------------------+---------------------------------+
534
| :const:`2 (VER_NT_DOMAIN_CONTROLLER)` | The system is a domain |
536
+---------------------------------------+---------------------------------+
537
| :const:`3 (VER_NT_SERVER)` | The system is a server, but not |
538
| | a domain controller. |
539
+---------------------------------------+---------------------------------+
542
This function wraps the Win32 :c:func:`GetVersionEx` function; see the
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Microsoft documentation on :c:func:`OSVERSIONINFOEX` for more information
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Availability: Windows.
548
.. versionadded:: 2.3
549
.. versionchanged:: 2.7
550
Changed to a named tuple and added *service_pack_minor*,
551
*service_pack_major*, *suite_mask*, and *product_type*.
556
The version number encoded as a single integer. This is guaranteed to increase
557
with each version, including proper support for non-production releases. For
558
example, to test that the Python interpreter is at least version 1.5.2, use::
560
if sys.hexversion >= 0x010502F0:
561
# use some advanced feature
564
# use an alternative implementation or warn the user
567
This is called ``hexversion`` since it only really looks meaningful when viewed
568
as the result of passing it to the built-in :func:`hex` function. The
569
``version_info`` value may be used for a more human-friendly encoding of the
572
The ``hexversion`` is a 32-bit number with the following layout:
574
+-------------------------+------------------------------------------------+
575
| Bits (big endian order) | Meaning |
576
+=========================+================================================+
577
| :const:`1-8` | ``PY_MAJOR_VERSION`` (the ``2`` in |
579
+-------------------------+------------------------------------------------+
580
| :const:`9-16` | ``PY_MINOR_VERSION`` (the ``1`` in |
582
+-------------------------+------------------------------------------------+
583
| :const:`17-24` | ``PY_MICRO_VERSION`` (the ``0`` in |
585
+-------------------------+------------------------------------------------+
586
| :const:`25-28` | ``PY_RELEASE_LEVEL`` (``0xA`` for alpha, |
587
| | ``0xB`` for beta, ``0xC`` for release |
588
| | candidate and ``0xF`` for final) |
589
+-------------------------+------------------------------------------------+
590
| :const:`29-32` | ``PY_RELEASE_SERIAL`` (the ``3`` in |
591
| | ``2.1.0a3``, zero for final releases) |
592
+-------------------------+------------------------------------------------+
594
Thus ``2.1.0a3`` is hexversion ``0x020100a3``.
596
.. versionadded:: 1.5.2
601
A struct sequence that holds information about Python's
602
internal representation of integers. The attributes are read only.
604
.. tabularcolumns:: |l|L|
606
+-------------------------+----------------------------------------------+
607
| Attribute | Explanation |
608
+=========================+==============================================+
609
| :const:`bits_per_digit` | number of bits held in each digit. Python |
610
| | integers are stored internally in base |
611
| | ``2**long_info.bits_per_digit`` |
612
+-------------------------+----------------------------------------------+
613
| :const:`sizeof_digit` | size in bytes of the C type used to |
614
| | represent a digit |
615
+-------------------------+----------------------------------------------+
617
.. versionadded:: 2.7
624
These three variables are not always defined; they are set when an exception is
625
not handled and the interpreter prints an error message and a stack traceback.
626
Their intended use is to allow an interactive user to import a debugger module
627
and engage in post-mortem debugging without having to re-execute the command
628
that caused the error. (Typical use is ``import pdb; pdb.pm()`` to enter the
629
post-mortem debugger; see chapter :ref:`debugger` for
632
The meaning of the variables is the same as that of the return values from
633
:func:`exc_info` above. (Since there is only one interactive thread,
634
thread-safety is not a concern for these variables, unlike for ``exc_type``
640
The largest positive integer supported by Python's regular integer type. This
641
is at least 2\*\*31-1. The largest negative integer is ``-maxint-1`` --- the
642
asymmetry results from the use of 2's complement binary arithmetic.
646
The largest positive integer supported by the platform's Py_ssize_t type,
647
and thus the maximum size lists, strings, dicts, and many other containers
652
An integer giving the largest supported code point for a Unicode character. The
653
value of this depends on the configuration option that specifies whether Unicode
654
characters are stored as UCS-2 or UCS-4.
659
A list of :term:`finder` objects that have their :meth:`find_module`
660
methods called to see if one of the objects can find the module to be
661
imported. The :meth:`find_module` method is called at least with the
662
absolute name of the module being imported. If the module to be imported is
663
contained in package then the parent package's :attr:`__path__` attribute
664
is passed in as a second argument. The method returns ``None`` if
665
the module cannot be found, else returns a :term:`loader`.
667
:data:`sys.meta_path` is searched before any implicit default finders or
670
See :pep:`302` for the original specification.
675
.. index:: builtin: reload
677
This is a dictionary that maps module names to modules which have already been
678
loaded. This can be manipulated to force reloading of modules and other tricks.
679
Note that removing a module from this dictionary is *not* the same as calling
680
:func:`reload` on the corresponding module object.
685
.. index:: triple: module; search; path
687
A list of strings that specifies the search path for modules. Initialized from
688
the environment variable :envvar:`PYTHONPATH`, plus an installation-dependent
691
As initialized upon program startup, the first item of this list, ``path[0]``,
692
is the directory containing the script that was used to invoke the Python
693
interpreter. If the script directory is not available (e.g. if the interpreter
694
is invoked interactively or if the script is read from standard input),
695
``path[0]`` is the empty string, which directs Python to search modules in the
696
current directory first. Notice that the script directory is inserted *before*
697
the entries inserted as a result of :envvar:`PYTHONPATH`.
699
A program is free to modify this list for its own purposes.
701
.. versionchanged:: 2.3
702
Unicode strings are no longer ignored.
705
Module :mod:`site` This describes how to use .pth files to extend
711
A list of callables that take a path argument to try to create a
712
:term:`finder` for the path. If a finder can be created, it is to be
713
returned by the callable, else raise :exc:`ImportError`.
715
Originally specified in :pep:`302`.
718
.. data:: path_importer_cache
720
A dictionary acting as a cache for :term:`finder` objects. The keys are
721
paths that have been passed to :data:`sys.path_hooks` and the values are
722
the finders that are found. If a path is a valid file system path but no
723
explicit finder is found on :data:`sys.path_hooks` then ``None`` is
724
stored to represent the implicit default finder should be used. If the path
725
is not an existing path then :class:`imp.NullImporter` is set.
727
Originally specified in :pep:`302`.
732
This string contains a platform identifier that can be used to append
733
platform-specific components to :data:`sys.path`, for instance.
735
For most Unix systems, this is the lowercased OS name as returned by ``uname
736
-s`` with the first part of the version as returned by ``uname -r`` appended,
737
e.g. ``'sunos5'``, *at the time when Python was built*. Unless you want to
738
test for a specific system version, it is therefore recommended to use the
741
if sys.platform.startswith('freebsd'):
742
# FreeBSD-specific code here...
743
elif sys.platform.startswith('linux'):
744
# Linux-specific code here...
746
.. versionchanged:: 2.7.3
747
Since lots of code check for ``sys.platform == 'linux2'``, and there is
748
no essential change between Linux 2.x and 3.x, ``sys.platform`` is always
749
set to ``'linux2'``, even on Linux 3.x. In Python 3.3 and later, the
750
value will always be set to ``'linux'``, so it is recommended to always
751
use the ``startswith`` idiom presented above.
753
For other systems, the values are:
755
===================== ===========================
756
System :data:`platform` value
757
===================== ===========================
758
Linux (2.x *and* 3.x) ``'linux2'``
760
Windows/Cygwin ``'cygwin'``
761
Mac OS X ``'darwin'``
763
OS/2 EMX ``'os2emx'``
766
===================== ===========================
769
:attr:`os.name` has a coarser granularity. :func:`os.uname` gives
770
system-dependent version information.
772
The :mod:`platform` module provides detailed checks for the
777
A string giving the site-specific directory prefix where the platform
778
independent Python files are installed; by default, this is the string
779
``'/usr/local'``. This can be set at build time with the ``--prefix``
780
argument to the :program:`configure` script. The main collection of Python
781
library modules is installed in the directory :file:`{prefix}/lib/python{X.Y}`
782
while the platform independent header files (all except :file:`pyconfig.h`) are
783
stored in :file:`{prefix}/include/python{X.Y}`, where *X.Y* is the version
784
number of Python, for example ``2.7``.
791
single: interpreter prompts
792
single: prompts, interpreter
794
Strings specifying the primary and secondary prompt of the interpreter. These
795
are only defined if the interpreter is in interactive mode. Their initial
796
values in this case are ``'>>> '`` and ``'... '``. If a non-string object is
797
assigned to either variable, its :func:`str` is re-evaluated each time the
798
interpreter prepares to read a new interactive command; this can be used to
799
implement a dynamic prompt.
802
.. data:: py3kwarning
804
Bool containing the status of the Python 3 warning flag. It's ``True``
805
when Python is started with the -3 option. (This should be considered
806
read-only; setting it to a different value doesn't have an effect on
809
.. versionadded:: 2.6
812
.. function:: setcheckinterval(interval)
814
Set the interpreter's "check interval". This integer value determines how often
815
the interpreter checks for periodic things such as thread switches and signal
816
handlers. The default is ``100``, meaning the check is performed every 100
817
Python virtual instructions. Setting it to a larger value may increase
818
performance for programs using threads. Setting it to a value ``<=`` 0 checks
819
every virtual instruction, maximizing responsiveness as well as overhead.
822
.. function:: setdefaultencoding(name)
824
Set the current default string encoding used by the Unicode implementation. If
825
*name* does not match any available encoding, :exc:`LookupError` is raised.
826
This function is only intended to be used by the :mod:`site` module
827
implementation and, where needed, by :mod:`sitecustomize`. Once used by the
828
:mod:`site` module, it is removed from the :mod:`sys` module's namespace.
830
.. Note that :mod:`site` is not imported if the :option:`-S` option is passed
831
to the interpreter, in which case this function will remain available.
833
.. versionadded:: 2.0
836
.. function:: setdlopenflags(n)
838
Set the flags used by the interpreter for :c:func:`dlopen` calls, such as when
839
the interpreter loads extension modules. Among other things, this will enable a
840
lazy resolving of symbols when importing a module, if called as
841
``sys.setdlopenflags(0)``. To share symbols across extension modules, call as
842
``sys.setdlopenflags(dl.RTLD_NOW | dl.RTLD_GLOBAL)``. Symbolic names for the
843
flag modules can be either found in the :mod:`dl` module, or in the :mod:`DLFCN`
844
module. If :mod:`DLFCN` is not available, it can be generated from
845
:file:`/usr/include/dlfcn.h` using the :program:`h2py` script. Availability:
848
.. versionadded:: 2.2
851
.. function:: setprofile(profilefunc)
854
single: profile function
857
Set the system's profile function, which allows you to implement a Python source
858
code profiler in Python. See chapter :ref:`profile` for more information on the
859
Python profiler. The system's profile function is called similarly to the
860
system's trace function (see :func:`settrace`), but it isn't called for each
861
executed line of code (only on call and return, but the return event is reported
862
even when an exception has been set). The function is thread-specific, but
863
there is no way for the profiler to know about context switches between threads,
864
so it does not make sense to use this in the presence of multiple threads. Also,
865
its return value is not used, so it can simply return ``None``.
868
.. function:: setrecursionlimit(limit)
870
Set the maximum depth of the Python interpreter stack to *limit*. This limit
871
prevents infinite recursion from causing an overflow of the C stack and crashing
874
The highest possible limit is platform-dependent. A user may need to set the
875
limit higher when she has a program that requires deep recursion and a platform
876
that supports a higher limit. This should be done with care, because a too-high
877
limit can lead to a crash.
880
.. function:: settrace(tracefunc)
883
single: trace function
886
Set the system's trace function, which allows you to implement a Python
887
source code debugger in Python. The function is thread-specific; for a
888
debugger to support multiple threads, it must be registered using
889
:func:`settrace` for each thread being debugged.
891
Trace functions should have three arguments: *frame*, *event*, and
892
*arg*. *frame* is the current stack frame. *event* is a string: ``'call'``,
893
``'line'``, ``'return'``, ``'exception'``, ``'c_call'``, ``'c_return'``, or
894
``'c_exception'``. *arg* depends on the event type.
896
The trace function is invoked (with *event* set to ``'call'``) whenever a new
897
local scope is entered; it should return a reference to a local trace
898
function to be used that scope, or ``None`` if the scope shouldn't be traced.
900
The local trace function should return a reference to itself (or to another
901
function for further tracing in that scope), or ``None`` to turn off tracing
904
The events have the following meaning:
907
A function is called (or some other code block entered). The
908
global trace function is called; *arg* is ``None``; the return value
909
specifies the local trace function.
912
The interpreter is about to execute a new line of code or re-execute the
913
condition of a loop. The local trace function is called; *arg* is
914
``None``; the return value specifies the new local trace function. See
915
:file:`Objects/lnotab_notes.txt` for a detailed explanation of how this
919
A function (or other code block) is about to return. The local trace
920
function is called; *arg* is the value that will be returned, or ``None``
921
if the event is caused by an exception being raised. The trace function's
922
return value is ignored.
925
An exception has occurred. The local trace function is called; *arg* is a
926
tuple ``(exception, value, traceback)``; the return value specifies the
927
new local trace function.
930
A C function is about to be called. This may be an extension function or
931
a built-in. *arg* is the C function object.
934
A C function has returned. *arg* is the C function object.
937
A C function has raised an exception. *arg* is the C function object.
939
Note that as an exception is propagated down the chain of callers, an
940
``'exception'`` event is generated at each level.
942
For more information on code and frame objects, refer to :ref:`types`.
946
The :func:`settrace` function is intended only for implementing debuggers,
947
profilers, coverage tools and the like. Its behavior is part of the
948
implementation platform, rather than part of the language definition, and
949
thus may not be available in all Python implementations.
952
.. function:: settscdump(on_flag)
954
Activate dumping of VM measurements using the Pentium timestamp counter, if
955
*on_flag* is true. Deactivate these dumps if *on_flag* is off. The function is
956
available only if Python was compiled with ``--with-tsc``. To understand
957
the output of this dump, read :file:`Python/ceval.c` in the Python sources.
959
.. versionadded:: 2.4
963
This function is intimately bound to CPython implementation details and
964
thus not likely to be implemented elsewhere.
975
File objects corresponding to the interpreter's standard input, output and error
976
streams. ``stdin`` is used for all interpreter input except for scripts but
977
including calls to :func:`input` and :func:`raw_input`. ``stdout`` is used for
978
the output of :keyword:`print` and :term:`expression` statements and for the
979
prompts of :func:`input` and :func:`raw_input`. The interpreter's own prompts
980
and (almost all of) its error messages go to ``stderr``. ``stdout`` and
981
``stderr`` needn't be built-in file objects: any object is acceptable as long
982
as it has a :meth:`write` method that takes a string argument. (Changing these
983
objects doesn't affect the standard I/O streams of processes executed by
984
:func:`os.popen`, :func:`os.system` or the :func:`exec\*` family of functions in
985
the :mod:`os` module.)
992
These objects contain the original values of ``stdin``, ``stderr`` and
993
``stdout`` at the start of the program. They are used during finalization,
994
and could be useful to print to the actual standard stream no matter if the
995
``sys.std*`` object has been redirected.
997
It can also be used to restore the actual files to known working file objects
998
in case they have been overwritten with a broken object. However, the
999
preferred way to do this is to explicitly save the previous stream before
1000
replacing it, and restore the saved object.
1003
.. data:: subversion
1005
A triple (repo, branch, version) representing the Subversion information of the
1006
Python interpreter. *repo* is the name of the repository, ``'CPython'``.
1007
*branch* is a string of one of the forms ``'trunk'``, ``'branches/name'`` or
1008
``'tags/name'``. *version* is the output of ``svnversion``, if the interpreter
1009
was built from a Subversion checkout; it contains the revision number (range)
1010
and possibly a trailing 'M' if there were local modifications. If the tree was
1011
exported (or svnversion was not available), it is the revision of
1012
``Include/patchlevel.h`` if the branch is a tag. Otherwise, it is ``None``.
1014
.. versionadded:: 2.5
1017
Python is now `developed <https://docs.python.org/devguide/>`_ using
1018
Mercurial. In recent Python 2.7 bugfix releases, :data:`subversion`
1019
therefore contains placeholder information. It is removed in Python
1023
.. data:: tracebacklimit
1025
When this variable is set to an integer value, it determines the maximum number
1026
of levels of traceback information printed when an unhandled exception occurs.
1027
The default is ``1000``. When set to ``0`` or less, all traceback information
1028
is suppressed and only the exception type and value are printed.
1033
A string containing the version number of the Python interpreter plus additional
1034
information on the build number and compiler used. This string is displayed
1035
when the interactive interpreter is started. Do not extract version information
1036
out of it, rather, use :data:`version_info` and the functions provided by the
1037
:mod:`platform` module.
1040
.. data:: api_version
1042
The C API version for this interpreter. Programmers may find this useful when
1043
debugging version conflicts between Python and extension modules.
1045
.. versionadded:: 2.3
1048
.. data:: version_info
1050
A tuple containing the five components of the version number: *major*, *minor*,
1051
*micro*, *releaselevel*, and *serial*. All values except *releaselevel* are
1052
integers; the release level is ``'alpha'``, ``'beta'``, ``'candidate'``, or
1053
``'final'``. The ``version_info`` value corresponding to the Python version 2.0
1054
is ``(2, 0, 0, 'final', 0)``. The components can also be accessed by name,
1055
so ``sys.version_info[0]`` is equivalent to ``sys.version_info.major``
1058
.. versionadded:: 2.0
1059
.. versionchanged:: 2.7
1060
Added named component attributes
1063
.. data:: warnoptions
1065
This is an implementation detail of the warnings framework; do not modify this
1066
value. Refer to the :mod:`warnings` module for more information on the warnings
1072
The version number used to form registry keys on Windows platforms. This is
1073
stored as string resource 1000 in the Python DLL. The value is normally the
1074
first three characters of :const:`version`. It is provided in the :mod:`sys`
1075
module for informational purposes; modifying this value has no effect on the
1076
registry keys used by Python. Availability: Windows.
1078
.. rubric:: Citations
1080
.. [C99] ISO/IEC 9899:1999. "Programming languages -- C." A public draft of this standard is available at http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf\ .
added
removed
python-2.7.11-docs-html/_sources/library/sys.txt
