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Georg Brandl116aa622007-08-15 14:28:22 +00001:mod:`sys` --- System-specific parameters and functions
2=======================================================
3
4.. module:: sys
5 :synopsis: Access system-specific parameters and functions.
6
7
8This module provides access to some variables used or maintained by the
9interpreter and to functions that interact strongly with the interpreter. It is
10always available.
11
12
13.. data:: argv
14
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.
20
21 To loop over the standard input, or the list of files given on the
22 command line, see the :mod:`fileinput` module.
23
24
25.. data:: byteorder
26
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.
30
Georg Brandl116aa622007-08-15 14:28:22 +000031
32.. data:: subversion
33
34 A triple (repo, branch, version) representing the Subversion information of the
35 Python interpreter. *repo* is the name of the repository, ``'CPython'``.
36 *branch* is a string of one of the forms ``'trunk'``, ``'branches/name'`` or
37 ``'tags/name'``. *version* is the output of ``svnversion``, if the interpreter
38 was built from a Subversion checkout; it contains the revision number (range)
39 and possibly a trailing 'M' if there were local modifications. If the tree was
40 exported (or svnversion was not available), it is the revision of
41 ``Include/patchlevel.h`` if the branch is a tag. Otherwise, it is ``None``.
42
Georg Brandl116aa622007-08-15 14:28:22 +000043
44.. data:: builtin_module_names
45
46 A tuple of strings giving the names of all modules that are compiled into this
47 Python interpreter. (This information is not available in any other way ---
48 ``modules.keys()`` only lists the imported modules.)
49
50
51.. data:: copyright
52
53 A string containing the copyright pertaining to the Python interpreter.
54
55
Christian Heimes15ebc882008-02-04 18:48:49 +000056.. function:: _clear_type_cache()
57
58 Clear the internal type cache. The type cache is used to speed up attribute
59 and method lookups. Use the function *only* to drop unnecessary references
60 during reference leak debugging.
61
62 This function should be used for internal and specialized purposes only.
Christian Heimes26855632008-01-27 23:50:43 +000063
Christian Heimes26855632008-01-27 23:50:43 +000064
Georg Brandl116aa622007-08-15 14:28:22 +000065.. function:: _current_frames()
66
67 Return a dictionary mapping each thread's identifier to the topmost stack frame
68 currently active in that thread at the time the function is called. Note that
69 functions in the :mod:`traceback` module can build the call stack given such a
70 frame.
71
72 This is most useful for debugging deadlock: this function does not require the
73 deadlocked threads' cooperation, and such threads' call stacks are frozen for as
74 long as they remain deadlocked. The frame returned for a non-deadlocked thread
75 may bear no relationship to that thread's current activity by the time calling
76 code examines the frame.
77
78 This function should be used for internal and specialized purposes only.
79
Georg Brandl116aa622007-08-15 14:28:22 +000080
81.. data:: dllhandle
82
83 Integer specifying the handle of the Python DLL. Availability: Windows.
84
85
86.. function:: displayhook(value)
87
88 If *value* is not ``None``, this function prints it to ``sys.stdout``, and saves
Georg Brandl1a3284e2007-12-02 09:40:06 +000089 it in ``builtins._``.
Georg Brandl116aa622007-08-15 14:28:22 +000090
Christian Heimesd8654cf2007-12-02 15:22:16 +000091 ``sys.displayhook`` is called on the result of evaluating an :term:`expression`
92 entered in an interactive Python session. The display of these values can be
93 customized by assigning another one-argument function to ``sys.displayhook``.
Georg Brandl116aa622007-08-15 14:28:22 +000094
95
96.. function:: excepthook(type, value, traceback)
97
98 This function prints out a given traceback and exception to ``sys.stderr``.
99
100 When an exception is raised and uncaught, the interpreter calls
101 ``sys.excepthook`` with three arguments, the exception class, exception
102 instance, and a traceback object. In an interactive session this happens just
103 before control is returned to the prompt; in a Python program this happens just
104 before the program exits. The handling of such top-level exceptions can be
105 customized by assigning another three-argument function to ``sys.excepthook``.
106
107
108.. data:: __displayhook__
109 __excepthook__
110
111 These objects contain the original values of ``displayhook`` and ``excepthook``
112 at the start of the program. They are saved so that ``displayhook`` and
113 ``excepthook`` can be restored in case they happen to get replaced with broken
114 objects.
115
116
117.. function:: exc_info()
118
119 This function returns a tuple of three values that give information about the
120 exception that is currently being handled. The information returned is specific
121 both to the current thread and to the current stack frame. If the current stack
122 frame is not handling an exception, the information is taken from the calling
123 stack frame, or its caller, and so on until a stack frame is found that is
124 handling an exception. Here, "handling an exception" is defined as "executing
Benjamin Petersoneec3d712008-06-11 15:59:43 +0000125 an except clause." For any stack frame, only information about the exception
126 being currently handled is accessible.
Georg Brandl116aa622007-08-15 14:28:22 +0000127
128 .. index:: object: traceback
129
130 If no exception is being handled anywhere on the stack, a tuple containing three
131 ``None`` values is returned. Otherwise, the values returned are ``(type, value,
132 traceback)``. Their meaning is: *type* gets the exception type of the exception
133 being handled (a class object); *value* gets the exception parameter (its
134 :dfn:`associated value` or the second argument to :keyword:`raise`, which is
135 always a class instance if the exception type is a class object); *traceback*
136 gets a traceback object (see the Reference Manual) which encapsulates the call
137 stack at the point where the exception originally occurred.
138
139 .. warning::
140
Georg Brandle6bcc912008-05-12 18:05:20 +0000141 Assigning the *traceback* return value to a local variable in a function
142 that is handling an exception will cause a circular reference. Since most
143 functions don't need access to the traceback, the best solution is to use
144 something like ``exctype, value = sys.exc_info()[:2]`` to extract only the
145 exception type and value. If you do need the traceback, make sure to
146 delete it after use (best done with a :keyword:`try`
147 ... :keyword:`finally` statement) or to call :func:`exc_info` in a
148 function that does not itself handle an exception.
Georg Brandl116aa622007-08-15 14:28:22 +0000149
Georg Brandle6bcc912008-05-12 18:05:20 +0000150 Such cycles are normally automatically reclaimed when garbage collection
151 is enabled and they become unreachable, but it remains more efficient to
152 avoid creating cycles.
Georg Brandl116aa622007-08-15 14:28:22 +0000153
154
155.. data:: exec_prefix
156
157 A string giving the site-specific directory prefix where the platform-dependent
158 Python files are installed; by default, this is also ``'/usr/local'``. This can
159 be set at build time with the :option:`--exec-prefix` argument to the
160 :program:`configure` script. Specifically, all configuration files (e.g. the
161 :file:`pyconfig.h` header file) are installed in the directory ``exec_prefix +
162 '/lib/pythonversion/config'``, and shared library modules are installed in
163 ``exec_prefix + '/lib/pythonversion/lib-dynload'``, where *version* is equal to
164 ``version[:3]``.
165
166
167.. data:: executable
168
169 A string giving the name of the executable binary for the Python interpreter, on
170 systems where this makes sense.
171
172
173.. function:: exit([arg])
174
175 Exit from Python. This is implemented by raising the :exc:`SystemExit`
176 exception, so cleanup actions specified by finally clauses of :keyword:`try`
177 statements are honored, and it is possible to intercept the exit attempt at an
178 outer level. The optional argument *arg* can be an integer giving the exit
179 status (defaulting to zero), or another type of object. If it is an integer,
180 zero is considered "successful termination" and any nonzero value is considered
181 "abnormal termination" by shells and the like. Most systems require it to be in
182 the range 0-127, and produce undefined results otherwise. Some systems have a
183 convention for assigning specific meanings to specific exit codes, but these are
184 generally underdeveloped; Unix programs generally use 2 for command line syntax
185 errors and 1 for all other kind of errors. If another type of object is passed,
186 ``None`` is equivalent to passing zero, and any other object is printed to
187 ``sys.stderr`` and results in an exit code of 1. In particular,
188 ``sys.exit("some error message")`` is a quick way to exit a program when an
189 error occurs.
190
191
Christian Heimesd32ed6f2008-01-14 18:49:24 +0000192.. data:: flags
193
194 The struct sequence *flags* exposes the status of command line flags. The
195 attributes are read only.
196
197 +------------------------------+------------------------------------------+
198 | attribute | flag |
199 +==============================+==========================================+
200 | :const:`debug` | -d |
201 +------------------------------+------------------------------------------+
202 | :const:`py3k_warning` | -3 |
203 +------------------------------+------------------------------------------+
204 | :const:`division_warning` | -Q |
205 +------------------------------+------------------------------------------+
206 | :const:`division_new` | -Qnew |
207 +------------------------------+------------------------------------------+
208 | :const:`inspect` | -i |
209 +------------------------------+------------------------------------------+
210 | :const:`interactive` | -i |
211 +------------------------------+------------------------------------------+
212 | :const:`optimize` | -O or -OO |
213 +------------------------------+------------------------------------------+
214 | :const:`dont_write_bytecode` | -B |
215 +------------------------------+------------------------------------------+
216 | :const:`no_site` | -S |
217 +------------------------------+------------------------------------------+
Guido van Rossum7736b5b2008-01-15 21:44:53 +0000218 | :const:`ignore_environment` | -E |
Christian Heimesd32ed6f2008-01-14 18:49:24 +0000219 +------------------------------+------------------------------------------+
Christian Heimesd32ed6f2008-01-14 18:49:24 +0000220 | :const:`verbose` | -v |
221 +------------------------------+------------------------------------------+
222 | :const:`unicode` | -U |
223 +------------------------------+------------------------------------------+
224
Christian Heimesd32ed6f2008-01-14 18:49:24 +0000225
Christian Heimes93852662007-12-01 12:22:32 +0000226.. data:: float_info
227
Christian Heimesd32ed6f2008-01-14 18:49:24 +0000228 A structseq holding information about the float type. It contains low level
Christian Heimes93852662007-12-01 12:22:32 +0000229 information about the precision and internal representation. Please study
230 your system's :file:`float.h` for more information.
231
232 +---------------------+--------------------------------------------------+
Christian Heimesd32ed6f2008-01-14 18:49:24 +0000233 | attribute | explanation |
Christian Heimes93852662007-12-01 12:22:32 +0000234 +=====================+==================================================+
235 | :const:`epsilon` | Difference between 1 and the next representable |
236 | | floating point number |
237 +---------------------+--------------------------------------------------+
238 | :const:`dig` | digits (see :file:`float.h`) |
239 +---------------------+--------------------------------------------------+
240 | :const:`mant_dig` | mantissa digits (see :file:`float.h`) |
241 +---------------------+--------------------------------------------------+
242 | :const:`max` | maximum representable finite float |
243 +---------------------+--------------------------------------------------+
244 | :const:`max_exp` | maximum int e such that radix**(e-1) is in the |
245 | | range of finite representable floats |
246 +---------------------+--------------------------------------------------+
247 | :const:`max_10_exp` | maximum int e such that 10**e is in the |
248 | | range of finite representable floats |
249 +---------------------+--------------------------------------------------+
250 | :const:`min` | Minimum positive normalizer float |
251 +---------------------+--------------------------------------------------+
252 | :const:`min_exp` | minimum int e such that radix**(e-1) is a |
253 | | normalized float |
254 +---------------------+--------------------------------------------------+
255 | :const:`min_10_exp` | minimum int e such that 10**e is a normalized |
256 | | float |
257 +---------------------+--------------------------------------------------+
258 | :const:`radix` | radix of exponent |
259 +---------------------+--------------------------------------------------+
260 | :const:`rounds` | addition rounds (see :file:`float.h`) |
261 +---------------------+--------------------------------------------------+
262
263 .. note::
264
265 The information in the table is simplified.
266
267
Mark Dickinsonb08a53a2009-04-16 19:52:09 +0000268.. data:: float_repr_style
269
270 A string indicating how the :func:`repr` function behaves for
271 floats. If the string has value ``'short'`` then for a finite
272 float ``x``, ``repr(x)`` aims to produce a short string with the
273 property that ``float(repr(x)) == x``. This is the usual behaviour
274 in Python 3.1 and later. Otherwise, ``float_repr_style`` has value
275 ``'legacy'`` and ``repr(x)`` behaves in the same way as it did in
276 versions of Python prior to 3.1.
277
278 .. versionadded:: 3.1
279
280
Georg Brandl116aa622007-08-15 14:28:22 +0000281.. function:: getcheckinterval()
282
283 Return the interpreter's "check interval"; see :func:`setcheckinterval`.
284
Georg Brandl116aa622007-08-15 14:28:22 +0000285
286.. function:: getdefaultencoding()
287
288 Return the name of the current default string encoding used by the Unicode
289 implementation.
290
Georg Brandl116aa622007-08-15 14:28:22 +0000291
292.. function:: getdlopenflags()
293
294 Return the current value of the flags that are used for :cfunc:`dlopen` calls.
Neal Norwitz6cf49cf2008-03-24 06:22:57 +0000295 The flag constants are defined in the :mod:`ctypes` and :mod:`DLFCN` modules.
Georg Brandl116aa622007-08-15 14:28:22 +0000296 Availability: Unix.
297
Georg Brandl116aa622007-08-15 14:28:22 +0000298
299.. function:: getfilesystemencoding()
300
301 Return the name of the encoding used to convert Unicode filenames into system
302 file names, or ``None`` if the system default encoding is used. The result value
303 depends on the operating system:
304
305 * On Windows 9x, the encoding is "mbcs".
306
307 * On Mac OS X, the encoding is "utf-8".
308
309 * On Unix, the encoding is the user's preference according to the result of
310 nl_langinfo(CODESET), or :const:`None` if the ``nl_langinfo(CODESET)`` failed.
311
312 * On Windows NT+, file names are Unicode natively, so no conversion is
313 performed. :func:`getfilesystemencoding` still returns ``'mbcs'``, as this is
314 the encoding that applications should use when they explicitly want to convert
315 Unicode strings to byte strings that are equivalent when used as file names.
316
Georg Brandl116aa622007-08-15 14:28:22 +0000317
318.. function:: getrefcount(object)
319
320 Return the reference count of the *object*. The count returned is generally one
321 higher than you might expect, because it includes the (temporary) reference as
322 an argument to :func:`getrefcount`.
323
324
325.. function:: getrecursionlimit()
326
327 Return the current value of the recursion limit, the maximum depth of the Python
328 interpreter stack. This limit prevents infinite recursion from causing an
329 overflow of the C stack and crashing Python. It can be set by
330 :func:`setrecursionlimit`.
331
332
Robert Schuppeniesfbe94c52008-07-14 10:13:31 +0000333.. function:: getsizeof(object[, default])
Martin v. Löwis00709aa2008-06-04 14:18:43 +0000334
335 Return the size of an object in bytes. The object can be any type of
336 object. All built-in objects will return correct results, but this
Robert Schuppeniesfbe94c52008-07-14 10:13:31 +0000337 does not have to hold true for third-party extensions as it is implementation
Martin v. Löwis00709aa2008-06-04 14:18:43 +0000338 specific.
339
Robert Schuppeniesfbe94c52008-07-14 10:13:31 +0000340 The *default* argument allows to define a value which will be returned
341 if the object type does not provide means to retrieve the size and would
Georg Brandl48310cd2009-01-03 21:18:54 +0000342 cause a `TypeError`.
Robert Schuppeniesfbe94c52008-07-14 10:13:31 +0000343
Brett Cannon2df81132009-09-22 20:08:09 +0000344 :func:`getsizeof` calls the object's __sizeof__ method and adds an additional
Robert Schuppeniesfbe94c52008-07-14 10:13:31 +0000345 garbage collector overhead if the object is managed by the garbage collector.
346
Martin v. Löwis00709aa2008-06-04 14:18:43 +0000347
Georg Brandl116aa622007-08-15 14:28:22 +0000348.. function:: _getframe([depth])
349
350 Return a frame object from the call stack. If optional integer *depth* is
351 given, return the frame object that many calls below the top of the stack. If
352 that is deeper than the call stack, :exc:`ValueError` is raised. The default
353 for *depth* is zero, returning the frame at the top of the call stack.
354
355 This function should be used for internal and specialized purposes only.
356
357
Christian Heimes9bd667a2008-01-20 15:14:11 +0000358.. function:: getprofile()
359
360 .. index::
361 single: profile function
362 single: profiler
363
364 Get the profiler function as set by :func:`setprofile`.
365
Christian Heimes9bd667a2008-01-20 15:14:11 +0000366
367.. function:: gettrace()
368
369 .. index::
370 single: trace function
371 single: debugger
372
373 Get the trace function as set by :func:`settrace`.
374
375 .. note::
376
377 The :func:`gettrace` function is intended only for implementing debuggers,
378 profilers, coverage tools and the like. Its behavior is part of the
379 implementation platform, rather than part of the language definition,
380 and thus may not be available in all Python implementations.
381
Christian Heimes9bd667a2008-01-20 15:14:11 +0000382
Georg Brandl116aa622007-08-15 14:28:22 +0000383.. function:: getwindowsversion()
384
385 Return a tuple containing five components, describing the Windows version
386 currently running. The elements are *major*, *minor*, *build*, *platform*, and
387 *text*. *text* contains a string while all other values are integers.
388
389 *platform* may be one of the following values:
390
Christian Heimes81ee3ef2008-05-04 22:42:01 +0000391 +-----------------------------------------+-------------------------+
392 | Constant | Platform |
393 +=========================================+=========================+
394 | :const:`0 (VER_PLATFORM_WIN32s)` | Win32s on Windows 3.1 |
395 +-----------------------------------------+-------------------------+
396 | :const:`1 (VER_PLATFORM_WIN32_WINDOWS)` | Windows 95/98/ME |
397 +-----------------------------------------+-------------------------+
398 | :const:`2 (VER_PLATFORM_WIN32_NT)` | Windows NT/2000/XP/x64 |
399 +-----------------------------------------+-------------------------+
400 | :const:`3 (VER_PLATFORM_WIN32_CE)` | Windows CE |
401 +-----------------------------------------+-------------------------+
Georg Brandl116aa622007-08-15 14:28:22 +0000402
403 This function wraps the Win32 :cfunc:`GetVersionEx` function; see the Microsoft
404 documentation for more information about these fields.
405
406 Availability: Windows.
407
Georg Brandl116aa622007-08-15 14:28:22 +0000408
409.. data:: hexversion
410
411 The version number encoded as a single integer. This is guaranteed to increase
412 with each version, including proper support for non-production releases. For
413 example, to test that the Python interpreter is at least version 1.5.2, use::
414
415 if sys.hexversion >= 0x010502F0:
416 # use some advanced feature
417 ...
418 else:
419 # use an alternative implementation or warn the user
420 ...
421
422 This is called ``hexversion`` since it only really looks meaningful when viewed
423 as the result of passing it to the built-in :func:`hex` function. The
424 ``version_info`` value may be used for a more human-friendly encoding of the
425 same information.
426
Georg Brandl116aa622007-08-15 14:28:22 +0000427
Mark Dickinsonbd792642009-03-18 20:06:12 +0000428.. data:: int_info
429
430 A struct sequence that holds information about Python's
431 internal representation of integers. The attributes are read only.
432
433 +-------------------------+----------------------------------------------+
434 | attribute | explanation |
435 +=========================+==============================================+
436 | :const:`bits_per_digit` | number of bits held in each digit. Python |
437 | | integers are stored internally in base |
438 | | ``2**int_info.bits_per_digit`` |
439 +-------------------------+----------------------------------------------+
440 | :const:`sizeof_digit` | size in bytes of the C type used to |
441 | | represent a digit |
442 +-------------------------+----------------------------------------------+
443
Mark Dickinsond72c7b62009-03-20 16:00:49 +0000444 .. versionadded:: 3.1
445
Mark Dickinsonbd792642009-03-18 20:06:12 +0000446
Georg Brandl116aa622007-08-15 14:28:22 +0000447.. function:: intern(string)
448
449 Enter *string* in the table of "interned" strings and return the interned string
450 -- which is *string* itself or a copy. Interning strings is useful to gain a
451 little performance on dictionary lookup -- if the keys in a dictionary are
452 interned, and the lookup key is interned, the key comparisons (after hashing)
453 can be done by a pointer compare instead of a string compare. Normally, the
454 names used in Python programs are automatically interned, and the dictionaries
455 used to hold module, class or instance attributes have interned keys.
456
Georg Brandl55ac8f02007-09-01 13:51:09 +0000457 Interned strings are not immortal; you must keep a reference to the return
458 value of :func:`intern` around to benefit from it.
Georg Brandl116aa622007-08-15 14:28:22 +0000459
460
461.. data:: last_type
462 last_value
463 last_traceback
464
465 These three variables are not always defined; they are set when an exception is
466 not handled and the interpreter prints an error message and a stack traceback.
467 Their intended use is to allow an interactive user to import a debugger module
468 and engage in post-mortem debugging without having to re-execute the command
469 that caused the error. (Typical use is ``import pdb; pdb.pm()`` to enter the
470 post-mortem debugger; see chapter :ref:`debugger` for
471 more information.)
472
473 The meaning of the variables is the same as that of the return values from
474 :func:`exc_info` above. (Since there is only one interactive thread,
475 thread-safety is not a concern for these variables, unlike for ``exc_type``
476 etc.)
477
478
Christian Heimesa37d4c62007-12-04 23:02:19 +0000479.. data:: maxsize
Georg Brandl116aa622007-08-15 14:28:22 +0000480
Georg Brandl33770552007-12-15 09:55:35 +0000481 An integer giving the maximum value a variable of type :ctype:`Py_ssize_t` can
482 take. It's usually ``2**31 - 1`` on a 32-bit platform and ``2**63 - 1`` on a
483 64-bit platform.
Christian Heimesa37d4c62007-12-04 23:02:19 +0000484
Georg Brandl116aa622007-08-15 14:28:22 +0000485
486.. data:: maxunicode
487
488 An integer giving the largest supported code point for a Unicode character. The
489 value of this depends on the configuration option that specifies whether Unicode
490 characters are stored as UCS-2 or UCS-4.
491
492
Brett Cannone43b0602009-03-21 03:11:16 +0000493.. data:: meta_path
494
495 A list of :term:`finder` objects that have their :meth:`find_module`
496 methods called to see if one of the objects can find the module to be
497 imported. The :meth:`find_module` method is called at least with the
498 absolute name of the module being imported. If the module to be imported is
499 contained in package then the parent package's :attr:`__path__` attribute
500 is passed in as a second argument. The method returns :keyword:`None` if
501 the module cannot be found, else returns a :term:`loader`.
502
503 :data:`sys.meta_path` is searched before any implicit default finders or
504 :data:`sys.path`.
505
506 See :pep:`302` for the original specification.
507
508
Georg Brandl116aa622007-08-15 14:28:22 +0000509.. data:: modules
510
511 This is a dictionary that maps module names to modules which have already been
512 loaded. This can be manipulated to force reloading of modules and other tricks.
513
514
515.. data:: path
516
517 .. index:: triple: module; search; path
518
519 A list of strings that specifies the search path for modules. Initialized from
520 the environment variable :envvar:`PYTHONPATH`, plus an installation-dependent
521 default.
522
523 As initialized upon program startup, the first item of this list, ``path[0]``,
524 is the directory containing the script that was used to invoke the Python
525 interpreter. If the script directory is not available (e.g. if the interpreter
526 is invoked interactively or if the script is read from standard input),
527 ``path[0]`` is the empty string, which directs Python to search modules in the
528 current directory first. Notice that the script directory is inserted *before*
529 the entries inserted as a result of :envvar:`PYTHONPATH`.
530
531 A program is free to modify this list for its own purposes.
532
Georg Brandl116aa622007-08-15 14:28:22 +0000533
Benjamin Peterson058e31e2009-01-16 03:54:08 +0000534 .. seealso::
535 Module :mod:`site` This describes how to use .pth files to extend
536 :data:`sys.path`.
537
538
Brett Cannone43b0602009-03-21 03:11:16 +0000539.. data:: path_hooks
540
541 A list of callables that take a path argument to try to create a
542 :term:`finder` for the path. If a finder can be created, it is to be
543 returned by the callable, else raise :exc:`ImportError`.
544
545 Originally specified in :pep:`302`.
546
547
548.. data:: path_importer_cache
549
550 A dictionary acting as a cache for :term:`finder` objects. The keys are
551 paths that have been passed to :data:`sys.path_hooks` and the values are
552 the finders that are found. If a path is a valid file system path but no
553 explicit finder is found on :data:`sys.path_hooks` then :keyword:`None` is
554 stored to represent the implicit default finder should be used. If the path
555 is not an existing path then :class:`imp.NullImporter` is set.
556
557 Originally specified in :pep:`302`.
558
559
Georg Brandl116aa622007-08-15 14:28:22 +0000560.. data:: platform
561
Christian Heimes9bd667a2008-01-20 15:14:11 +0000562 This string contains a platform identifier that can be used to append
563 platform-specific components to :data:`sys.path`, for instance.
564
565 For Unix systems, this is the lowercased OS name as returned by ``uname -s``
566 with the first part of the version as returned by ``uname -r`` appended,
567 e.g. ``'sunos5'`` or ``'linux2'``, *at the time when Python was built*.
568 For other systems, the values are:
569
570 ================ ===========================
571 System :data:`platform` value
572 ================ ===========================
573 Windows ``'win32'``
574 Windows/Cygwin ``'cygwin'``
Georg Brandlc575c902008-09-13 17:46:05 +0000575 Mac OS X ``'darwin'``
Christian Heimes9bd667a2008-01-20 15:14:11 +0000576 OS/2 ``'os2'``
577 OS/2 EMX ``'os2emx'``
Christian Heimes9bd667a2008-01-20 15:14:11 +0000578 AtheOS ``'atheos'``
579 ================ ===========================
Georg Brandl116aa622007-08-15 14:28:22 +0000580
581
582.. data:: prefix
583
584 A string giving the site-specific directory prefix where the platform
585 independent Python files are installed; by default, this is the string
586 ``'/usr/local'``. This can be set at build time with the :option:`--prefix`
587 argument to the :program:`configure` script. The main collection of Python
588 library modules is installed in the directory ``prefix + '/lib/pythonversion'``
589 while the platform independent header files (all except :file:`pyconfig.h`) are
590 stored in ``prefix + '/include/pythonversion'``, where *version* is equal to
591 ``version[:3]``.
592
593
594.. data:: ps1
595 ps2
596
597 .. index::
598 single: interpreter prompts
599 single: prompts, interpreter
600
601 Strings specifying the primary and secondary prompt of the interpreter. These
602 are only defined if the interpreter is in interactive mode. Their initial
603 values in this case are ``'>>> '`` and ``'... '``. If a non-string object is
604 assigned to either variable, its :func:`str` is re-evaluated each time the
605 interpreter prepares to read a new interactive command; this can be used to
606 implement a dynamic prompt.
607
608
Christian Heimes790c8232008-01-07 21:14:23 +0000609.. data:: dont_write_bytecode
610
611 If this is true, Python won't try to write ``.pyc`` or ``.pyo`` files on the
612 import of source modules. This value is initially set to ``True`` or ``False``
613 depending on the ``-B`` command line option and the ``PYTHONDONTWRITEBYTECODE``
614 environment variable, but you can set it yourself to control bytecode file
615 generation.
616
Christian Heimes790c8232008-01-07 21:14:23 +0000617
Georg Brandl116aa622007-08-15 14:28:22 +0000618.. function:: setcheckinterval(interval)
619
620 Set the interpreter's "check interval". This integer value determines how often
621 the interpreter checks for periodic things such as thread switches and signal
622 handlers. The default is ``100``, meaning the check is performed every 100
623 Python virtual instructions. Setting it to a larger value may increase
624 performance for programs using threads. Setting it to a value ``<=`` 0 checks
625 every virtual instruction, maximizing responsiveness as well as overhead.
626
627
628.. function:: setdefaultencoding(name)
629
630 Set the current default string encoding used by the Unicode implementation. If
631 *name* does not match any available encoding, :exc:`LookupError` is raised.
632 This function is only intended to be used by the :mod:`site` module
633 implementation and, where needed, by :mod:`sitecustomize`. Once used by the
634 :mod:`site` module, it is removed from the :mod:`sys` module's namespace.
635
Christian Heimes5b5e81c2007-12-31 16:14:33 +0000636 .. Note that :mod:`site` is not imported if the :option:`-S` option is passed
637 to the interpreter, in which case this function will remain available.
Georg Brandl116aa622007-08-15 14:28:22 +0000638
Georg Brandl116aa622007-08-15 14:28:22 +0000639
640.. function:: setdlopenflags(n)
641
642 Set the flags used by the interpreter for :cfunc:`dlopen` calls, such as when
643 the interpreter loads extension modules. Among other things, this will enable a
644 lazy resolving of symbols when importing a module, if called as
645 ``sys.setdlopenflags(0)``. To share symbols across extension modules, call as
Neal Norwitz6cf49cf2008-03-24 06:22:57 +0000646 ``sys.setdlopenflags(ctypes.RTLD_GLOBAL)``. Symbolic names for the
647 flag modules can be either found in the :mod:`ctypes` module, or in the :mod:`DLFCN`
Georg Brandl116aa622007-08-15 14:28:22 +0000648 module. If :mod:`DLFCN` is not available, it can be generated from
649 :file:`/usr/include/dlfcn.h` using the :program:`h2py` script. Availability:
650 Unix.
651
Martin v. Löwis04dc25c2008-10-03 16:09:28 +0000652.. function:: setfilesystemencoding(enc)
653
654 Set the encoding used when converting Python strings to file names to *enc*.
655 By default, Python tries to determine the encoding it should use automatically
656 on Unix; on Windows, it avoids such conversion completely. This function can
657 be used when Python's determination of the encoding needs to be overwritten,
658 e.g. when not all file names on disk can be decoded using the encoding that
659 Python had chosen.
Georg Brandl116aa622007-08-15 14:28:22 +0000660
661.. function:: setprofile(profilefunc)
662
663 .. index::
664 single: profile function
665 single: profiler
666
667 Set the system's profile function, which allows you to implement a Python source
668 code profiler in Python. See chapter :ref:`profile` for more information on the
669 Python profiler. The system's profile function is called similarly to the
670 system's trace function (see :func:`settrace`), but it isn't called for each
671 executed line of code (only on call and return, but the return event is reported
672 even when an exception has been set). The function is thread-specific, but
673 there is no way for the profiler to know about context switches between threads,
674 so it does not make sense to use this in the presence of multiple threads. Also,
675 its return value is not used, so it can simply return ``None``.
676
677
678.. function:: setrecursionlimit(limit)
679
680 Set the maximum depth of the Python interpreter stack to *limit*. This limit
681 prevents infinite recursion from causing an overflow of the C stack and crashing
682 Python.
683
684 The highest possible limit is platform-dependent. A user may need to set the
685 limit higher when she has a program that requires deep recursion and a platform
686 that supports a higher limit. This should be done with care, because a too-high
687 limit can lead to a crash.
688
689
690.. function:: settrace(tracefunc)
691
692 .. index::
693 single: trace function
694 single: debugger
695
696 Set the system's trace function, which allows you to implement a Python
Amaury Forgeot d'Arcb0c29162008-11-22 22:18:04 +0000697 source code debugger in Python. The function is thread-specific; for a
Georg Brandl116aa622007-08-15 14:28:22 +0000698 debugger to support multiple threads, it must be registered using
699 :func:`settrace` for each thread being debugged.
700
Amaury Forgeot d'Arcb0c29162008-11-22 22:18:04 +0000701 Trace functions should have three arguments: *frame*, *event*, and
702 *arg*. *frame* is the current stack frame. *event* is a string: ``'call'``,
703 ``'line'``, ``'return'``, ``'exception'``, ``'c_call'``, ``'c_return'``, or
704 ``'c_exception'``. *arg* depends on the event type.
705
706 The trace function is invoked (with *event* set to ``'call'``) whenever a new
707 local scope is entered; it should return a reference to a local trace
708 function to be used that scope, or ``None`` if the scope shouldn't be traced.
709
710 The local trace function should return a reference to itself (or to another
711 function for further tracing in that scope), or ``None`` to turn off tracing
712 in that scope.
713
714 The events have the following meaning:
715
Georg Brandl48310cd2009-01-03 21:18:54 +0000716 ``'call'``
Amaury Forgeot d'Arcb0c29162008-11-22 22:18:04 +0000717 A function is called (or some other code block entered). The
718 global trace function is called; *arg* is ``None``; the return value
719 specifies the local trace function.
720
721 ``'line'``
Alexandre Vassalotti7b82b402009-07-21 04:30:03 +0000722 The interpreter is about to execute a new line of code or re-execute the
723 condition of a loop. The local trace function is called; *arg* is
724 ``None``; the return value specifies the new local trace function. See
725 :file:`Objects/lnotab_notes.txt` for a detailed explanation of how this
726 works.
Amaury Forgeot d'Arcb0c29162008-11-22 22:18:04 +0000727
728 ``'return'``
729 A function (or other code block) is about to return. The local trace
730 function is called; *arg* is the value that will be returned. The trace
731 function's return value is ignored.
732
733 ``'exception'``
734 An exception has occurred. The local trace function is called; *arg* is a
735 tuple ``(exception, value, traceback)``; the return value specifies the
736 new local trace function.
737
738 ``'c_call'``
739 A C function is about to be called. This may be an extension function or
Georg Brandl22b34312009-07-26 14:54:51 +0000740 a built-in. *arg* is the C function object.
Amaury Forgeot d'Arcb0c29162008-11-22 22:18:04 +0000741
742 ``'c_return'``
743 A C function has returned. *arg* is ``None``.
744
745 ``'c_exception'``
746 A C function has thrown an exception. *arg* is ``None``.
747
748 Note that as an exception is propagated down the chain of callers, an
749 ``'exception'`` event is generated at each level.
750
751 For more information on code and frame objects, refer to :ref:`types`.
752
Georg Brandl116aa622007-08-15 14:28:22 +0000753 .. note::
754
755 The :func:`settrace` function is intended only for implementing debuggers,
756 profilers, coverage tools and the like. Its behavior is part of the
757 implementation platform, rather than part of the language definition, and thus
758 may not be available in all Python implementations.
759
760
761.. function:: settscdump(on_flag)
762
763 Activate dumping of VM measurements using the Pentium timestamp counter, if
764 *on_flag* is true. Deactivate these dumps if *on_flag* is off. The function is
765 available only if Python was compiled with :option:`--with-tsc`. To understand
766 the output of this dump, read :file:`Python/ceval.c` in the Python sources.
767
Georg Brandl116aa622007-08-15 14:28:22 +0000768
769.. data:: stdin
770 stdout
771 stderr
772
773 File objects corresponding to the interpreter's standard input, output and error
Christian Heimesd8654cf2007-12-02 15:22:16 +0000774 streams. ``stdin`` is used for all interpreter input except for scripts but
775 including calls to :func:`input`. ``stdout`` is used for
776 the output of :func:`print` and :term:`expression` statements and for the
777 prompts of :func:`input`. The interpreter's own prompts
778 and (almost all of) its error messages go to ``stderr``. ``stdout`` and
779 ``stderr`` needn't be built-in file objects: any object is acceptable as long
Georg Brandl48310cd2009-01-03 21:18:54 +0000780 as it has a :meth:`write` method that takes a string argument. (Changing these
Christian Heimesd8654cf2007-12-02 15:22:16 +0000781 objects doesn't affect the standard I/O streams of processes executed by
782 :func:`os.popen`, :func:`os.system` or the :func:`exec\*` family of functions in
783 the :mod:`os` module.)
Georg Brandl116aa622007-08-15 14:28:22 +0000784
Benjamin Peterson3261fa52009-05-12 03:01:51 +0000785 The standard streams are in text mode by default. To write or read binary
786 data to these, use the underlying binary buffer. For example, to write bytes
787 to :data:`stdout`, use ``sys.stdout.buffer.write(b'abc')``. Using
Benjamin Peterson995bb472009-06-14 18:41:18 +0000788 :meth:`io.TextIOBase.detach` streams can be made binary by default. This
789 function sets :data:`stdin` and :data:`stdout` to binary::
Benjamin Peterson4199d602009-05-12 20:47:57 +0000790
791 def make_streams_binary():
792 sys.stdin = sys.stdin.detach()
Benjamin Peterson4487f532009-05-13 21:15:03 +0000793 sys.stdout = sys.stdout.detach()
Benjamin Peterson995bb472009-06-14 18:41:18 +0000794
795 Note that the streams can be replaced with objects (like
796 :class:`io.StringIO`) that do not support the
797 :attr:`~io.BufferedIOBase.buffer` attribute or the
798 :meth:`~io.BufferedIOBase.detach` method and can raise :exc:`AttributeError`
799 or :exc:`io.UnsupportedOperation`.
Benjamin Petersoneb9fc522008-12-07 14:58:03 +0000800
Georg Brandl116aa622007-08-15 14:28:22 +0000801
802.. data:: __stdin__
803 __stdout__
804 __stderr__
805
806 These objects contain the original values of ``stdin``, ``stderr`` and
Benjamin Petersond23f8222009-04-05 19:13:16 +0000807 ``stdout`` at the start of the program. They are used during finalization,
808 and could be useful to print to the actual standard stream no matter if the
809 ``sys.std*`` object has been redirected.
Georg Brandl116aa622007-08-15 14:28:22 +0000810
Benjamin Petersond23f8222009-04-05 19:13:16 +0000811 It can also be used to restore the actual files to known working file objects
812 in case they have been overwritten with a broken object. However, the
813 preferred way to do this is to explicitly save the previous stream before
814 replacing it, and restore the saved object.
Christian Heimes58cb1b82007-11-13 02:19:40 +0000815
Benjamin Petersond23f8222009-04-05 19:13:16 +0000816 .. note::
817 Under some conditions ``stdin``, ``stdout`` and ``stderr`` as well as the
818 original values ``__stdin__``, ``__stdout__`` and ``__stderr__`` can be
819 None. It is usually the case for Windows GUI apps that aren't connected
820 to a console and Python apps started with :program:`pythonw`.
Christian Heimes58cb1b82007-11-13 02:19:40 +0000821
Georg Brandl116aa622007-08-15 14:28:22 +0000822
823.. data:: tracebacklimit
824
825 When this variable is set to an integer value, it determines the maximum number
826 of levels of traceback information printed when an unhandled exception occurs.
827 The default is ``1000``. When set to ``0`` or less, all traceback information
828 is suppressed and only the exception type and value are printed.
829
830
831.. data:: version
832
833 A string containing the version number of the Python interpreter plus additional
834 information on the build number and compiler used. It has a value of the form
835 ``'version (#build_number, build_date, build_time) [compiler]'``. The first
836 three characters are used to identify the version in the installation
837 directories (where appropriate on each platform). An example::
838
839 >>> import sys
840 >>> sys.version
841 '1.5.2 (#0 Apr 13 1999, 10:51:12) [MSC 32 bit (Intel)]'
842
843
844.. data:: api_version
845
846 The C API version for this interpreter. Programmers may find this useful when
847 debugging version conflicts between Python and extension modules.
848
Georg Brandl116aa622007-08-15 14:28:22 +0000849
850.. data:: version_info
851
852 A tuple containing the five components of the version number: *major*, *minor*,
853 *micro*, *releaselevel*, and *serial*. All values except *releaselevel* are
854 integers; the release level is ``'alpha'``, ``'beta'``, ``'candidate'``, or
855 ``'final'``. The ``version_info`` value corresponding to the Python version 2.0
Eric Smith0e5b5622009-02-06 01:32:42 +0000856 is ``(2, 0, 0, 'final', 0)``. The components can also be accessed by name,
857 so ``sys.version_info[0]`` is equivalent to ``sys.version_info.major``
858 and so on.
Georg Brandl116aa622007-08-15 14:28:22 +0000859
Raymond Hettinger35a88362009-04-09 00:08:24 +0000860 .. versionchanged:: 3.1
Eric Smith0e5b5622009-02-06 01:32:42 +0000861 Added named component attributes
Georg Brandl116aa622007-08-15 14:28:22 +0000862
863.. data:: warnoptions
864
865 This is an implementation detail of the warnings framework; do not modify this
866 value. Refer to the :mod:`warnings` module for more information on the warnings
867 framework.
868
869
870.. data:: winver
871
872 The version number used to form registry keys on Windows platforms. This is
873 stored as string resource 1000 in the Python DLL. The value is normally the
874 first three characters of :const:`version`. It is provided in the :mod:`sys`
875 module for informational purposes; modifying this value has no effect on the
876 registry keys used by Python. Availability: Windows.