2 :mod:`sys` --- System-specific parameters and functions
3 =======================================================
6 :synopsis: Access system-specific parameters and functions.
9 This module provides access to some variables used or maintained by the
10 interpreter and to functions that interact strongly with the interpreter. It is
16 The list of command line arguments passed to a Python script. ``argv[0]`` is the
17 script name (it is operating system dependent whether this is a full pathname or
18 not). If the command was executed using the :option:`-c` command line option to
19 the interpreter, ``argv[0]`` is set to the string ``'-c'``. If no script name
20 was passed to the Python interpreter, ``argv[0]`` is the empty string.
22 To loop over the standard input, or the list of files given on the
23 command line, see the :mod:`fileinput` module.
28 An indicator of the native byte order. This will have the value ``'big'`` on
29 big-endian (most-significant byte first) platforms, and ``'little'`` on
30 little-endian (least-significant byte first) platforms.
37 A triple (repo, branch, version) representing the Subversion information of the
38 Python interpreter. *repo* is the name of the repository, ``'CPython'``.
39 *branch* is a string of one of the forms ``'trunk'``, ``'branches/name'`` or
40 ``'tags/name'``. *version* is the output of ``svnversion``, if the interpreter
41 was built from a Subversion checkout; it contains the revision number (range)
42 and possibly a trailing 'M' if there were local modifications. If the tree was
43 exported (or svnversion was not available), it is the revision of
44 ``Include/patchlevel.h`` if the branch is a tag. Otherwise, it is ``None``.
49 .. data:: builtin_module_names
51 A tuple of strings giving the names of all modules that are compiled into this
52 Python interpreter. (This information is not available in any other way ---
53 ``modules.keys()`` only lists the imported modules.)
58 A string containing the copyright pertaining to the Python interpreter.
61 .. function:: _clear_type_cache()
63 Clear the internal type cache. The type cache is used to speed up attribute
64 and method lookups. Use the function *only* to drop unnecessary references
65 during reference leak debugging.
67 This function should be used for internal and specialized purposes only.
72 .. function:: _current_frames()
74 Return a dictionary mapping each thread's identifier to the topmost stack frame
75 currently active in that thread at the time the function is called. Note that
76 functions in the :mod:`traceback` module can build the call stack given such a
79 This is most useful for debugging deadlock: this function does not require the
80 deadlocked threads' cooperation, and such threads' call stacks are frozen for as
81 long as they remain deadlocked. The frame returned for a non-deadlocked thread
82 may bear no relationship to that thread's current activity by the time calling
83 code examines the frame.
85 This function should be used for internal and specialized purposes only.
92 Integer specifying the handle of the Python DLL. Availability: Windows.
95 .. function:: displayhook(value)
97 If *value* is not ``None``, this function prints it to ``sys.stdout``, and saves
98 it in ``__builtin__._``.
100 ``sys.displayhook`` is called on the result of evaluating an :term:`expression`
101 entered in an interactive Python session. The display of these values can be
102 customized by assigning another one-argument function to ``sys.displayhook``.
105 .. function:: excepthook(type, value, traceback)
107 This function prints out a given traceback and exception to ``sys.stderr``.
109 When an exception is raised and uncaught, the interpreter calls
110 ``sys.excepthook`` with three arguments, the exception class, exception
111 instance, and a traceback object. In an interactive session this happens just
112 before control is returned to the prompt; in a Python program this happens just
113 before the program exits. The handling of such top-level exceptions can be
114 customized by assigning another three-argument function to ``sys.excepthook``.
117 .. data:: __displayhook__
120 These objects contain the original values of ``displayhook`` and ``excepthook``
121 at the start of the program. They are saved so that ``displayhook`` and
122 ``excepthook`` can be restored in case they happen to get replaced with broken
126 .. function:: exc_info()
128 This function returns a tuple of three values that give information about the
129 exception that is currently being handled. The information returned is specific
130 both to the current thread and to the current stack frame. If the current stack
131 frame is not handling an exception, the information is taken from the calling
132 stack frame, or its caller, and so on until a stack frame is found that is
133 handling an exception. Here, "handling an exception" is defined as "executing
134 or having executed an except clause." For any stack frame, only information
135 about the most recently handled exception is accessible.
137 .. index:: object: traceback
139 If no exception is being handled anywhere on the stack, a tuple containing three
140 ``None`` values is returned. Otherwise, the values returned are ``(type, value,
141 traceback)``. Their meaning is: *type* gets the exception type of the exception
142 being handled (a class object); *value* gets the exception parameter (its
143 :dfn:`associated value` or the second argument to :keyword:`raise`, which is
144 always a class instance if the exception type is a class object); *traceback*
145 gets a traceback object (see the Reference Manual) which encapsulates the call
146 stack at the point where the exception originally occurred.
148 If :func:`exc_clear` is called, this function will return three ``None`` values
149 until either another exception is raised in the current thread or the execution
150 stack returns to a frame where another exception is being handled.
154 Assigning the *traceback* return value to a local variable in a function that is
155 handling an exception will cause a circular reference. This will prevent
156 anything referenced by a local variable in the same function or by the traceback
157 from being garbage collected. Since most functions don't need access to the
158 traceback, the best solution is to use something like ``exctype, value =
159 sys.exc_info()[:2]`` to extract only the exception type and value. If you do
160 need the traceback, make sure to delete it after use (best done with a
161 :keyword:`try` ... :keyword:`finally` statement) or to call :func:`exc_info` in
162 a function that does not itself handle an exception.
166 Beginning with Python 2.2, such cycles are automatically reclaimed when garbage
167 collection is enabled and they become unreachable, but it remains more efficient
168 to avoid creating cycles.
171 .. function:: exc_clear()
173 This function clears all information relating to the current or last exception
174 that occurred in the current thread. After calling this function,
175 :func:`exc_info` will return three ``None`` values until another exception is
176 raised in the current thread or the execution stack returns to a frame where
177 another exception is being handled.
179 This function is only needed in only a few obscure situations. These include
180 logging and error handling systems that report information on the last or
181 current exception. This function can also be used to try to free resources and
182 trigger object finalization, though no guarantee is made as to what objects will
185 .. versionadded:: 2.3
193 Use :func:`exc_info` instead.
195 Since they are global variables, they are not specific to the current thread, so
196 their use is not safe in a multi-threaded program. When no exception is being
197 handled, ``exc_type`` is set to ``None`` and the other two are undefined.
200 .. data:: exec_prefix
202 A string giving the site-specific directory prefix where the platform-dependent
203 Python files are installed; by default, this is also ``'/usr/local'``. This can
204 be set at build time with the :option:`--exec-prefix` argument to the
205 :program:`configure` script. Specifically, all configuration files (e.g. the
206 :file:`pyconfig.h` header file) are installed in the directory ``exec_prefix +
207 '/lib/pythonversion/config'``, and shared library modules are installed in
208 ``exec_prefix + '/lib/pythonversion/lib-dynload'``, where *version* is equal to
214 A string giving the name of the executable binary for the Python interpreter, on
215 systems where this makes sense.
218 .. function:: exit([arg])
220 Exit from Python. This is implemented by raising the :exc:`SystemExit`
221 exception, so cleanup actions specified by finally clauses of :keyword:`try`
222 statements are honored, and it is possible to intercept the exit attempt at an
223 outer level. The optional argument *arg* can be an integer giving the exit
224 status (defaulting to zero), or another type of object. If it is an integer,
225 zero is considered "successful termination" and any nonzero value is considered
226 "abnormal termination" by shells and the like. Most systems require it to be in
227 the range 0-127, and produce undefined results otherwise. Some systems have a
228 convention for assigning specific meanings to specific exit codes, but these are
229 generally underdeveloped; Unix programs generally use 2 for command line syntax
230 errors and 1 for all other kind of errors. If another type of object is passed,
231 ``None`` is equivalent to passing zero, and any other object is printed to
232 ``sys.stderr`` and results in an exit code of 1. In particular,
233 ``sys.exit("some error message")`` is a quick way to exit a program when an
239 This value is not actually defined by the module, but can be set by the user (or
240 by a program) to specify a clean-up action at program exit. When set, it should
241 be a parameterless function. This function will be called when the interpreter
242 exits. Only one function may be installed in this way; to allow multiple
243 functions which will be called at termination, use the :mod:`atexit` module.
247 The exit function is not called when the program is killed by a signal, when a
248 Python fatal internal error is detected, or when ``os._exit()`` is called.
251 Use :mod:`atexit` instead.
256 The struct sequence *flags* exposes the status of command line flags. The
257 attributes are read only.
259 +------------------------------+------------------------------------------+
261 +==============================+==========================================+
262 | :const:`debug` | -d |
263 +------------------------------+------------------------------------------+
264 | :const:`py3k_warning` | -3 |
265 +------------------------------+------------------------------------------+
266 | :const:`division_warning` | -Q |
267 +------------------------------+------------------------------------------+
268 | :const:`division_new` | -Qnew |
269 +------------------------------+------------------------------------------+
270 | :const:`inspect` | -i |
271 +------------------------------+------------------------------------------+
272 | :const:`interactive` | -i |
273 +------------------------------+------------------------------------------+
274 | :const:`optimize` | -O or -OO |
275 +------------------------------+------------------------------------------+
276 | :const:`dont_write_bytecode` | -B |
277 +------------------------------+------------------------------------------+
278 | :const:`no_site` | -S |
279 +------------------------------+------------------------------------------+
280 | :const:`ignore_environment` | -E |
281 +------------------------------+------------------------------------------+
282 | :const:`tabcheck` | -t or -tt |
283 +------------------------------+------------------------------------------+
284 | :const:`verbose` | -v |
285 +------------------------------+------------------------------------------+
286 | :const:`unicode` | -U |
287 +------------------------------+------------------------------------------+
289 .. versionadded:: 2.6
294 A structseq holding information about the float type. It contains low level
295 information about the precision and internal representation. Please study
296 your system's :file:`float.h` for more information.
298 +---------------------+--------------------------------------------------+
299 | attribute | explanation |
300 +=====================+==================================================+
301 | :const:`epsilon` | Difference between 1 and the next representable |
302 | | floating point number |
303 +---------------------+--------------------------------------------------+
304 | :const:`dig` | digits (see :file:`float.h`) |
305 +---------------------+--------------------------------------------------+
306 | :const:`mant_dig` | mantissa digits (see :file:`float.h`) |
307 +---------------------+--------------------------------------------------+
308 | :const:`max` | maximum representable finite float |
309 +---------------------+--------------------------------------------------+
310 | :const:`max_exp` | maximum int e such that radix**(e-1) is in the |
311 | | range of finite representable floats |
312 +---------------------+--------------------------------------------------+
313 | :const:`max_10_exp` | maximum int e such that 10**e is in the |
314 | | range of finite representable floats |
315 +---------------------+--------------------------------------------------+
316 | :const:`min` | Minimum positive normalizer float |
317 +---------------------+--------------------------------------------------+
318 | :const:`min_exp` | minimum int e such that radix**(e-1) is a |
319 | | normalized float |
320 +---------------------+--------------------------------------------------+
321 | :const:`min_10_exp` | minimum int e such that 10**e is a normalized |
323 +---------------------+--------------------------------------------------+
324 | :const:`radix` | radix of exponent |
325 +---------------------+--------------------------------------------------+
326 | :const:`rounds` | addition rounds (see :file:`float.h`) |
327 +---------------------+--------------------------------------------------+
331 The information in the table is simplified.
333 .. versionadded:: 2.6
335 .. data:: float_repr_style
337 A string indicating how the :func:`repr` function behaves for
338 floats. If the string has value ``'short'`` then for a finite
339 float ``x``, ``repr(x)`` aims to produce a short string with the
340 property that ``float(repr(x)) == x``. This is the usual behaviour
341 in Python 2.7 and later. Otherwise, ``float_repr_style`` has value
342 ``'legacy'`` and ``repr(x)`` behaves in the same way as it did in
343 versions of Python prior to 2.7.
345 .. versionadded:: 2.7
348 .. function:: getcheckinterval()
350 Return the interpreter's "check interval"; see :func:`setcheckinterval`.
352 .. versionadded:: 2.3
355 .. function:: getdefaultencoding()
357 Return the name of the current default string encoding used by the Unicode
360 .. versionadded:: 2.0
363 .. function:: getdlopenflags()
365 Return the current value of the flags that are used for :cfunc:`dlopen` calls.
366 The flag constants are defined in the :mod:`dl` and :mod:`DLFCN` modules.
369 .. versionadded:: 2.2
372 .. function:: getfilesystemencoding()
374 Return the name of the encoding used to convert Unicode filenames into system
375 file names, or ``None`` if the system default encoding is used. The result value
376 depends on the operating system:
378 * On Windows 9x, the encoding is "mbcs".
380 * On Mac OS X, the encoding is "utf-8".
382 * On Unix, the encoding is the user's preference according to the result of
383 nl_langinfo(CODESET), or :const:`None` if the ``nl_langinfo(CODESET)`` failed.
385 * On Windows NT+, file names are Unicode natively, so no conversion is
386 performed. :func:`getfilesystemencoding` still returns ``'mbcs'``, as this is
387 the encoding that applications should use when they explicitly want to convert
388 Unicode strings to byte strings that are equivalent when used as file names.
390 .. versionadded:: 2.3
393 .. function:: getrefcount(object)
395 Return the reference count of the *object*. The count returned is generally one
396 higher than you might expect, because it includes the (temporary) reference as
397 an argument to :func:`getrefcount`.
400 .. function:: getrecursionlimit()
402 Return the current value of the recursion limit, the maximum depth of the Python
403 interpreter stack. This limit prevents infinite recursion from causing an
404 overflow of the C stack and crashing Python. It can be set by
405 :func:`setrecursionlimit`.
408 .. function:: getsizeof(object[, default])
410 Return the size of an object in bytes. The object can be any type of
411 object. All built-in objects will return correct results, but this
412 does not have to hold true for third-party extensions as it is implementation
415 If given, *default* will be returned if the object does not provide means to
416 retrieve the size. Otherwise a `TypeError` will be raised.
418 :func:`getsizeof` calls the object's ``__sizeof__`` method and adds an
419 additional garbage collector overhead if the object is managed by the garbage
422 .. versionadded:: 2.6
425 .. function:: _getframe([depth])
427 Return a frame object from the call stack. If optional integer *depth* is
428 given, return the frame object that many calls below the top of the stack. If
429 that is deeper than the call stack, :exc:`ValueError` is raised. The default
430 for *depth* is zero, returning the frame at the top of the call stack.
434 This function should be used for internal and specialized purposes only.
435 It is not guaranteed to exist in all implementations of Python.
438 .. function:: getprofile()
441 single: profile function
444 Get the profiler function as set by :func:`setprofile`.
446 .. versionadded:: 2.6
449 .. function:: gettrace()
452 single: trace function
455 Get the trace function as set by :func:`settrace`.
459 The :func:`gettrace` function is intended only for implementing debuggers,
460 profilers, coverage tools and the like. Its behavior is part of the
461 implementation platform, rather than part of the language definition, and
462 thus may not be available in all Python implementations.
464 .. versionadded:: 2.6
467 .. function:: getwindowsversion()
469 Return a tuple containing five components, describing the Windows version
470 currently running. The elements are *major*, *minor*, *build*, *platform*, and
471 *text*. *text* contains a string while all other values are integers.
473 *platform* may be one of the following values:
475 +-----------------------------------------+-------------------------+
476 | Constant | Platform |
477 +=========================================+=========================+
478 | :const:`0 (VER_PLATFORM_WIN32s)` | Win32s on Windows 3.1 |
479 +-----------------------------------------+-------------------------+
480 | :const:`1 (VER_PLATFORM_WIN32_WINDOWS)` | Windows 95/98/ME |
481 +-----------------------------------------+-------------------------+
482 | :const:`2 (VER_PLATFORM_WIN32_NT)` | Windows NT/2000/XP/x64 |
483 +-----------------------------------------+-------------------------+
484 | :const:`3 (VER_PLATFORM_WIN32_CE)` | Windows CE |
485 +-----------------------------------------+-------------------------+
487 This function wraps the Win32 :cfunc:`GetVersionEx` function; see the Microsoft
488 documentation for more information about these fields.
490 Availability: Windows.
492 .. versionadded:: 2.3
497 The version number encoded as a single integer. This is guaranteed to increase
498 with each version, including proper support for non-production releases. For
499 example, to test that the Python interpreter is at least version 1.5.2, use::
501 if sys.hexversion >= 0x010502F0:
502 # use some advanced feature
505 # use an alternative implementation or warn the user
508 This is called ``hexversion`` since it only really looks meaningful when viewed
509 as the result of passing it to the built-in :func:`hex` function. The
510 ``version_info`` value may be used for a more human-friendly encoding of the
513 .. versionadded:: 1.5.2
518 A struct sequence that holds information about Python's
519 internal representation of integers. The attributes are read only.
521 +-------------------------+----------------------------------------------+
522 | attribute | explanation |
523 +=========================+==============================================+
524 | :const:`bits_per_digit` | number of bits held in each digit. Python |
525 | | integers are stored internally in base |
526 | | ``2**long_info.bits_per_digit`` |
527 +-------------------------+----------------------------------------------+
528 | :const:`sizeof_digit` | size in bytes of the C type used to |
529 | | represent a digit |
530 +-------------------------+----------------------------------------------+
532 .. versionadded:: 2.7
539 These three variables are not always defined; they are set when an exception is
540 not handled and the interpreter prints an error message and a stack traceback.
541 Their intended use is to allow an interactive user to import a debugger module
542 and engage in post-mortem debugging without having to re-execute the command
543 that caused the error. (Typical use is ``import pdb; pdb.pm()`` to enter the
544 post-mortem debugger; see chapter :ref:`debugger` for
547 The meaning of the variables is the same as that of the return values from
548 :func:`exc_info` above. (Since there is only one interactive thread,
549 thread-safety is not a concern for these variables, unlike for ``exc_type``
555 The largest positive integer supported by Python's regular integer type. This
556 is at least 2\*\*31-1. The largest negative integer is ``-maxint-1`` --- the
557 asymmetry results from the use of 2's complement binary arithmetic.
561 The largest positive integer supported by the platform's Py_ssize_t type,
562 and thus the maximum size lists, strings, dicts, and many other containers
567 An integer giving the largest supported code point for a Unicode character. The
568 value of this depends on the configuration option that specifies whether Unicode
569 characters are stored as UCS-2 or UCS-4.
574 A list of :term:`finder` objects that have their :meth:`find_module`
575 methods called to see if one of the objects can find the module to be
576 imported. The :meth:`find_module` method is called at least with the
577 absolute name of the module being imported. If the module to be imported is
578 contained in package then the parent package's :attr:`__path__` attribute
579 is passed in as a second argument. The method returns :keyword:`None` if
580 the module cannot be found, else returns a :term:`loader`.
582 :data:`sys.meta_path` is searched before any implicit default finders or
585 See :pep:`302` for the original specification.
590 .. index:: builtin: reload
592 This is a dictionary that maps module names to modules which have already been
593 loaded. This can be manipulated to force reloading of modules and other tricks.
594 Note that removing a module from this dictionary is *not* the same as calling
595 :func:`reload` on the corresponding module object.
600 .. index:: triple: module; search; path
602 A list of strings that specifies the search path for modules. Initialized from
603 the environment variable :envvar:`PYTHONPATH`, plus an installation-dependent
606 As initialized upon program startup, the first item of this list, ``path[0]``,
607 is the directory containing the script that was used to invoke the Python
608 interpreter. If the script directory is not available (e.g. if the interpreter
609 is invoked interactively or if the script is read from standard input),
610 ``path[0]`` is the empty string, which directs Python to search modules in the
611 current directory first. Notice that the script directory is inserted *before*
612 the entries inserted as a result of :envvar:`PYTHONPATH`.
614 A program is free to modify this list for its own purposes.
616 .. versionchanged:: 2.3
617 Unicode strings are no longer ignored.
620 Module :mod:`site` This describes how to use .pth files to extend
626 A list of callables that take a path argument to try to create a
627 :term:`finder` for the path. If a finder can be created, it is to be
628 returned by the callable, else raise :exc:`ImportError`.
630 Originally specified in :pep:`302`.
633 .. data:: path_importer_cache
635 A dictionary acting as a cache for :term:`finder` objects. The keys are
636 paths that have been passed to :data:`sys.path_hooks` and the values are
637 the finders that are found. If a path is a valid file system path but no
638 explicit finder is found on :data:`sys.path_hooks` then :keyword:`None` is
639 stored to represent the implicit default finder should be used. If the path
640 is not an existing path then :class:`imp.NullImporter` is set.
642 Originally specified in :pep:`302`.
647 This string contains a platform identifier that can be used to append
648 platform-specific components to :data:`sys.path`, for instance.
650 For Unix systems, this is the lowercased OS name as returned by ``uname -s``
651 with the first part of the version as returned by ``uname -r`` appended,
652 e.g. ``'sunos5'`` or ``'linux2'``, *at the time when Python was built*.
653 For other systems, the values are:
655 ================ ===========================
656 System :data:`platform` value
657 ================ ===========================
659 Windows/Cygwin ``'cygwin'``
660 Mac OS X ``'darwin'``
662 OS/2 EMX ``'os2emx'``
665 ================ ===========================
670 A string giving the site-specific directory prefix where the platform
671 independent Python files are installed; by default, this is the string
672 ``'/usr/local'``. This can be set at build time with the :option:`--prefix`
673 argument to the :program:`configure` script. The main collection of Python
674 library modules is installed in the directory ``prefix + '/lib/pythonversion'``
675 while the platform independent header files (all except :file:`pyconfig.h`) are
676 stored in ``prefix + '/include/pythonversion'``, where *version* is equal to
684 single: interpreter prompts
685 single: prompts, interpreter
687 Strings specifying the primary and secondary prompt of the interpreter. These
688 are only defined if the interpreter is in interactive mode. Their initial
689 values in this case are ``'>>> '`` and ``'... '``. If a non-string object is
690 assigned to either variable, its :func:`str` is re-evaluated each time the
691 interpreter prepares to read a new interactive command; this can be used to
692 implement a dynamic prompt.
695 .. data:: py3kwarning
697 Bool containing the status of the Python 3.0 warning flag. It's ``True``
698 when Python is started with the -3 option. (This should be considered
699 read-only; setting it to a different value doesn't have an effect on
700 Python 3.0 warnings.)
702 .. versionadded:: 2.6
705 .. data:: dont_write_bytecode
707 If this is true, Python won't try to write ``.pyc`` or ``.pyo`` files on the
708 import of source modules. This value is initially set to ``True`` or ``False``
709 depending on the ``-B`` command line option and the ``PYTHONDONTWRITEBYTECODE``
710 environment variable, but you can set it yourself to control bytecode file
713 .. versionadded:: 2.6
716 .. function:: setcheckinterval(interval)
718 Set the interpreter's "check interval". This integer value determines how often
719 the interpreter checks for periodic things such as thread switches and signal
720 handlers. The default is ``100``, meaning the check is performed every 100
721 Python virtual instructions. Setting it to a larger value may increase
722 performance for programs using threads. Setting it to a value ``<=`` 0 checks
723 every virtual instruction, maximizing responsiveness as well as overhead.
726 .. function:: setdefaultencoding(name)
728 Set the current default string encoding used by the Unicode implementation. If
729 *name* does not match any available encoding, :exc:`LookupError` is raised.
730 This function is only intended to be used by the :mod:`site` module
731 implementation and, where needed, by :mod:`sitecustomize`. Once used by the
732 :mod:`site` module, it is removed from the :mod:`sys` module's namespace.
734 .. Note that :mod:`site` is not imported if the :option:`-S` option is passed
735 to the interpreter, in which case this function will remain available.
737 .. versionadded:: 2.0
740 .. function:: setdlopenflags(n)
742 Set the flags used by the interpreter for :cfunc:`dlopen` calls, such as when
743 the interpreter loads extension modules. Among other things, this will enable a
744 lazy resolving of symbols when importing a module, if called as
745 ``sys.setdlopenflags(0)``. To share symbols across extension modules, call as
746 ``sys.setdlopenflags(dl.RTLD_NOW | dl.RTLD_GLOBAL)``. Symbolic names for the
747 flag modules can be either found in the :mod:`dl` module, or in the :mod:`DLFCN`
748 module. If :mod:`DLFCN` is not available, it can be generated from
749 :file:`/usr/include/dlfcn.h` using the :program:`h2py` script. Availability:
752 .. versionadded:: 2.2
755 .. function:: setprofile(profilefunc)
758 single: profile function
761 Set the system's profile function, which allows you to implement a Python source
762 code profiler in Python. See chapter :ref:`profile` for more information on the
763 Python profiler. The system's profile function is called similarly to the
764 system's trace function (see :func:`settrace`), but it isn't called for each
765 executed line of code (only on call and return, but the return event is reported
766 even when an exception has been set). The function is thread-specific, but
767 there is no way for the profiler to know about context switches between threads,
768 so it does not make sense to use this in the presence of multiple threads. Also,
769 its return value is not used, so it can simply return ``None``.
772 .. function:: setrecursionlimit(limit)
774 Set the maximum depth of the Python interpreter stack to *limit*. This limit
775 prevents infinite recursion from causing an overflow of the C stack and crashing
778 The highest possible limit is platform-dependent. A user may need to set the
779 limit higher when she has a program that requires deep recursion and a platform
780 that supports a higher limit. This should be done with care, because a too-high
781 limit can lead to a crash.
784 .. function:: settrace(tracefunc)
787 single: trace function
790 Set the system's trace function, which allows you to implement a Python
791 source code debugger in Python. The function is thread-specific; for a
792 debugger to support multiple threads, it must be registered using
793 :func:`settrace` for each thread being debugged.
795 Trace functions should have three arguments: *frame*, *event*, and
796 *arg*. *frame* is the current stack frame. *event* is a string: ``'call'``,
797 ``'line'``, ``'return'``, ``'exception'``, ``'c_call'``, ``'c_return'``, or
798 ``'c_exception'``. *arg* depends on the event type.
800 The trace function is invoked (with *event* set to ``'call'``) whenever a new
801 local scope is entered; it should return a reference to a local trace
802 function to be used that scope, or ``None`` if the scope shouldn't be traced.
804 The local trace function should return a reference to itself (or to another
805 function for further tracing in that scope), or ``None`` to turn off tracing
808 The events have the following meaning:
811 A function is called (or some other code block entered). The
812 global trace function is called; *arg* is ``None``; the return value
813 specifies the local trace function.
816 The interpreter is about to execute a new line of code or re-execute the
817 condition of a loop. The local trace function is called; *arg* is
818 ``None``; the return value specifies the new local trace function. See
819 :file:`Objects/lnotab_notes.txt` for a detailed explanation of how this
823 A function (or other code block) is about to return. The local trace
824 function is called; *arg* is the value that will be returned. The trace
825 function's return value is ignored.
828 An exception has occurred. The local trace function is called; *arg* is a
829 tuple ``(exception, value, traceback)``; the return value specifies the
830 new local trace function.
833 A C function is about to be called. This may be an extension function or
834 a built-in. *arg* is the C function object.
837 A C function has returned. *arg* is ``None``.
840 A C function has thrown an exception. *arg* is ``None``.
842 Note that as an exception is propagated down the chain of callers, an
843 ``'exception'`` event is generated at each level.
845 For more information on code and frame objects, refer to :ref:`types`.
849 The :func:`settrace` function is intended only for implementing debuggers,
850 profilers, coverage tools and the like. Its behavior is part of the
851 implementation platform, rather than part of the language definition, and
852 thus may not be available in all Python implementations.
855 .. function:: settscdump(on_flag)
857 Activate dumping of VM measurements using the Pentium timestamp counter, if
858 *on_flag* is true. Deactivate these dumps if *on_flag* is off. The function is
859 available only if Python was compiled with :option:`--with-tsc`. To understand
860 the output of this dump, read :file:`Python/ceval.c` in the Python sources.
862 .. versionadded:: 2.4
873 File objects corresponding to the interpreter's standard input, output and error
874 streams. ``stdin`` is used for all interpreter input except for scripts but
875 including calls to :func:`input` and :func:`raw_input`. ``stdout`` is used for
876 the output of :keyword:`print` and :term:`expression` statements and for the
877 prompts of :func:`input` and :func:`raw_input`. The interpreter's own prompts
878 and (almost all of) its error messages go to ``stderr``. ``stdout`` and
879 ``stderr`` needn't be built-in file objects: any object is acceptable as long
880 as it has a :meth:`write` method that takes a string argument. (Changing these
881 objects doesn't affect the standard I/O streams of processes executed by
882 :func:`os.popen`, :func:`os.system` or the :func:`exec\*` family of functions in
883 the :mod:`os` module.)
890 These objects contain the original values of ``stdin``, ``stderr`` and
891 ``stdout`` at the start of the program. They are used during finalization,
892 and could be useful to print to the actual standard stream no matter if the
893 ``sys.std*`` object has been redirected.
895 It can also be used to restore the actual files to known working file objects
896 in case they have been overwritten with a broken object. However, the
897 preferred way to do this is to explicitly save the previous stream before
898 replacing it, and restore the saved object.
901 .. data:: tracebacklimit
903 When this variable is set to an integer value, it determines the maximum number
904 of levels of traceback information printed when an unhandled exception occurs.
905 The default is ``1000``. When set to ``0`` or less, all traceback information
906 is suppressed and only the exception type and value are printed.
911 A string containing the version number of the Python interpreter plus additional
912 information on the build number and compiler used. It has a value of the form
913 ``'version (#build_number, build_date, build_time) [compiler]'``. The first
914 three characters are used to identify the version in the installation
915 directories (where appropriate on each platform). An example::
919 '1.5.2 (#0 Apr 13 1999, 10:51:12) [MSC 32 bit (Intel)]'
922 .. data:: api_version
924 The C API version for this interpreter. Programmers may find this useful when
925 debugging version conflicts between Python and extension modules.
927 .. versionadded:: 2.3
930 .. data:: version_info
932 A tuple containing the five components of the version number: *major*, *minor*,
933 *micro*, *releaselevel*, and *serial*. All values except *releaselevel* are
934 integers; the release level is ``'alpha'``, ``'beta'``, ``'candidate'``, or
935 ``'final'``. The ``version_info`` value corresponding to the Python version 2.0
936 is ``(2, 0, 0, 'final', 0)``. The components can also be accessed by name,
937 so ``sys.version_info[0]`` is equivalent to ``sys.version_info.major``
940 .. versionadded:: 2.0
941 .. versionchanged:: 2.7
942 Added named component attributes
945 .. data:: warnoptions
947 This is an implementation detail of the warnings framework; do not modify this
948 value. Refer to the :mod:`warnings` module for more information on the warnings
954 The version number used to form registry keys on Windows platforms. This is
955 stored as string resource 1000 in the Python DLL. The value is normally the
956 first three characters of :const:`version`. It is provided in the :mod:`sys`
957 module for informational purposes; modifying this value has no effect on the
958 registry keys used by Python. Availability: Windows.