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
336 .. function:: getcheckinterval()
338 Return the interpreter's "check interval"; see :func:`setcheckinterval`.
340 .. versionadded:: 2.3
343 .. function:: getdefaultencoding()
345 Return the name of the current default string encoding used by the Unicode
348 .. versionadded:: 2.0
351 .. function:: getdlopenflags()
353 Return the current value of the flags that are used for :cfunc:`dlopen` calls.
354 The flag constants are defined in the :mod:`dl` and :mod:`DLFCN` modules.
357 .. versionadded:: 2.2
360 .. function:: getfilesystemencoding()
362 Return the name of the encoding used to convert Unicode filenames into system
363 file names, or ``None`` if the system default encoding is used. The result value
364 depends on the operating system:
366 * On Windows 9x, the encoding is "mbcs".
368 * On Mac OS X, the encoding is "utf-8".
370 * On Unix, the encoding is the user's preference according to the result of
371 nl_langinfo(CODESET), or :const:`None` if the ``nl_langinfo(CODESET)`` failed.
373 * On Windows NT+, file names are Unicode natively, so no conversion is
374 performed. :func:`getfilesystemencoding` still returns ``'mbcs'``, as this is
375 the encoding that applications should use when they explicitly want to convert
376 Unicode strings to byte strings that are equivalent when used as file names.
378 .. versionadded:: 2.3
381 .. function:: getrefcount(object)
383 Return the reference count of the *object*. The count returned is generally one
384 higher than you might expect, because it includes the (temporary) reference as
385 an argument to :func:`getrefcount`.
388 .. function:: getrecursionlimit()
390 Return the current value of the recursion limit, the maximum depth of the Python
391 interpreter stack. This limit prevents infinite recursion from causing an
392 overflow of the C stack and crashing Python. It can be set by
393 :func:`setrecursionlimit`.
396 .. function:: getsizeof(object[, default])
398 Return the size of an object in bytes. The object can be any type of
399 object. All built-in objects will return correct results, but this
400 does not have to hold true for third-party extensions as it is implementation
403 The *default* argument allows to define a value which will be returned
404 if the object type does not provide means to retrieve the size and would
407 func:`getsizeof` calls the object's __sizeof__ method and adds an additional
408 garbage collector overhead if the object is managed by the garbage collector.
410 .. versionadded:: 2.6
413 .. function:: _getframe([depth])
415 Return a frame object from the call stack. If optional integer *depth* is
416 given, return the frame object that many calls below the top of the stack. If
417 that is deeper than the call stack, :exc:`ValueError` is raised. The default
418 for *depth* is zero, returning the frame at the top of the call stack.
420 This function should be used for internal and specialized purposes only.
423 .. function:: getprofile()
426 single: profile function
429 Get the profiler function as set by :func:`setprofile`.
431 .. versionadded:: 2.6
434 .. function:: gettrace()
437 single: trace function
440 Get the trace function as set by :func:`settrace`.
444 The :func:`gettrace` function is intended only for implementing debuggers,
445 profilers, coverage tools and the like. Its behavior is part of the
446 implementation platform, rather than part of the language definition,
447 and thus may not be available in all Python implementations.
449 .. versionadded:: 2.6
452 .. function:: getwindowsversion()
454 Return a tuple containing five components, describing the Windows version
455 currently running. The elements are *major*, *minor*, *build*, *platform*, and
456 *text*. *text* contains a string while all other values are integers.
458 *platform* may be one of the following values:
460 +-----------------------------------------+-------------------------+
461 | Constant | Platform |
462 +=========================================+=========================+
463 | :const:`0 (VER_PLATFORM_WIN32s)` | Win32s on Windows 3.1 |
464 +-----------------------------------------+-------------------------+
465 | :const:`1 (VER_PLATFORM_WIN32_WINDOWS)` | Windows 95/98/ME |
466 +-----------------------------------------+-------------------------+
467 | :const:`2 (VER_PLATFORM_WIN32_NT)` | Windows NT/2000/XP/x64 |
468 +-----------------------------------------+-------------------------+
469 | :const:`3 (VER_PLATFORM_WIN32_CE)` | Windows CE |
470 +-----------------------------------------+-------------------------+
472 This function wraps the Win32 :cfunc:`GetVersionEx` function; see the Microsoft
473 documentation for more information about these fields.
475 Availability: Windows.
477 .. versionadded:: 2.3
482 The version number encoded as a single integer. This is guaranteed to increase
483 with each version, including proper support for non-production releases. For
484 example, to test that the Python interpreter is at least version 1.5.2, use::
486 if sys.hexversion >= 0x010502F0:
487 # use some advanced feature
490 # use an alternative implementation or warn the user
493 This is called ``hexversion`` since it only really looks meaningful when viewed
494 as the result of passing it to the built-in :func:`hex` function. The
495 ``version_info`` value may be used for a more human-friendly encoding of the
498 .. versionadded:: 1.5.2
503 A struct sequence that holds information about Python's
504 internal representation of integers. The attributes are read only.
506 +-------------------------+----------------------------------------------+
507 | attribute | explanation |
508 +=========================+==============================================+
509 | :const:`bits_per_digit` | number of bits held in each digit. Python |
510 | | integers are stored internally in base |
511 | | ``2**long_info.bits_per_digit`` |
512 +-------------------------+----------------------------------------------+
513 | :const:`sizeof_digit` | size in bytes of the C type used to |
514 | | represent a digit |
515 +-------------------------+----------------------------------------------+
517 .. versionadded:: 2.7
524 These three variables are not always defined; they are set when an exception is
525 not handled and the interpreter prints an error message and a stack traceback.
526 Their intended use is to allow an interactive user to import a debugger module
527 and engage in post-mortem debugging without having to re-execute the command
528 that caused the error. (Typical use is ``import pdb; pdb.pm()`` to enter the
529 post-mortem debugger; see chapter :ref:`debugger` for
532 The meaning of the variables is the same as that of the return values from
533 :func:`exc_info` above. (Since there is only one interactive thread,
534 thread-safety is not a concern for these variables, unlike for ``exc_type``
540 The largest positive integer supported by Python's regular integer type. This
541 is at least 2\*\*31-1. The largest negative integer is ``-maxint-1`` --- the
542 asymmetry results from the use of 2's complement binary arithmetic.
546 The largest positive integer supported by the platform's Py_ssize_t type,
547 and thus the maximum size lists, strings, dicts, and many other containers
552 An integer giving the largest supported code point for a Unicode character. The
553 value of this depends on the configuration option that specifies whether Unicode
554 characters are stored as UCS-2 or UCS-4.
559 A list of :term:`finder` objects that have their :meth:`find_module`
560 methods called to see if one of the objects can find the module to be
561 imported. The :meth:`find_module` method is called at least with the
562 absolute name of the module being imported. If the module to be imported is
563 contained in package then the parent package's :attr:`__path__` attribute
564 is passed in as a second argument. The method returns :keyword:`None` if
565 the module cannot be found, else returns a :term:`loader`.
567 :data:`sys.meta_path` is searched before any implicit default finders or
570 See :pep:`302` for the original specification.
575 .. index:: builtin: reload
577 This is a dictionary that maps module names to modules which have already been
578 loaded. This can be manipulated to force reloading of modules and other tricks.
579 Note that removing a module from this dictionary is *not* the same as calling
580 :func:`reload` on the corresponding module object.
585 .. index:: triple: module; search; path
587 A list of strings that specifies the search path for modules. Initialized from
588 the environment variable :envvar:`PYTHONPATH`, plus an installation-dependent
591 As initialized upon program startup, the first item of this list, ``path[0]``,
592 is the directory containing the script that was used to invoke the Python
593 interpreter. If the script directory is not available (e.g. if the interpreter
594 is invoked interactively or if the script is read from standard input),
595 ``path[0]`` is the empty string, which directs Python to search modules in the
596 current directory first. Notice that the script directory is inserted *before*
597 the entries inserted as a result of :envvar:`PYTHONPATH`.
599 A program is free to modify this list for its own purposes.
601 .. versionchanged:: 2.3
602 Unicode strings are no longer ignored.
605 Module :mod:`site` This describes how to use .pth files to extend
611 A list of callables that take a path argument to try to create a
612 :term:`finder` for the path. If a finder can be created, it is to be
613 returned by the callable, else raise :exc:`ImportError`.
615 Originally specified in :pep:`302`.
618 .. data:: path_importer_cache
620 A dictionary acting as a cache for :term:`finder` objects. The keys are
621 paths that have been passed to :data:`sys.path_hooks` and the values are
622 the finders that are found. If a path is a valid file system path but no
623 explicit finder is found on :data:`sys.path_hooks` then :keyword:`None` is
624 stored to represent the implicit default finder should be used. If the path
625 is not an existing path then :class:`imp.NullImporter` is set.
627 Originally specified in :pep:`302`.
632 This string contains a platform identifier that can be used to append
633 platform-specific components to :data:`sys.path`, for instance.
635 For Unix systems, this is the lowercased OS name as returned by ``uname -s``
636 with the first part of the version as returned by ``uname -r`` appended,
637 e.g. ``'sunos5'`` or ``'linux2'``, *at the time when Python was built*.
638 For other systems, the values are:
640 ================ ===========================
641 System :data:`platform` value
642 ================ ===========================
644 Windows/Cygwin ``'cygwin'``
645 Mac OS X ``'darwin'``
647 OS/2 EMX ``'os2emx'``
650 ================ ===========================
655 A string giving the site-specific directory prefix where the platform
656 independent Python files are installed; by default, this is the string
657 ``'/usr/local'``. This can be set at build time with the :option:`--prefix`
658 argument to the :program:`configure` script. The main collection of Python
659 library modules is installed in the directory ``prefix + '/lib/pythonversion'``
660 while the platform independent header files (all except :file:`pyconfig.h`) are
661 stored in ``prefix + '/include/pythonversion'``, where *version* is equal to
669 single: interpreter prompts
670 single: prompts, interpreter
672 Strings specifying the primary and secondary prompt of the interpreter. These
673 are only defined if the interpreter is in interactive mode. Their initial
674 values in this case are ``'>>> '`` and ``'... '``. If a non-string object is
675 assigned to either variable, its :func:`str` is re-evaluated each time the
676 interpreter prepares to read a new interactive command; this can be used to
677 implement a dynamic prompt.
680 .. data:: py3kwarning
682 Bool containing the status of the Python 3.0 warning flag. It's ``True``
683 when Python is started with the -3 option. (This should be considered
684 read-only; setting it to a different value doesn't have an effect on
685 Python 3.0 warnings.)
687 .. versionadded:: 2.6
690 .. data:: dont_write_bytecode
692 If this is true, Python won't try to write ``.pyc`` or ``.pyo`` files on the
693 import of source modules. This value is initially set to ``True`` or ``False``
694 depending on the ``-B`` command line option and the ``PYTHONDONTWRITEBYTECODE``
695 environment variable, but you can set it yourself to control bytecode file
698 .. versionadded:: 2.6
701 .. function:: setcheckinterval(interval)
703 Set the interpreter's "check interval". This integer value determines how often
704 the interpreter checks for periodic things such as thread switches and signal
705 handlers. The default is ``100``, meaning the check is performed every 100
706 Python virtual instructions. Setting it to a larger value may increase
707 performance for programs using threads. Setting it to a value ``<=`` 0 checks
708 every virtual instruction, maximizing responsiveness as well as overhead.
711 .. function:: setdefaultencoding(name)
713 Set the current default string encoding used by the Unicode implementation. If
714 *name* does not match any available encoding, :exc:`LookupError` is raised.
715 This function is only intended to be used by the :mod:`site` module
716 implementation and, where needed, by :mod:`sitecustomize`. Once used by the
717 :mod:`site` module, it is removed from the :mod:`sys` module's namespace.
719 .. Note that :mod:`site` is not imported if the :option:`-S` option is passed
720 to the interpreter, in which case this function will remain available.
722 .. versionadded:: 2.0
725 .. function:: setdlopenflags(n)
727 Set the flags used by the interpreter for :cfunc:`dlopen` calls, such as when
728 the interpreter loads extension modules. Among other things, this will enable a
729 lazy resolving of symbols when importing a module, if called as
730 ``sys.setdlopenflags(0)``. To share symbols across extension modules, call as
731 ``sys.setdlopenflags(dl.RTLD_NOW | dl.RTLD_GLOBAL)``. Symbolic names for the
732 flag modules can be either found in the :mod:`dl` module, or in the :mod:`DLFCN`
733 module. If :mod:`DLFCN` is not available, it can be generated from
734 :file:`/usr/include/dlfcn.h` using the :program:`h2py` script. Availability:
737 .. versionadded:: 2.2
740 .. function:: setprofile(profilefunc)
743 single: profile function
746 Set the system's profile function, which allows you to implement a Python source
747 code profiler in Python. See chapter :ref:`profile` for more information on the
748 Python profiler. The system's profile function is called similarly to the
749 system's trace function (see :func:`settrace`), but it isn't called for each
750 executed line of code (only on call and return, but the return event is reported
751 even when an exception has been set). The function is thread-specific, but
752 there is no way for the profiler to know about context switches between threads,
753 so it does not make sense to use this in the presence of multiple threads. Also,
754 its return value is not used, so it can simply return ``None``.
757 .. function:: setrecursionlimit(limit)
759 Set the maximum depth of the Python interpreter stack to *limit*. This limit
760 prevents infinite recursion from causing an overflow of the C stack and crashing
763 The highest possible limit is platform-dependent. A user may need to set the
764 limit higher when she has a program that requires deep recursion and a platform
765 that supports a higher limit. This should be done with care, because a too-high
766 limit can lead to a crash.
769 .. function:: settrace(tracefunc)
772 single: trace function
775 Set the system's trace function, which allows you to implement a Python
776 source code debugger in Python. The function is thread-specific; for a
777 debugger to support multiple threads, it must be registered using
778 :func:`settrace` for each thread being debugged.
780 Trace functions should have three arguments: *frame*, *event*, and
781 *arg*. *frame* is the current stack frame. *event* is a string: ``'call'``,
782 ``'line'``, ``'return'``, ``'exception'``, ``'c_call'``, ``'c_return'``, or
783 ``'c_exception'``. *arg* depends on the event type.
785 The trace function is invoked (with *event* set to ``'call'``) whenever a new
786 local scope is entered; it should return a reference to a local trace
787 function to be used that scope, or ``None`` if the scope shouldn't be traced.
789 The local trace function should return a reference to itself (or to another
790 function for further tracing in that scope), or ``None`` to turn off tracing
793 The events have the following meaning:
796 A function is called (or some other code block entered). The
797 global trace function is called; *arg* is ``None``; the return value
798 specifies the local trace function.
801 The interpreter is about to execute a new line of code (sometimes multiple
802 line events on one line exist). The local trace function is called; *arg*
803 is ``None``; the return value specifies the new local trace function.
806 A function (or other code block) is about to return. The local trace
807 function is called; *arg* is the value that will be returned. The trace
808 function's return value is ignored.
811 An exception has occurred. The local trace function is called; *arg* is a
812 tuple ``(exception, value, traceback)``; the return value specifies the
813 new local trace function.
816 A C function is about to be called. This may be an extension function or
817 a builtin. *arg* is the C function object.
820 A C function has returned. *arg* is ``None``.
823 A C function has thrown an exception. *arg* is ``None``.
825 Note that as an exception is propagated down the chain of callers, an
826 ``'exception'`` event is generated at each level.
828 For more information on code and frame objects, refer to :ref:`types`.
832 The :func:`settrace` function is intended only for implementing debuggers,
833 profilers, coverage tools and the like. Its behavior is part of the
834 implementation platform, rather than part of the language definition, and thus
835 may not be available in all Python implementations.
838 .. function:: settscdump(on_flag)
840 Activate dumping of VM measurements using the Pentium timestamp counter, if
841 *on_flag* is true. Deactivate these dumps if *on_flag* is off. The function is
842 available only if Python was compiled with :option:`--with-tsc`. To understand
843 the output of this dump, read :file:`Python/ceval.c` in the Python sources.
845 .. versionadded:: 2.4
856 File objects corresponding to the interpreter's standard input, output and error
857 streams. ``stdin`` is used for all interpreter input except for scripts but
858 including calls to :func:`input` and :func:`raw_input`. ``stdout`` is used for
859 the output of :keyword:`print` and :term:`expression` statements and for the
860 prompts of :func:`input` and :func:`raw_input`. The interpreter's own prompts
861 and (almost all of) its error messages go to ``stderr``. ``stdout`` and
862 ``stderr`` needn't be built-in file objects: any object is acceptable as long
863 as it has a :meth:`write` method that takes a string argument. (Changing these
864 objects doesn't affect the standard I/O streams of processes executed by
865 :func:`os.popen`, :func:`os.system` or the :func:`exec\*` family of functions in
866 the :mod:`os` module.)
873 These objects contain the original values of ``stdin``, ``stderr`` and
874 ``stdout`` at the start of the program. They are used during finalization,
875 and could be useful to print to the actual standard stream no matter if the
876 ``sys.std*`` object has been redirected.
878 It can also be used to restore the actual files to known working file objects
879 in case they have been overwritten with a broken object. However, the
880 preferred way to do this is to explicitly save the previous stream before
881 replacing it, and restore the saved object.
884 .. data:: tracebacklimit
886 When this variable is set to an integer value, it determines the maximum number
887 of levels of traceback information printed when an unhandled exception occurs.
888 The default is ``1000``. When set to ``0`` or less, all traceback information
889 is suppressed and only the exception type and value are printed.
894 A string containing the version number of the Python interpreter plus additional
895 information on the build number and compiler used. It has a value of the form
896 ``'version (#build_number, build_date, build_time) [compiler]'``. The first
897 three characters are used to identify the version in the installation
898 directories (where appropriate on each platform). An example::
902 '1.5.2 (#0 Apr 13 1999, 10:51:12) [MSC 32 bit (Intel)]'
905 .. data:: api_version
907 The C API version for this interpreter. Programmers may find this useful when
908 debugging version conflicts between Python and extension modules.
910 .. versionadded:: 2.3
913 .. data:: version_info
915 A tuple containing the five components of the version number: *major*, *minor*,
916 *micro*, *releaselevel*, and *serial*. All values except *releaselevel* are
917 integers; the release level is ``'alpha'``, ``'beta'``, ``'candidate'``, or
918 ``'final'``. The ``version_info`` value corresponding to the Python version 2.0
919 is ``(2, 0, 0, 'final', 0)``. The components can also be accessed by name,
920 so ``sys.version_info[0]`` is equivalent to ``sys.version_info.major``
923 .. versionadded:: 2.0
924 .. versionchanged:: 2.7
925 Added named component attributes
928 .. data:: warnoptions
930 This is an implementation detail of the warnings framework; do not modify this
931 value. Refer to the :mod:`warnings` module for more information on the warnings
937 The version number used to form registry keys on Windows platforms. This is
938 stored as string resource 1000 in the Python DLL. The value is normally the
939 first three characters of :const:`version`. It is provided in the :mod:`sys`
940 module for informational purposes; modifying this value has no effect on the
941 registry keys used by Python. Availability: Windows.