Doc/library/sys.rst

:mod:`sys` --- System-specific parameters and functions
=======================================================

.. module:: sys
   :synopsis: Access system-specific parameters and functions.


This module provides access to some variables used or maintained by the
interpreter and to functions that interact strongly with the interpreter. It is
always available.


.. data:: argv

   The list of command line arguments passed to a Python script. ``argv[0]`` is the
   script name (it is operating system dependent whether this is a full pathname or
   not).  If the command was executed using the :option:`-c` command line option to
   the interpreter, ``argv[0]`` is set to the string ``'-c'``.  If no script name
   was passed to the Python interpreter, ``argv[0]`` is the empty string.

   To loop over the standard input, or the list of files given on the
   command line, see the :mod:`fileinput` module.


.. data:: byteorder

   An indicator of the native byte order.  This will have the value ``'big'`` on
   big-endian (most-significant byte first) platforms, and ``'little'`` on
   little-endian (least-significant byte first) platforms.

   .. versionadded:: 2.0


.. data:: builtin_module_names

   A tuple of strings giving the names of all modules that are compiled into this
   Python interpreter.  (This information is not available in any other way ---
   ``modules.keys()`` only lists the imported modules.)


.. function:: call_tracing(func, args)

   Call ``func(*args)``, while tracing is enabled.  The tracing state is saved,
   and restored afterwards.  This is intended to be called from a debugger from
   a checkpoint, to recursively debug some other code.


.. data:: copyright

   A string containing the copyright pertaining to the Python interpreter.


.. function:: _clear_type_cache()

   Clear the internal type cache. The type cache is used to speed up attribute
   and method lookups. Use the function *only* to drop unnecessary references
   during reference leak debugging.

   This function should be used for internal and specialized purposes only.

   .. versionadded:: 2.6


.. function:: _current_frames()

   Return a dictionary mapping each thread's identifier to the topmost stack frame
   currently active in that thread at the time the function is called. Note that
   functions in the :mod:`traceback` module can build the call stack given such a
   frame.

   This is most useful for debugging deadlock:  this function does not require the
   deadlocked threads' cooperation, and such threads' call stacks are frozen for as
   long as they remain deadlocked.  The frame returned for a non-deadlocked thread
   may bear no relationship to that thread's current activity by the time calling
   code examines the frame.

   This function should be used for internal and specialized purposes only.

   .. versionadded:: 2.5


.. data:: dllhandle

   Integer specifying the handle of the Python DLL. Availability: Windows.


.. function:: displayhook(value)

   If *value* is not ``None``, this function prints it to ``sys.stdout``, and saves
   it in ``__builtin__._``.

   ``sys.displayhook`` is called on the result of evaluating an :term:`expression`
   entered in an interactive Python session.  The display of these values can be
   customized by assigning another one-argument function to ``sys.displayhook``.


.. function:: excepthook(type, value, traceback)

   This function prints out a given traceback and exception to ``sys.stderr``.

   When an exception is raised and uncaught, the interpreter calls
   ``sys.excepthook`` with three arguments, the exception class, exception
   instance, and a traceback object.  In an interactive session this happens just
   before control is returned to the prompt; in a Python program this happens just
   before the program exits.  The handling of such top-level exceptions can be
   customized by assigning another three-argument function to ``sys.excepthook``.


.. data:: __displayhook__
          __excepthook__

   These objects contain the original values of ``displayhook`` and ``excepthook``
   at the start of the program.  They are saved so that ``displayhook`` and
   ``excepthook`` can be restored in case they happen to get replaced with broken
   objects.


.. function:: exc_info()

   This function returns a tuple of three values that give information about the
   exception that is currently being handled.  The information returned is specific
   both to the current thread and to the current stack frame.  If the current stack
   frame is not handling an exception, the information is taken from the calling
   stack frame, or its caller, and so on until a stack frame is found that is
   handling an exception.  Here, "handling an exception" is defined as "executing
   or having executed an except clause."  For any stack frame, only information
   about the most recently handled exception is accessible.

   .. index:: object: traceback

   If no exception is being handled anywhere on the stack, a tuple containing three
   ``None`` values is returned.  Otherwise, the values returned are ``(type, value,
   traceback)``.  Their meaning is: *type* gets the exception type of the exception
   being handled (a class object); *value* gets the exception parameter (its
   :dfn:`associated value` or the second argument to :keyword:`raise`, which is
   always a class instance if the exception type is a class object); *traceback*
   gets a traceback object (see the Reference Manual) which encapsulates the call
   stack at the point where the exception originally occurred.

   If :func:`exc_clear` is called, this function will return three ``None`` values
   until either another exception is raised in the current thread or the execution
   stack returns to a frame where another exception is being handled.

   .. warning::

      Assigning the *traceback* return value to a local variable in a function that is
      handling an exception will cause a circular reference.  This will prevent
      anything referenced by a local variable in the same function or by the traceback
      from being garbage collected.  Since most functions don't need access to the
      traceback, the best solution is to use something like ``exctype, value =
      sys.exc_info()[:2]`` to extract only the exception type and value.  If you do
      need the traceback, make sure to delete it after use (best done with a
      :keyword:`try` ... :keyword:`finally` statement) or to call :func:`exc_info` in
      a function that does not itself handle an exception.

   .. note::

      Beginning with Python 2.2, such cycles are automatically reclaimed when garbage
      collection is enabled and they become unreachable, but it remains more efficient
      to avoid creating cycles.


.. function:: exc_clear()

   This function clears all information relating to the current or last exception
   that occurred in the current thread.  After calling this function,
   :func:`exc_info` will return three ``None`` values until another exception is
   raised in the current thread or the execution stack returns to a frame where
   another exception is being handled.

   This function is only needed in only a few obscure situations.  These include
   logging and error handling systems that report information on the last or
   current exception.  This function can also be used to try to free resources and
   trigger object finalization, though no guarantee is made as to what objects will
   be freed, if any.

   .. versionadded:: 2.3


.. data:: exc_type
          exc_value
          exc_traceback

   .. deprecated:: 1.5
      Use :func:`exc_info` instead.

   Since they are global variables, they are not specific to the current thread, so
   their use is not safe in a multi-threaded program.  When no exception is being
   handled, ``exc_type`` is set to ``None`` and the other two are undefined.


.. data:: exec_prefix

   A string giving the site-specific directory prefix where the platform-dependent
   Python files are installed; by default, this is also ``'/usr/local'``.  This can
   be set at build time with the ``--exec-prefix`` argument to the
   :program:`configure` script.  Specifically, all configuration files (e.g. the
   :file:`pyconfig.h` header file) are installed in the directory ``exec_prefix +
   '/lib/pythonversion/config'``, and shared library modules are installed in
   ``exec_prefix + '/lib/pythonversion/lib-dynload'``, where *version* is equal to
   ``version[:3]``.


.. data:: executable

   A string giving the name of the executable binary for the Python interpreter, on
   systems where this makes sense.


.. function:: exit([arg])

   Exit from Python.  This is implemented by raising the :exc:`SystemExit`
   exception, so cleanup actions specified by finally clauses of :keyword:`try`
   statements are honored, and it is possible to intercept the exit attempt at
   an outer level.

   The optional argument *arg* can be an integer giving the exit status
   (defaulting to zero), or another type of object.  If it is an integer, zero
   is considered "successful termination" and any nonzero value is considered
   "abnormal termination" by shells and the like.  Most systems require it to be
   in the range 0-127, and produce undefined results otherwise.  Some systems
   have a convention for assigning specific meanings to specific exit codes, but
   these are generally underdeveloped; Unix programs generally use 2 for command
   line syntax errors and 1 for all other kind of errors.  If another type of
   object is passed, ``None`` is equivalent to passing zero, and any other
   object is printed to :data:`stderr` and results in an exit code of 1.  In
   particular, ``sys.exit("some error message")`` is a quick way to exit a
   program when an error occurs.

   Since :func:`exit` ultimately "only" raises an exception, it will only exit
   the process when called from the main thread, and the exception is not
   intercepted.


.. data:: exitfunc

   This value is not actually defined by the module, but can be set by the user (or
   by a program) to specify a clean-up action at program exit.  When set, it should
   be a parameterless function.  This function will be called when the interpreter
   exits.  Only one function may be installed in this way; to allow multiple
   functions which will be called at termination, use the :mod:`atexit` module.

   .. note::

      The exit function is not called when the program is killed by a signal, when a
      Python fatal internal error is detected, or when ``os._exit()`` is called.

   .. deprecated:: 2.4
      Use :mod:`atexit` instead.


.. data:: flags

   The struct sequence *flags* exposes the status of command line flags. The
   attributes are read only.

   ============================= ===================================
   attribute                     flag
   ============================= ===================================
   :const:`debug`                :option:`-d`
   :const:`py3k_warning`         :option:`-3`
   :const:`division_warning`     :option:`-Q`
   :const:`division_new`         :option:`-Qnew <-Q>`
   :const:`inspect`              :option:`-i`
   :const:`interactive`          :option:`-i`
   :const:`optimize`             :option:`-O` or :option:`-OO`
   :const:`dont_write_bytecode`  :option:`-B`
   :const:`no_user_site`         :option:`-s`
   :const:`no_site`              :option:`-S`
   :const:`ignore_environment`   :option:`-E`
   :const:`tabcheck`             :option:`-t` or :option:`-tt <-t>`
   :const:`verbose`              :option:`-v`
   :const:`unicode`              :option:`-U`
   :const:`bytes_warning`        :option:`-b`
   ============================= ===================================

   .. versionadded:: 2.6


.. data:: float_info

   A structseq holding information about the float type. It contains low level
   information about the precision and internal representation.  The values
   correspond to the various floating-point constants defined in the standard
   header file :file:`float.h` for the 'C' programming language; see section
   5.2.4.2.2 of the 1999 ISO/IEC C standard [C99]_, 'Characteristics of
   floating types', for details.

   +---------------------+----------------+--------------------------------------------------+
   | attribute           | float.h macro  | explanation                                      |
   +=====================+================+==================================================+
   | :const:`epsilon`    | DBL_EPSILON    | difference between 1 and the least value greater |
   |                     |                | than 1 that is representable as a float          |
   +---------------------+----------------+--------------------------------------------------+
   | :const:`dig`        | DBL_DIG        | maximum number of decimal digits that can be     |
   |                     |                | faithfully represented in a float;  see below    |
   +---------------------+----------------+--------------------------------------------------+
   | :const:`mant_dig`   | DBL_MANT_DIG   | float precision: the number of base-``radix``    |
   |                     |                | digits in the significand of a float             |
   +---------------------+----------------+--------------------------------------------------+
   | :const:`max`        | DBL_MAX        | maximum representable finite float               |
   +---------------------+----------------+--------------------------------------------------+
   | :const:`max_exp`    | DBL_MAX_EXP    | maximum integer e such that ``radix**(e-1)`` is  |
   |                     |                | a representable finite float                     |
   +---------------------+----------------+--------------------------------------------------+
   | :const:`max_10_exp` | DBL_MAX_10_EXP | maximum integer e such that ``10**e`` is in the  |
   |                     |                | range of representable finite floats             |
   +---------------------+----------------+--------------------------------------------------+
   | :const:`min`        | DBL_MIN        | minimum positive normalized float                |
   +---------------------+----------------+--------------------------------------------------+
   | :const:`min_exp`    | DBL_MIN_EXP    | minimum integer e such that ``radix**(e-1)`` is  |
   |                     |                | a normalized float                               |
   +---------------------+----------------+--------------------------------------------------+
   | :const:`min_10_exp` | DBL_MIN_10_EXP | minimum integer e such that ``10**e`` is a       |
   |                     |                | normalized float                                 |
   +---------------------+----------------+--------------------------------------------------+
   | :const:`radix`      | FLT_RADIX      | radix of exponent representation                 |
   +---------------------+----------------+--------------------------------------------------+
   | :const:`rounds`     | FLT_ROUNDS     | constant representing rounding mode              |
   |                     |                | used for arithmetic operations                   |
   +---------------------+----------------+--------------------------------------------------+

   The attribute :attr:`sys.float_info.dig` needs further explanation.  If
   ``s`` is any string representing a decimal number with at most
   :attr:`sys.float_info.dig` significant digits, then converting ``s`` to a
   float and back again will recover a string representing the same decimal
   value::

      >>> import sys
      >>> sys.float_info.dig
      15
      >>> s = '3.14159265358979'    # decimal string with 15 significant digits
      >>> format(float(s), '.15g')  # convert to float and back -> same value
      '3.14159265358979'

   But for strings with more than :attr:`sys.float_info.dig` significant digits,
   this isn't always true::

      >>> s = '9876543211234567'    # 16 significant digits is too many!
      >>> format(float(s), '.16g')  # conversion changes value
      '9876543211234568'

   .. versionadded:: 2.6

.. data:: float_repr_style

   A string indicating how the :func:`repr` function behaves for
   floats.  If the string has value ``'short'`` then for a finite
   float ``x``, ``repr(x)`` aims to produce a short string with the
   property that ``float(repr(x)) == x``.  This is the usual behaviour
   in Python 2.7 and later.  Otherwise, ``float_repr_style`` has value
   ``'legacy'`` and ``repr(x)`` behaves in the same way as it did in
   versions of Python prior to 2.7.

   .. versionadded:: 2.7


.. function:: getcheckinterval()

   Return the interpreter's "check interval"; see :func:`setcheckinterval`.

   .. versionadded:: 2.3


.. function:: getdefaultencoding()

   Return the name of the current default string encoding used by the Unicode
   implementation.

   .. versionadded:: 2.0


.. function:: getdlopenflags()

   Return the current value of the flags that are used for :cfunc:`dlopen` calls.
   The flag constants are defined in the :mod:`dl` and :mod:`DLFCN` modules.
   Availability: Unix.

   .. versionadded:: 2.2


.. function:: getfilesystemencoding()

   Return the name of the encoding used to convert Unicode filenames into system
   file names, or ``None`` if the system default encoding is used. The result value
   depends on the operating system:

   * On Mac OS X, the encoding is ``'utf-8'``.

   * On Unix, the encoding is the user's preference according to the result of
     nl_langinfo(CODESET), or ``None`` if the ``nl_langinfo(CODESET)``
     failed.

   * On Windows NT+, file names are Unicode natively, so no conversion is
     performed. :func:`getfilesystemencoding` still returns ``'mbcs'``, as
     this is the encoding that applications should use when they explicitly
     want to convert Unicode strings to byte strings that are equivalent when
     used as file names.

   * On Windows 9x, the encoding is ``'mbcs'``.

   .. versionadded:: 2.3


.. function:: getrefcount(object)

   Return the reference count of the *object*.  The count returned is generally one
   higher than you might expect, because it includes the (temporary) reference as
   an argument to :func:`getrefcount`.


.. function:: getrecursionlimit()

   Return the current value of the recursion limit, the maximum depth of the Python
   interpreter stack.  This limit prevents infinite recursion from causing an
   overflow of the C stack and crashing Python.  It can be set by
   :func:`setrecursionlimit`.


.. function:: getsizeof(object[, default])

   Return the size of an object in bytes. The object can be any type of
   object. All built-in objects will return correct results, but this
   does not have to hold true for third-party extensions as it is implementation
   specific.

   If given, *default* will be returned if the object does not provide means to
   retrieve the size.  Otherwise a :exc:`TypeError` will be raised.

   :func:`getsizeof` calls the object's ``__sizeof__`` method and adds an
   additional garbage collector overhead if the object is managed by the garbage
   collector.

   .. versionadded:: 2.6


.. function:: _getframe([depth])

   Return a frame object from the call stack.  If optional integer *depth* is
   given, return the frame object that many calls below the top of the stack.  If
   that is deeper than the call stack, :exc:`ValueError` is raised.  The default
   for *depth* is zero, returning the frame at the top of the call stack.

   .. impl-detail::

      This function should be used for internal and specialized purposes only.
      It is not guaranteed to exist in all implementations of Python.


.. function:: getprofile()

   .. index::
      single: profile function
      single: profiler

   Get the profiler function as set by :func:`setprofile`.

   .. versionadded:: 2.6


.. function:: gettrace()

   .. index::
      single: trace function
      single: debugger

   Get the trace function as set by :func:`settrace`.

   .. impl-detail::

      The :func:`gettrace` function is intended only for implementing debuggers,
      profilers, coverage tools and the like.  Its behavior is part of the
      implementation platform, rather than part of the language definition, and
      thus may not be available in all Python implementations.

   .. versionadded:: 2.6


.. function:: getwindowsversion()

   Return a named tuple describing the Windows version
   currently running.  The named elements are *major*, *minor*,
   *build*, *platform*, *service_pack*, *service_pack_minor*,
   *service_pack_major*, *suite_mask*, and *product_type*.
   *service_pack* contains a string while all other values are
   integers. The components can also be accessed by name, so
   ``sys.getwindowsversion()[0]`` is equivalent to
   ``sys.getwindowsversion().major``. For compatibility with prior
   versions, only the first 5 elements are retrievable by indexing.

   *platform* may be one of the following values:

   +-----------------------------------------+-------------------------+
   | Constant                                | Platform                |
   +=========================================+=========================+
   | :const:`0 (VER_PLATFORM_WIN32s)`        | Win32s on Windows 3.1   |
   +-----------------------------------------+-------------------------+
   | :const:`1 (VER_PLATFORM_WIN32_WINDOWS)` | Windows 95/98/ME        |
   +-----------------------------------------+-------------------------+
   | :const:`2 (VER_PLATFORM_WIN32_NT)`      | Windows NT/2000/XP/x64  |
   +-----------------------------------------+-------------------------+
   | :const:`3 (VER_PLATFORM_WIN32_CE)`      | Windows CE              |
   +-----------------------------------------+-------------------------+

   *product_type* may be one of the following values:

   +---------------------------------------+---------------------------------+
   | Constant                              | Meaning                         |
   +=======================================+=================================+
   | :const:`1 (VER_NT_WORKSTATION)`       | The system is a workstation.    |
   +---------------------------------------+---------------------------------+
   | :const:`2 (VER_NT_DOMAIN_CONTROLLER)` | The system is a domain          |
   |                                       | controller.                     |
   +---------------------------------------+---------------------------------+
   | :const:`3 (VER_NT_SERVER)`            | The system is a server, but not |
   |                                       | a domain controller.            |
   +---------------------------------------+---------------------------------+


   This function wraps the Win32 :cfunc:`GetVersionEx` function; see the
   Microsoft documentation on :cfunc:`OSVERSIONINFOEX` for more information
   about these fields.

   Availability: Windows.

   .. versionadded:: 2.3
   .. versionchanged:: 2.7
      Changed to a named tuple and added *service_pack_minor*,
      *service_pack_major*, *suite_mask*, and *product_type*.


.. data:: hexversion

   The version number encoded as a single integer.  This is guaranteed to increase
   with each version, including proper support for non-production releases.  For
   example, to test that the Python interpreter is at least version 1.5.2, use::

      if sys.hexversion >= 0x010502F0:
          # use some advanced feature
          ...
      else:
          # use an alternative implementation or warn the user
          ...

   This is called ``hexversion`` since it only really looks meaningful when viewed
   as the result of passing it to the built-in :func:`hex` function.  The
   ``version_info`` value may be used for a more human-friendly encoding of the
   same information.

   The ``hexversion`` is a 32-bit number with the following layout:

   +-------------------------+------------------------------------------------+
   | Bits (big endian order) | Meaning                                        |
   +=========================+================================================+
   | :const:`1-8`            |  ``PY_MAJOR_VERSION``  (the ``2`` in           |
   |                         |  ``2.1.0a3``)                                  |
   +-------------------------+------------------------------------------------+
   | :const:`9-16`           |  ``PY_MINOR_VERSION``  (the ``1`` in           |
   |                         |  ``2.1.0a3``)                                  |
   +-------------------------+------------------------------------------------+
   | :const:`17-24`          |  ``PY_MICRO_VERSION``  (the ``0`` in           |
   |                         |  ``2.1.0a3``)                                  |
   +-------------------------+------------------------------------------------+
   | :const:`25-28`          |  ``PY_RELEASE_LEVEL``  (``0xA`` for alpha,     |
   |                         |  ``0xB`` for beta, ``0xC`` for release         |
   |                         |  candidate and ``0xF`` for final)              |
   +-------------------------+------------------------------------------------+
   | :const:`29-32`          |  ``PY_RELEASE_SERIAL``  (the ``3`` in          |
   |                         |  ``2.1.0a3``, zero for final releases)         |
   +-------------------------+------------------------------------------------+

   Thus ``2.1.0a3`` is hexversion ``0x020100a3``.

   .. versionadded:: 1.5.2


.. data:: long_info

   A struct sequence that holds information about Python's
   internal representation of integers.  The attributes are read only.

   +-------------------------+----------------------------------------------+
   | Attribute               | Explanation                                  |
   +=========================+==============================================+
   | :const:`bits_per_digit` | number of bits held in each digit.  Python   |
   |                         | integers are stored internally in base       |
   |                         | ``2**long_info.bits_per_digit``              |
   +-------------------------+----------------------------------------------+
   | :const:`sizeof_digit`   | size in bytes of the C type used to          |
   |                         | represent a digit                            |
   +-------------------------+----------------------------------------------+

   .. versionadded:: 2.7


.. data:: last_type
          last_value
          last_traceback

   These three variables are not always defined; they are set when an exception is
   not handled and the interpreter prints an error message and a stack traceback.
   Their intended use is to allow an interactive user to import a debugger module
   and engage in post-mortem debugging without having to re-execute the command
   that caused the error.  (Typical use is ``import pdb; pdb.pm()`` to enter the
   post-mortem debugger; see chapter :ref:`debugger` for
   more information.)

   The meaning of the variables is the same as that of the return values from
   :func:`exc_info` above.  (Since there is only one interactive thread,
   thread-safety is not a concern for these variables, unlike for ``exc_type``
   etc.)


.. data:: maxint

   The largest positive integer supported by Python's regular integer type.  This
   is at least 2\*\*31-1.  The largest negative integer is ``-maxint-1`` --- the
   asymmetry results from the use of 2's complement binary arithmetic.

.. data:: maxsize

   The largest positive integer supported by the platform's Py_ssize_t type,
   and thus the maximum size lists, strings, dicts, and many other containers
   can have.

.. data:: maxunicode

   An integer giving the largest supported code point for a Unicode character.  The
   value of this depends on the configuration option that specifies whether Unicode
   characters are stored as UCS-2 or UCS-4.


.. data:: meta_path

    A list of :term:`finder` objects that have their :meth:`find_module`
    methods called to see if one of the objects can find the module to be
    imported. The :meth:`find_module` method is called at least with the
    absolute name of the module being imported. If the module to be imported is
    contained in package then the parent package's :attr:`__path__` attribute
    is passed in as a second argument. The method returns :keyword:`None` if
    the module cannot be found, else returns a :term:`loader`.

    :data:`sys.meta_path` is searched before any implicit default finders or
    :data:`sys.path`.

    See :pep:`302` for the original specification.


.. data:: modules

   .. index:: builtin: reload

   This is a dictionary that maps module names to modules which have already been
   loaded.  This can be manipulated to force reloading of modules and other tricks.
   Note that removing a module from this dictionary is *not* the same as calling
   :func:`reload` on the corresponding module object.


.. data:: path

   .. index:: triple: module; search; path

   A list of strings that specifies the search path for modules. Initialized from
   the environment variable :envvar:`PYTHONPATH`, plus an installation-dependent
   default.

   As initialized upon program startup, the first item of this list, ``path[0]``,
   is the directory containing the script that was used to invoke the Python
   interpreter.  If the script directory is not available (e.g.  if the interpreter
   is invoked interactively or if the script is read from standard input),
   ``path[0]`` is the empty string, which directs Python to search modules in the
   current directory first.  Notice that the script directory is inserted *before*
   the entries inserted as a result of :envvar:`PYTHONPATH`.

   A program is free to modify this list for its own purposes.

   .. versionchanged:: 2.3
      Unicode strings are no longer ignored.

   .. seealso::
      Module :mod:`site` This describes how to use .pth files to extend
      :data:`sys.path`.


.. data:: path_hooks

    A list of callables that take a path argument to try to create a
    :term:`finder` for the path. If a finder can be created, it is to be
    returned by the callable, else raise :exc:`ImportError`.

    Originally specified in :pep:`302`.


.. data:: path_importer_cache

    A dictionary acting as a cache for :term:`finder` objects. The keys are
    paths that have been passed to :data:`sys.path_hooks` and the values are
    the finders that are found. If a path is a valid file system path but no
    explicit finder is found on :data:`sys.path_hooks` then :keyword:`None` is
    stored to represent the implicit default finder should be used. If the path
    is not an existing path then :class:`imp.NullImporter` is set.

    Originally specified in :pep:`302`.


.. data:: platform

   This string contains a platform identifier that can be used to append
   platform-specific components to :data:`sys.path`, for instance.

   For Unix systems, this is the lowercased OS name as returned by ``uname -s``
   with the first part of the version as returned by ``uname -r`` appended,
   e.g. ``'sunos5'`` or ``'linux2'``, *at the time when Python was built*.
   Unless you want to test for a specific system version, it is therefore
   recommended to use the following idiom::

      if sys.platform.startswith('linux'):
          # Linux-specific code here...

   For other systems, the values are:

   ================ ===========================
   System           :data:`platform` value
   ================ ===========================
   Windows          ``'win32'``
   Windows/Cygwin   ``'cygwin'``
   Mac OS X         ``'darwin'``
   OS/2             ``'os2'``
   OS/2 EMX         ``'os2emx'``
   RiscOS           ``'riscos'``
   AtheOS           ``'atheos'``
   ================ ===========================

   .. seealso::
      :attr:`os.name` has a coarser granularity.  :func:`os.uname` gives
      system-dependent version information.

      The :mod:`platform` module provides detailed checks for the
      system's identity.

.. data:: prefix

   A string giving the site-specific directory prefix where the platform
   independent Python files are installed; by default, this is the string
   ``'/usr/local'``.  This can be set at build time with the ``--prefix``
   argument to the :program:`configure` script.  The main collection of Python
   library modules is installed in the directory ``prefix + '/lib/pythonversion'``
   while the platform independent header files (all except :file:`pyconfig.h`) are
   stored in ``prefix + '/include/pythonversion'``, where *version* is equal to
   ``version[:3]``.


.. data:: ps1
          ps2

   .. index::
      single: interpreter prompts
      single: prompts, interpreter

   Strings specifying the primary and secondary prompt of the interpreter.  These
   are only defined if the interpreter is in interactive mode.  Their initial
   values in this case are ``'>>> '`` and ``'... '``.  If a non-string object is
   assigned to either variable, its :func:`str` is re-evaluated each time the
   interpreter prepares to read a new interactive command; this can be used to
   implement a dynamic prompt.


.. data:: py3kwarning

   Bool containing the status of the Python 3.0 warning flag. It's ``True``
   when Python is started with the -3 option.  (This should be considered
   read-only; setting it to a different value doesn't have an effect on
   Python 3.0 warnings.)

   .. versionadded:: 2.6


.. data:: dont_write_bytecode

   If this is true, Python won't try to write ``.pyc`` or ``.pyo`` files on the
   import of source modules.  This value is initially set to ``True`` or ``False``
   depending on the ``-B`` command line option and the ``PYTHONDONTWRITEBYTECODE``
   environment variable, but you can set it yourself to control bytecode file
   generation.

   .. versionadded:: 2.6


.. function:: setcheckinterval(interval)

   Set the interpreter's "check interval".  This integer value determines how often
   the interpreter checks for periodic things such as thread switches and signal
   handlers.  The default is ``100``, meaning the check is performed every 100
   Python virtual instructions. Setting it to a larger value may increase
   performance for programs using threads.  Setting it to a value ``<=`` 0 checks
   every virtual instruction, maximizing responsiveness as well as overhead.


.. function:: setdefaultencoding(name)

   Set the current default string encoding used by the Unicode implementation.  If
   *name* does not match any available encoding, :exc:`LookupError` is raised.
   This function is only intended to be used by the :mod:`site` module
   implementation and, where needed, by :mod:`sitecustomize`.  Once used by the
   :mod:`site` module, it is removed from the :mod:`sys` module's namespace.

   .. Note that :mod:`site` is not imported if the :option:`-S` option is passed
      to the interpreter, in which case this function will remain available.

   .. versionadded:: 2.0


.. function:: setdlopenflags(n)

   Set the flags used by the interpreter for :cfunc:`dlopen` calls, such as when
   the interpreter loads extension modules.  Among other things, this will enable a
   lazy resolving of symbols when importing a module, if called as
   ``sys.setdlopenflags(0)``.  To share symbols across extension modules, call as
   ``sys.setdlopenflags(dl.RTLD_NOW | dl.RTLD_GLOBAL)``.  Symbolic names for the
   flag modules can be either found in the :mod:`dl` module, or in the :mod:`DLFCN`
   module. If :mod:`DLFCN` is not available, it can be generated from
   :file:`/usr/include/dlfcn.h` using the :program:`h2py` script. Availability:
   Unix.

   .. versionadded:: 2.2


.. function:: setprofile(profilefunc)

   .. index::
      single: profile function
      single: profiler

   Set the system's profile function, which allows you to implement a Python source
   code profiler in Python.  See chapter :ref:`profile` for more information on the
   Python profiler.  The system's profile function is called similarly to the
   system's trace function (see :func:`settrace`), but it isn't called for each
   executed line of code (only on call and return, but the return event is reported
   even when an exception has been set).  The function is thread-specific, but
   there is no way for the profiler to know about context switches between threads,
   so it does not make sense to use this in the presence of multiple threads. Also,
   its return value is not used, so it can simply return ``None``.


.. function:: setrecursionlimit(limit)

   Set the maximum depth of the Python interpreter stack to *limit*.  This limit
   prevents infinite recursion from causing an overflow of the C stack and crashing
   Python.

   The highest possible limit is platform-dependent.  A user may need to set the
   limit higher when she has a program that requires deep recursion and a platform
   that supports a higher limit.  This should be done with care, because a too-high
   limit can lead to a crash.


.. function:: settrace(tracefunc)

   .. index::
      single: trace function
      single: debugger

   Set the system's trace function, which allows you to implement a Python
   source code debugger in Python.  The function is thread-specific; for a
   debugger to support multiple threads, it must be registered using
   :func:`settrace` for each thread being debugged.

   Trace functions should have three arguments: *frame*, *event*, and
   *arg*. *frame* is the current stack frame.  *event* is a string: ``'call'``,
   ``'line'``, ``'return'``, ``'exception'``, ``'c_call'``, ``'c_return'``, or
   ``'c_exception'``. *arg* depends on the event type.

   The trace function is invoked (with *event* set to ``'call'``) whenever a new
   local scope is entered; it should return a reference to a local trace
   function to be used that scope, or ``None`` if the scope shouldn't be traced.

   The local trace function should return a reference to itself (or to another
   function for further tracing in that scope), or ``None`` to turn off tracing
   in that scope.

   The events have the following meaning:

   ``'call'``
      A function is called (or some other code block entered).  The
      global trace function is called; *arg* is ``None``; the return value
      specifies the local trace function.

   ``'line'``
      The interpreter is about to execute a new line of code or re-execute the
      condition of a loop.  The local trace function is called; *arg* is
      ``None``; the return value specifies the new local trace function.  See
      :file:`Objects/lnotab_notes.txt` for a detailed explanation of how this
      works.

   ``'return'``
      A function (or other code block) is about to return.  The local trace
      function is called; *arg* is the value that will be returned, or ``None``
      if the event is caused by an exception being raised.  The trace function's
      return value is ignored.

   ``'exception'``
      An exception has occurred.  The local trace function is called; *arg* is a
      tuple ``(exception, value, traceback)``; the return value specifies the
      new local trace function.

   ``'c_call'``
      A C function is about to be called.  This may be an extension function or
      a built-in.  *arg* is the C function object.

   ``'c_return'``
      A C function has returned. *arg* is the C function object.

   ``'c_exception'``
      A C function has raised an exception.  *arg* is the C function object.

   Note that as an exception is propagated down the chain of callers, an
   ``'exception'`` event is generated at each level.

   For more information on code and frame objects, refer to :ref:`types`.

   .. impl-detail::

      The :func:`settrace` function is intended only for implementing debuggers,
      profilers, coverage tools and the like.  Its behavior is part of the
      implementation platform, rather than part of the language definition, and
      thus may not be available in all Python implementations.


.. function:: settscdump(on_flag)

   Activate dumping of VM measurements using the Pentium timestamp counter, if
   *on_flag* is true. Deactivate these dumps if *on_flag* is off. The function is
   available only if Python was compiled with ``--with-tsc``. To understand
   the output of this dump, read :file:`Python/ceval.c` in the Python sources.

   .. versionadded:: 2.4

   .. impl-detail::

      This function is intimately bound to CPython implementation details and
      thus not likely to be implemented elsewhere.


.. data:: stdin
          stdout
          stderr

   .. index::
      builtin: input
      builtin: raw_input

   File objects corresponding to the interpreter's standard input, output and error
   streams.  ``stdin`` is used for all interpreter input except for scripts but
   including calls to :func:`input` and :func:`raw_input`.  ``stdout`` is used for
   the output of :keyword:`print` and :term:`expression` statements and for the
   prompts of :func:`input` and :func:`raw_input`. The interpreter's own prompts
   and (almost all of) its error messages go to ``stderr``.  ``stdout`` and
   ``stderr`` needn't be built-in file objects: any object is acceptable as long
   as it has a :meth:`write` method that takes a string argument.  (Changing these
   objects doesn't affect the standard I/O streams of processes executed by
   :func:`os.popen`, :func:`os.system` or the :func:`exec\*` family of functions in
   the :mod:`os` module.)


.. data:: __stdin__
          __stdout__
          __stderr__

   These objects contain the original values of ``stdin``, ``stderr`` and
   ``stdout`` at the start of the program.  They are used during finalization,
   and could be useful to print to the actual standard stream no matter if the
   ``sys.std*`` object has been redirected.

   It can also be used to restore the actual files to known working file objects
   in case they have been overwritten with a broken object.  However, the
   preferred way to do this is to explicitly save the previous stream before
   replacing it, and restore the saved object.


.. data:: subversion

   A triple (repo, branch, version) representing the Subversion information of the
   Python interpreter. *repo* is the name of the repository, ``'CPython'``.
   *branch* is a string of one of the forms ``'trunk'``, ``'branches/name'`` or
   ``'tags/name'``. *version* is the output of ``svnversion``, if the interpreter
   was built from a Subversion checkout; it contains the revision number (range)
   and possibly a trailing 'M' if there were local modifications. If the tree was
   exported (or svnversion was not available), it is the revision of
   ``Include/patchlevel.h`` if the branch is a tag. Otherwise, it is ``None``.

   .. versionadded:: 2.5

   .. note::
      Python is now `developed <http://docs.python.org/devguide/>`_ using
      Mercurial.  In recent Python 2.7 bugfix releases, :data:`subversion`
      therefore contains placeholder information.  It is removed in Python
      3.3.


.. data:: tracebacklimit

   When this variable is set to an integer value, it determines the maximum number
   of levels of traceback information printed when an unhandled exception occurs.
   The default is ``1000``.  When set to ``0`` or less, all traceback information
   is suppressed and only the exception type and value are printed.


.. data:: version

   A string containing the version number of the Python interpreter plus additional
   information on the build number and compiler used.  This string is displayed
   when the interactive interpreter is started.  Do not extract version information
   out of it, rather, use :data:`version_info` and the functions provided by the
   :mod:`platform` module.


.. data:: api_version

   The C API version for this interpreter.  Programmers may find this useful when
   debugging version conflicts between Python and extension modules.

   .. versionadded:: 2.3


.. data:: version_info

   A tuple containing the five components of the version number: *major*, *minor*,
   *micro*, *releaselevel*, and *serial*.  All values except *releaselevel* are
   integers; the release level is ``'alpha'``, ``'beta'``, ``'candidate'``, or
   ``'final'``.  The ``version_info`` value corresponding to the Python version 2.0
   is ``(2, 0, 0, 'final', 0)``.  The components can also be accessed by name,
   so ``sys.version_info[0]`` is equivalent to ``sys.version_info.major``
   and so on.

   .. versionadded:: 2.0
   .. versionchanged:: 2.7
      Added named component attributes


.. data:: warnoptions

   This is an implementation detail of the warnings framework; do not modify this
   value.  Refer to the :mod:`warnings` module for more information on the warnings
   framework.


.. data:: winver

   The version number used to form registry keys on Windows platforms. This is
   stored as string resource 1000 in the Python DLL.  The value is normally the
   first three characters of :const:`version`.  It is provided in the :mod:`sys`
   module for informational purposes; modifying this value has no effect on the
   registry keys used by Python. Availability: Windows.

.. rubric:: Citations

.. [C99] ISO/IEC 9899:1999.  "Programming languages -- C."  A public draft of this standard is available at http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf .