: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. Unless explicitly noted otherwise, all variables are read-only.

.. data:: abi_info

.. versionadded:: 3.15

An object containing information about the ABI of the currently running Python interpreter. It should include information that affect the CPython ABI in ways that require a specific build of the interpreter chosen from variants that can co-exist on a single machine. For example, it does not encode the base OS (Linux or Windows), but does include pointer size since some systems support both 32- and 64-bit builds. The available entries are the same on all platforms; e.g. pointer_size is available even on 64-bit-only architectures.

The following attributes are available:

.. attribute:: abi_info.pointer_bits

  The width of pointers in bits, as an integer,
  equivalent to ``8 * sizeof(void *)``.
  Usually, this is  ``32`` or ``64``.

.. attribute:: abi_info.free_threaded

  A Boolean indicating whether the interpreter was built with
  :term:`free threading` support.
  This reflects either the presence of the :option:`--disable-gil`
  :file:`configure` option (on Unix)
  or setting the ``DisableGil`` property (on Windows).

.. attribute:: abi_info.debug

  A Boolean indicating whether the interpreter was built in
  :ref:`debug mode <debug-build>`.
  This reflects either the presence of the :option:`--with-pydebug`
  :file:`configure` option (on Unix)
  or the ``Debug`` configuration (on Windows).

.. attribute:: abi_info.byteorder

  A string indicating the native byte order,
  either ``'big'`` or ``'little'``.
  This is the same as the :data:`byteorder` attribute.

.. data:: abiflags

On POSIX systems where Python was built with the standard configure script, this contains the ABI flags as specified by :pep:3149.

.. versionadded:: 3.2

.. versionchanged:: 3.8 Default flags became an empty string (m flag for pymalloc has been removed).

.. availability:: Unix.

.. function:: addaudithook(hook)

Append the callable hook to the list of active auditing hooks for the current (sub)interpreter.

When an auditing event is raised through the :func:sys.audit function, each hook will be called in the order it was added with the event name and the tuple of arguments. Native hooks added by :c:func:PySys_AddAuditHook are called first, followed by hooks added in the current (sub)interpreter. Hooks can then log the event, raise an exception to abort the operation, or terminate the process entirely.

Note that audit hooks are primarily for collecting information about internal or otherwise unobservable actions, whether by Python or libraries written in Python. They are not suitable for implementing a "sandbox". In particular, malicious code can trivially disable or bypass hooks added using this function. At a minimum, any security-sensitive hooks must be added using the C API :c:func:PySys_AddAuditHook before initialising the runtime, and any modules allowing arbitrary memory modification (such as :mod:ctypes) should be completely removed or closely monitored.

.. audit-event:: sys.addaudithook "" sys.addaudithook

  Calling :func:`sys.addaudithook` will itself raise an auditing event
  named ``sys.addaudithook`` with no arguments. If any
  existing hooks raise an exception derived from :class:`RuntimeError`, the
  new hook will not be added and the exception suppressed. As a result,
  callers cannot assume that their hook has been added unless they control
  all existing hooks.

See the :ref:audit events table <audit-events> for all events raised by CPython, and :pep:578 for the original design discussion.

.. versionadded:: 3.8

.. versionchanged:: 3.8.1

  Exceptions derived from :class:`Exception` but not :class:`RuntimeError`
  are no longer suppressed.

.. impl-detail::

  When tracing is enabled (see :func:`settrace`), Python hooks are only
  traced if the callable has a ``__cantrace__`` member that is set to a
  true value. Otherwise, trace functions will skip the hook.

.. 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.

See also :data:sys.orig_argv.

.. note:: On Unix, command line arguments are passed by bytes from OS. Python decodes them with filesystem encoding and "surrogateescape" error handler. When you need original bytes, you can get it by [os.fsencode(arg) for arg in sys.argv].

.. _auditing:

.. function:: audit(event, *args)

.. index:: single: auditing

Raise an auditing event and trigger any active auditing hooks. event is a string identifying the event, and args may contain optional arguments with more information about the event. The number and types of arguments for a given event are considered a public and stable API and should not be modified between releases.

For example, one auditing event is named os.chdir. This event has one argument called path that will contain the requested new working directory.

:func:sys.audit will call the existing auditing hooks, passing the event name and arguments, and will re-raise the first exception from any hook. In general, if an exception is raised, it should not be handled and the process should be terminated as quickly as possible. This allows hook implementations to decide how to respond to particular events: they can merely log the event or abort the operation by raising an exception.

Hooks are added using the :func:sys.addaudithook or :c:func:PySys_AddAuditHook functions.

The native equivalent of this function is :c:func:PySys_Audit. Using the native function is preferred when possible.

See the :ref:audit events table <audit-events> for all events raised by CPython.

.. versionadded:: 3.8

.. data:: base_exec_prefix

Equivalent to :data:exec_prefix, but referring to the base Python installation.

When running under :ref:sys-path-init-virtual-environments, :data:exec_prefix gets overwritten to the virtual environment prefix. :data:base_exec_prefix, conversely, does not change, and always points to the base Python installation. Refer to :ref:sys-path-init-virtual-environments for more information.

.. versionadded:: 3.3

.. data:: base_prefix

Equivalent to :data:prefix, but referring to the base Python installation.

When running under :ref:virtual environment <venv-def>, :data:prefix gets overwritten to the virtual environment prefix. :data:base_prefix, conversely, does not change, and always points to the base Python installation. Refer to :ref:sys-path-init-virtual-environments for more information.

.. versionadded:: 3.3

.. 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.

.. data:: builtin_module_names

A tuple of strings containing 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.)

See also the :data:sys.stdlib_module_names list.

.. 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 or profile some other code.

Tracing is suspended while calling a tracing function set by :func:settrace or :func:setprofile to avoid infinite recursion. :func:!call_tracing enables explicit recursion of the tracing function.

.. 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.

.. deprecated:: 3.13 Use the more general :func:_clear_internal_caches function instead.

.. function:: _clear_internal_caches()

Clear all internal performance-related caches. Use this function only to release unnecessary references and memory blocks when hunting for leaks.

.. versionadded:: 3.13

.. 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.

.. audit-event:: sys._current_frames "" sys._current_frames

.. function:: _current_exceptions()

Return a dictionary mapping each thread's identifier to the topmost exception currently active in that thread at the time the function is called. If a thread is not currently handling an exception, it is not included in the result dictionary.

This is most useful for statistical profiling.

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

.. audit-event:: sys._current_exceptions "" sys._current_exceptions

.. versionchanged:: 3.12 Each value in the dictionary is now a single exception instance, rather than a 3-tuple as returned from sys.exc_info().

.. function:: breakpointhook()

This hook function is called by built-in :func:breakpoint. By default, it drops you into the :mod:pdb debugger, but it can be set to any other function so that you can choose which debugger gets used.

The signature of this function is dependent on what it calls. For example, the default binding (e.g. pdb.set_trace()) expects no arguments, but you might bind it to a function that expects additional arguments (positional and/or keyword). The built-in breakpoint() function passes its *args and **kws straight through. Whatever breakpointhooks() returns is returned from breakpoint().

The default implementation first consults the environment variable :envvar:PYTHONBREAKPOINT. If that is set to "0" then this function returns immediately; i.e. it is a no-op. If the environment variable is not set, or is set to the empty string, pdb.set_trace() is called. Otherwise this variable should name a function to run, using Python's dotted-import nomenclature, e.g. package.subpackage.module.function. In this case, package.subpackage.module would be imported and the resulting module must have a callable named function(). This is run, passing in *args and **kws, and whatever function() returns, sys.breakpointhook() returns to the built-in :func:breakpoint function.

Note that if anything goes wrong while importing the callable named by :envvar:PYTHONBREAKPOINT, a :exc:RuntimeWarning is reported and the breakpoint is ignored.

Also note that if sys.breakpointhook() is overridden programmatically, :envvar:PYTHONBREAKPOINT is not consulted.

.. versionadded:: 3.7

.. function:: _debugmallocstats()

Print low-level information to stderr about the state of CPython's memory allocator.

If Python is :ref:built in debug mode <debug-build> (:option:configure --with-pydebug option <--with-pydebug>), it also performs some expensive internal consistency checks.

.. versionadded:: 3.3

.. impl-detail::

  This function is specific to CPython.  The exact output format is not
  defined here, and may change.

.. data:: dllhandle

Integer specifying the handle of the Python DLL.

.. availability:: Windows.

.. function:: displayhook(value)

If value is not None, this function prints repr(value) to sys.stdout, and saves value in builtins._. If repr(value) is not encodable to sys.stdout.encoding with sys.stdout.errors error handler (which is probably 'strict'), encode it to sys.stdout.encoding with 'backslashreplace' error handler.

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.

Pseudo-code::

   def displayhook(value):
       if value is None:
           return
       # Set '_' to None to avoid recursion
       builtins._ = None
       text = repr(value)
       try:
           sys.stdout.write(text)
       except UnicodeEncodeError:
           bytes = text.encode(sys.stdout.encoding, 'backslashreplace')
           if hasattr(sys.stdout, 'buffer'):
               sys.stdout.buffer.write(bytes)
           else:
               text = bytes.decode(sys.stdout.encoding, 'strict')
               sys.stdout.write(text)
       sys.stdout.write("\n")
       builtins._ = value

.. versionchanged:: 3.2 Use 'backslashreplace' error handler on :exc:UnicodeEncodeError.

.. data:: dont_write_bytecode

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

.. data:: _emscripten_info

A :term:named tuple holding information about the environment on the wasm32-emscripten platform. The named tuple is provisional and may change in the future.

.. attribute:: _emscripten_info.emscripten_version

  Emscripten version as tuple of ints (major, minor, micro), e.g. ``(3, 1, 8)``.

.. attribute:: _emscripten_info.runtime

  Runtime string, e.g. browser user agent, ``'Node.js v14.18.2'``, or ``'UNKNOWN'``.

.. attribute:: _emscripten_info.pthreads

  ``True`` if Python is compiled with Emscripten pthreads support.

.. attribute:: _emscripten_info.shared_memory

  ``True`` if Python is compiled with shared memory support.

.. availability:: Emscripten.

.. versionadded:: 3.11

.. data:: pycache_prefix

If this is set (not None), Python will write bytecode-cache .pyc files to (and read them from) a parallel directory tree rooted at this directory, rather than from __pycache__ directories in the source code tree. Any __pycache__ directories in the source code tree will be ignored and new .pyc files written within the pycache prefix. Thus if you use :mod:compileall as a pre-build step, you must ensure you run it with the same pycache prefix (if any) that you will use at runtime.

A relative path is interpreted relative to the current working directory.

This value is initially set based on the value of the :option:-X pycache_prefix=PATH command-line option or the :envvar:PYTHONPYCACHEPREFIX environment variable (command-line takes precedence). If neither are set, it is None.

.. versionadded:: 3.8

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

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

When an exception other than :exc:SystemExit 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.

.. audit-event:: sys.excepthook hook,type,value,traceback sys.excepthook

  Raise an auditing event ``sys.excepthook`` with arguments ``hook``,
  ``type``, ``value``, ``traceback`` when an uncaught exception occurs.
  If no hook has been set, ``hook`` may be ``None``. If any hook raises
  an exception derived from :class:`RuntimeError` the call to the hook will
  be suppressed. Otherwise, the audit hook exception will be reported as
  unraisable and ``sys.excepthook`` will be called.

.. seealso::

  The :func:`sys.unraisablehook` function handles unraisable exceptions
  and the :func:`threading.excepthook` function handles exception raised
  by :func:`threading.Thread.run`.

.. data:: breakpointhook displayhook excepthook unraisablehook

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

.. versionadded:: 3.7 breakpointhook

.. versionadded:: 3.8 unraisablehook

.. function:: exception()

This function, when called while an exception handler is executing (such as an except or except* clause), returns the exception instance that was caught by this handler. When exception handlers are nested within one another, only the exception handled by the innermost handler is accessible.

If no exception handler is executing, this function returns None.

.. versionadded:: 3.11

.. function:: exc_info()

This function returns the old-style representation of the handled exception. If an exception e is currently handled (so :func:exception would return e), :func:exc_info returns the tuple (type(e), e, e.__traceback__). That is, a tuple containing the type of the exception (a subclass of :exc:BaseException), the exception itself, and a :ref:traceback object <traceback-objects> which typically encapsulates the call stack at the point where the exception last occurred.

.. index:: pair: object; traceback

If no exception is being handled anywhere on the stack, this function return a tuple containing three None values.

.. versionchanged:: 3.11 The type and traceback fields are now derived from the value (the exception instance), so when an exception is modified while it is being handled, the changes are reflected in the results of subsequent calls to :func:exc_info.

.. 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 :file:{exec_prefix}/lib/python{X.Y}/config, and shared library modules are installed in :file:{exec_prefix}/lib/python{X.Y}/lib-dynload, where X.Y is the version number of Python, for example 3.2.

.. note::

  If a :ref:`virtual environment <venv-def>` is in effect, this :data:`exec_prefix`
  will point to the virtual environment. The value for the Python installation
  will still be available, via :data:`base_exec_prefix`.
  Refer to :ref:`sys-path-init-virtual-environments` for more information.

.. versionchanged:: 3.14

  When running under a :ref:`virtual environment <venv-def>`,
  :data:`prefix` and :data:`exec_prefix` are now set to the virtual
  environment prefix by the :ref:`path initialization <sys-path-init>`,
  instead of :mod:`site`. This means that :data:`prefix` and
  :data:`exec_prefix` always point to the virtual environment, even when
  :mod:`site` is disabled (:option:`-S`).

.. data:: executable

A string giving the absolute path of the executable binary for the Python interpreter, on systems where this makes sense. If Python is unable to retrieve the real path to its executable, :data:sys.executable will be an empty string or None.

.. function:: exit([arg])

Raise a :exc:SystemExit exception, signaling an intention to exit the interpreter.

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 kinds 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. Cleanup actions specified by :keyword:finally clauses of :keyword:try statements are honored, and it is possible to intercept the exit attempt at an outer level.

.. versionchanged:: 3.6 If an error occurs in the cleanup after the Python interpreter has caught :exc:SystemExit (such as an error flushing buffered data in the standard streams), the exit status is changed to 120.

.. data:: flags

The :term:named tuple flags exposes the status of command line flags. Flags should only be accessed only by name and not by index. The attributes are read only.

.. list-table::

  * - .. attribute:: flags.debug
    - :option:`-d`

  * - .. attribute:: flags.inspect
    - :option:`-i`

  * - .. attribute:: flags.interactive
    - :option:`-i`

  * - .. attribute:: flags.isolated
    - :option:`-I`

  * - .. attribute:: flags.optimize
    - :option:`-O` or :option:`-OO`

  * - .. attribute:: flags.dont_write_bytecode
    - :option:`-B`

  * - .. attribute:: flags.no_user_site
    - :option:`-s`

  * - .. attribute:: flags.no_site
    - :option:`-S`

  * - .. attribute:: flags.ignore_environment
    - :option:`-E`

  * - .. attribute:: flags.verbose
    - :option:`-v`

  * - .. attribute:: flags.bytes_warning
    - :option:`-b`

  * - .. attribute:: flags.quiet
    - :option:`-q`

  * - .. attribute:: flags.hash_randomization
    - :option:`-R`

  * - .. attribute:: flags.dev_mode
    - :option:`-X dev <-X>` (:ref:`Python Development Mode <devmode>`)

  * - .. attribute:: flags.utf8_mode
    - :option:`-X utf8 <-X>`

  * - .. attribute:: flags.safe_path
    - :option:`-P`

  * - .. attribute:: flags.int_max_str_digits
    - :option:`-X int_max_str_digits <-X>`
      (:ref:`integer string conversion length limitation <int_max_str_digits>`)

  * - .. attribute:: flags.warn_default_encoding
    - :option:`-X warn_default_encoding <-X>`

  * - .. attribute:: flags.gil
    - :option:`-X gil <-X>` and :envvar:`PYTHON_GIL`

  * - .. attribute:: flags.thread_inherit_context
    - :option:`-X thread_inherit_context <-X>` and
      :envvar:`PYTHON_THREAD_INHERIT_CONTEXT`

  * - .. attribute:: flags.context_aware_warnings
    - :option:`-X context_aware_warnings <-X>` and
      :envvar:`PYTHON_CONTEXT_AWARE_WARNINGS`

.. versionchanged:: 3.2 Added quiet attribute for the new :option:-q flag.

.. versionadded:: 3.2.3 The hash_randomization attribute.

.. versionchanged:: 3.3 Removed obsolete division_warning attribute.

.. versionchanged:: 3.4 Added isolated attribute for :option:-I isolated flag.

.. versionchanged:: 3.7 Added the dev_mode attribute for the new :ref:Python Development Mode <devmode> and the utf8_mode attribute for the new :option:-X utf8 flag.

.. versionchanged:: 3.10 Added warn_default_encoding attribute for :option:-X warn_default_encoding flag.

.. versionchanged:: 3.11 Added the safe_path attribute for :option:-P option.

.. versionchanged:: 3.11 Added the int_max_str_digits attribute.

.. versionchanged:: 3.13 Added the gil attribute.

.. versionchanged:: 3.14 Added the thread_inherit_context attribute.

.. versionchanged:: 3.14 Added the context_aware_warnings attribute.

.. data:: float_info

A :term:named tuple 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.

.. list-table:: Attributes of the :data:!float_info :term:named tuple :header-rows: 1

  * - attribute
    - float.h macro
    - explanation

  * - .. attribute:: float_info.epsilon
    - :c:macro:`!DBL_EPSILON`
    - difference between 1.0 and the least value greater than 1.0 that is
      representable as a float.

      See also :func:`math.ulp`.

  * - .. attribute:: float_info.dig
    - :c:macro:`!DBL_DIG`
    - The maximum number of decimal digits that can be faithfully
      represented in a float; see below.

  * - .. attribute:: float_info.mant_dig
    - :c:macro:`!DBL_MANT_DIG`
    - Float precision: the number of base-``radix`` digits in the
      significand of a float.

  * - .. attribute:: float_info.max
    - :c:macro:`!DBL_MAX`
    - The maximum representable positive finite float.

  * - .. attribute:: float_info.max_exp
    - :c:macro:`!DBL_MAX_EXP`
    - The maximum integer *e* such that ``radix**(e-1)`` is a representable
      finite float.

  * - .. attribute:: float_info.max_10_exp
    - :c:macro:`!DBL_MAX_10_EXP`
    - The maximum integer *e* such that ``10**e`` is in the range of
      representable finite floats.

  * - .. attribute:: float_info.min
    - :c:macro:`!DBL_MIN`
    - The minimum representable positive *normalized* float.

      Use :func:`math.ulp(0.0) <math.ulp>` to get the smallest positive
      *denormalized* representable float.

  * - .. attribute:: float_info.min_exp
    - :c:macro:`!DBL_MIN_EXP`
    - The minimum integer *e* such that ``radix**(e-1)`` is a normalized
      float.

  * - .. attribute:: float_info.min_10_exp
    - :c:macro:`!DBL_MIN_10_EXP`
    - The minimum integer *e* such that ``10**e`` is a normalized float.

  * - .. attribute:: float_info.radix
    - :c:macro:`!FLT_RADIX`
    - The radix of exponent representation.

  * - .. attribute:: float_info.rounds
    - :c:macro:`!FLT_ROUNDS`
    - An integer representing the rounding mode for floating-point arithmetic.
      This reflects the value of the system :c:macro:`!FLT_ROUNDS` macro
      at interpreter startup time:

      * ``-1``: indeterminable
      * ``0``: toward zero
      * ``1``: to nearest
      * ``2``: toward positive infinity
      * ``3``: toward negative infinity

      All other values for :c:macro:`!FLT_ROUNDS` characterize
      implementation-defined rounding behavior.

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'

.. 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 3.1 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 3.1.

.. versionadded:: 3.1

.. function:: getallocatedblocks()

Return the number of memory blocks currently allocated by the interpreter, regardless of their size. This function is mainly useful for tracking and debugging memory leaks. Because of the interpreter's internal caches, the result can vary from call to call; you may have to call :func:_clear_internal_caches and :func:gc.collect to get more predictable results.

If a Python build or implementation cannot reasonably compute this information, :func:getallocatedblocks is allowed to return 0 instead.

.. versionadded:: 3.4

.. function:: getunicodeinternedsize()

Return the number of unicode objects that have been interned.

.. versionadded:: 3.12

.. function:: getandroidapilevel()

Return the build-time API level of Android as an integer. This represents the minimum version of Android this build of Python can run on. For runtime version information, see :func:platform.android_ver.

.. availability:: Android.

.. versionadded:: 3.7

.. function:: getdefaultencoding()

Return 'utf-8'. This is the name of the default string encoding, used in methods like :meth:str.encode.

.. function:: getdlopenflags()

Return the current value of the flags that are used for :c:func:dlopen calls. Symbolic names for the flag values can be found in the :mod:os module (:samp:RTLD_{xxx} constants, e.g. :const:os.RTLD_LAZY).

.. availability:: Unix.

.. function:: getfilesystemencoding()

Get the :term:filesystem encoding <filesystem encoding and error handler>: the encoding used with the :term:filesystem error handler <filesystem encoding and error handler> to convert between Unicode filenames and bytes filenames. The filesystem error handler is returned from :func:getfilesystemencodeerrors.

For best compatibility, str should be used for filenames in all cases, although representing filenames as bytes is also supported. Functions accepting or returning filenames should support either str or bytes and internally convert to the system's preferred representation.

:func:os.fsencode and :func:os.fsdecode should be used to ensure that the correct encoding and errors mode are used.

The :term:filesystem encoding and error handler are configured at Python startup by the :c:func:PyConfig_Read function: see :c:member:~PyConfig.filesystem_encoding and :c:member:~PyConfig.filesystem_errors members of :c:type:PyConfig.

.. versionchanged:: 3.2 :func:getfilesystemencoding result cannot be None anymore.

.. versionchanged:: 3.6 Windows is no longer guaranteed to return 'mbcs'. See :pep:529 and :func:_enablelegacywindowsfsencoding for more information.

.. versionchanged:: 3.7 Return 'utf-8' if the :ref:Python UTF-8 Mode <utf8-mode> is enabled.

.. function:: getfilesystemencodeerrors()

Get the :term:filesystem error handler <filesystem encoding and error handler>: the error handler used with the :term:filesystem encoding <filesystem encoding and error handler> to convert between Unicode filenames and bytes filenames. The filesystem encoding is returned from :func:getfilesystemencoding.

:func:os.fsencode and :func:os.fsdecode should be used to ensure that the correct encoding and errors mode are used.

.. versionadded:: 3.6

.. function:: get_int_max_str_digits()

Returns the current value for the :ref:integer string conversion length limitation <int_max_str_digits>. See also :func:set_int_max_str_digits.

.. versionadded:: 3.11

.. function:: get_lazy_imports()

Returns the current lazy imports mode as a string.

"normal": Only imports explicitly marked with the lazy keyword are lazy
"all": All top-level imports are potentially lazy
"none": All lazy imports are suppressed (even explicitly marked ones)

See also :func:set_lazy_imports and :pep:810.

.. versionadded:: 3.15

.. function:: get_lazy_imports_filter()

Returns the current lazy imports filter function, or None if no filter is set.

The filter function is called for every potentially lazy import to determine whether it should actually be lazy. See :func:set_lazy_imports_filter for details on the filter function signature.

.. versionadded:: 3.15

.. 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.

Note that the returned value may not actually reflect how many references to the object are actually held. For example, some objects are :term:immortal and have a very high refcount that does not reflect the actual number of references. Consequently, do not rely on the returned value to be accurate, other than a value of 0 or 1.

.. impl-detail::

  :term:`Immortal <immortal>` objects with a large reference count can be
  identified via :func:`_is_immortal`.

.. versionchanged:: 3.12 Immortal objects have very large refcounts that do not match the actual number of references to the object.

.. 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.

Only the memory consumption directly attributed to the object is accounted for, not the memory consumption of objects it refers to.

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.

See recursive sizeof recipe <https://code.activestate.com/recipes/577504-compute-memory-footprint-of-an-object-and-its-cont/>_ for an example of using :func:getsizeof recursively to find the size of containers and all their contents.

.. function:: getswitchinterval()

Return the interpreter's "thread switch interval" in seconds; see :func:setswitchinterval.

.. versionadded:: 3.2

.. 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.

.. audit-event:: sys._getframe frame sys._getframe

.. 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:: _getframemodulename([depth])

Return the name of a module from the call stack. If optional integer depth is given, return the module that many calls below the top of the stack. If that is deeper than the call stack, or if the module is unidentifiable, None is returned. The default for depth is zero, returning the module at the top of the call stack.

.. audit-event:: sys._getframemodulename depth sys._getframemodulename

.. impl-detail::

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

.. versionadded:: 3.12

.. function:: getobjects(limit[, type])

This function only exists if CPython was built using the specialized configure option :option:--with-trace-refs. It is intended only for debugging garbage-collection issues.

Return a list of up to limit dynamically allocated Python objects. If type is given, only objects of that exact type (not subtypes) are included.

Objects from the list are not safe to use. Specifically, the result will include objects from all interpreters that share their object allocator state (that is, ones created with :c:member:PyInterpreterConfig.use_main_obmalloc set to 1 or using :c:func:Py_NewInterpreter, and the :ref:main interpreter <sub-interpreter-support>). Mixing objects from different interpreters may lead to crashes or other unexpected behavior.

.. impl-detail::

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

.. versionchanged:: 3.14

  The result may include objects from other interpreters.

.. function:: getprofile()

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

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

.. 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.

.. 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, product_type and platform_version. service_pack contains a string, platform_version a 3-tuple and 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 will be 2 (VER_PLATFORM_WIN32_NT).

product_type may be one of the following values:

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

platform_version returns the major version, minor version and build number of the current operating system, rather than the version that is being emulated for the process. It is intended for use in logging rather than for feature detection.

.. note:: platform_version derives the version from kernel32.dll which can be of a different version than the OS version. Please use :mod:platform module for achieving accurate OS version.

.. availability:: Windows.

.. versionchanged:: 3.2 Changed to a named tuple and added service_pack_minor, service_pack_major, suite_mask, and product_type.

.. versionchanged:: 3.6 Added platform_version

.. function:: get_asyncgen_hooks()

Returns an asyncgen_hooks object, which is similar to a :class:~collections.namedtuple of the form (firstiter, finalizer), where firstiter and finalizer are expected to be either None or functions which take an :term:asynchronous generator iterator as an argument, and are used to schedule finalization of an asynchronous generator by an event loop.

.. versionadded:: 3.6 See :pep:525 for more details.

.. note:: This function has been added on a provisional basis (see :pep:411 for details.)

.. function:: get_coroutine_origin_tracking_depth()

Get the current coroutine origin tracking depth, as set by :func:set_coroutine_origin_tracking_depth.

.. versionadded:: 3.7

.. note:: This function has been added on a provisional basis (see :pep:411 for details.) Use it only for debugging purposes.

.. data:: hash_info

A :term:named tuple giving parameters of the numeric hash implementation. For more details about hashing of numeric types, see :ref:numeric-hash.

.. attribute:: hash_info.width

  The width in bits used for hash values

.. attribute:: hash_info.modulus

  The prime modulus P used for numeric hash scheme

.. attribute:: hash_info.inf

  The hash value returned for a positive infinity

.. attribute:: hash_info.nan

  (This attribute is no longer used)

.. attribute:: hash_info.imag

  The multiplier used for the imaginary part of a complex number

.. attribute:: hash_info.algorithm

  The name of the algorithm for hashing of str, bytes, and memoryview

.. attribute:: hash_info.hash_bits

  The internal output size of the hash algorithm

.. attribute:: hash_info.seed_bits

  The size of the seed key of the hash algorithm

.. attribute:: hash_info.cutoff

  Cutoff for small string DJBX33A optimization in range ``[1, cutoff)``.

.. versionadded:: 3.2

.. versionchanged:: 3.4 Added algorithm, hash_bits, seed_bits, and cutoff.

.. 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 :term:named tuple :data:sys.version_info may be used for a more human-friendly encoding of the same information.

More details of hexversion can be found at :ref:apiabiversion.

.. data:: implementation

An object containing information about the implementation of the currently running Python interpreter. The following attributes are required to exist in all Python implementations.

name is the implementation's identifier, e.g. 'cpython'. The actual string is defined by the Python implementation, but it is guaranteed to be lower case.

version is a named tuple, in the same format as :data:sys.version_info. It represents the version of the Python implementation. This has a distinct meaning from the specific version of the Python language to which the currently running interpreter conforms, which sys.version_info represents. For example, for PyPy 1.8 sys.implementation.version might be sys.version_info(1, 8, 0, 'final', 0), whereas sys.version_info would be sys.version_info(2, 7, 2, 'final', 0). For CPython they are the same value, since it is the reference implementation.

hexversion is the implementation version in hexadecimal format, like :data:sys.hexversion.

cache_tag is the tag used by the import machinery in the filenames of cached modules. By convention, it would be a composite of the implementation's name and version, like 'cpython-33'. However, a Python implementation may use some other value if appropriate. If cache_tag is set to None, it indicates that module caching should be disabled.

supports_isolated_interpreters is a boolean value, whether this implementation supports multiple isolated interpreters. It is True for CPython on most platforms. Platforms with this support implement the low-level :mod:!_interpreters module.

.. seealso::

  :pep:`684`, :pep:`734`, and :mod:`concurrent.interpreters`.

:data:sys.implementation may contain additional attributes specific to the Python implementation. These non-standard attributes must start with an underscore, and are not described here. Regardless of its contents, :data:sys.implementation will not change during a run of the interpreter, nor between implementation versions. (It may change between Python language versions, however.) See :pep:421 for more information.

.. versionadded:: 3.3

.. versionchanged:: 3.14 Added supports_isolated_interpreters field.

.. note::

  The addition of new required attributes must go through the normal PEP
  process. See :pep:`421` for more information.

.. data:: int_info

A :term:named tuple that holds information about Python's internal representation of integers. The attributes are read only.

.. attribute:: int_info.bits_per_digit

  The number of bits held in each digit.
  Python integers are stored internally in base ``2**int_info.bits_per_digit``.

.. attribute:: int_info.sizeof_digit

  The size in bytes of the C type used to represent a digit.

.. attribute:: int_info.default_max_str_digits

  The default value for :func:`sys.get_int_max_str_digits`
  when it is not otherwise explicitly configured.

.. attribute:: int_info.str_digits_check_threshold

  The minimum non-zero value for :func:`sys.set_int_max_str_digits`,
  :envvar:`PYTHONINTMAXSTRDIGITS`, or :option:`-X int_max_str_digits <-X>`.

.. versionadded:: 3.1

.. versionchanged:: 3.11

  Added :attr:`~int_info.default_max_str_digits` and
  :attr:`~int_info.str_digits_check_threshold`.

.. data:: interactivehook

When this attribute exists, its value is automatically called (with no arguments) when the interpreter is launched in :ref:interactive mode <tut-interactive>. This is done after the :envvar:PYTHONSTARTUP file is read, so that you can set this hook there. The :mod:site module :ref:sets this <rlcompleter-config>.

.. audit-event:: cpython.run_interactivehook hook sys.interactivehook

  Raises an :ref:`auditing event <auditing>`
  ``cpython.run_interactivehook`` with the hook object as the argument when
  the hook is called on startup.

.. versionadded:: 3.4

.. function:: intern(string)

Enter string in the table of "interned" strings and return the interned string -- which is string itself or a copy. Interning strings is useful to gain a little performance on dictionary lookup -- if the keys in a dictionary are interned, and the lookup key is interned, the key comparisons (after hashing) can be done by a pointer compare instead of a string compare. Normally, the names used in Python programs are automatically interned, and the dictionaries used to hold module, class or instance attributes have interned keys.

Interned strings are not :term:immortal; you must keep a reference to the return value of :func:intern around to benefit from it.

.. function:: _is_gil_enabled()

Return :const:True if the :term:GIL is enabled and :const:False if it is disabled.

.. versionadded:: 3.13

.. impl-detail::

  It is not guaranteed to exist in all implementations of Python.

.. function:: is_finalizing()

Return :const:True if the main Python interpreter is :term:shutting down <interpreter shutdown>. Return :const:False otherwise.

See also the :exc:PythonFinalizationError exception.

.. versionadded:: 3.5

.. data:: _jit

Utilities for observing just-in-time compilation.

.. impl-detail::

  JIT compilation is an *experimental implementation detail* of CPython.
  ``sys._jit`` is not guaranteed to exist or behave the same way in all
  Python implementations, versions, or build configurations.

.. versionadded:: 3.14

.. function:: _jit.is_available()

  Return ``True`` if the current Python executable supports JIT compilation,
  and ``False`` otherwise.  This can be controlled by building CPython with
  the ``--experimental-jit`` option on Windows, and the
  :option:`--enable-experimental-jit` option on all other platforms.

.. function:: _jit.is_enabled()

  Return ``True`` if JIT compilation is enabled for the current Python
  process (implies :func:`sys._jit.is_available`), and ``False`` otherwise.
  If JIT compilation is available, this can be controlled by setting the
  :envvar:`PYTHON_JIT` environment variable to ``0`` (disabled) or ``1``
  (enabled) at interpreter startup.

.. function:: _jit.is_active()

  Return ``True`` if the topmost Python frame is currently executing JIT
  code (implies :func:`sys._jit.is_enabled`), and ``False`` otherwise.

  .. note::

     This function is intended for testing and debugging the JIT itself.
     It should be avoided for any other purpose.

  .. note::

     Due to the nature of tracing JIT compilers, repeated calls to this
     function may give surprising results. For example, branching on its
     return value will likely lead to unexpected behavior (if doing so
     causes JIT code to be entered or exited):

     .. code-block:: pycon

        >>> for warmup in range(BIG_NUMBER):
        ...     # This line is "hot", and is eventually JIT-compiled:
        ...     if sys._jit.is_active():
        ...         # This line is "cold", and is run in the interpreter:
        ...         assert sys._jit.is_active()
        ...
        Traceback (most recent call last):
          File "<stdin>", line 5, in <module>
            assert sys._jit.is_active()
                   ~~~~~~~~~~~~~~~~~~^^
        AssertionError

.. data:: last_exc

This variable is not always defined; it is set to the exception instance when an exception is not handled and the interpreter prints an error message and a stack traceback. Its 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 :mod:pdb module for more information.)

.. versionadded:: 3.12

.. function:: _is_immortal(op)

Return :const:True if the given object is :term:immortal, :const:False otherwise.

.. note::

  Objects that are immortal (and thus return ``True`` upon being passed
  to this function) are not guaranteed to be immortal in future versions,
  and vice versa for mortal objects.

.. versionadded:: 3.14

.. impl-detail::

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

.. function:: _is_interned(string)

Return :const:True if the given string is "interned", :const:False otherwise.

.. versionadded:: 3.13

.. impl-detail::

  It is not guaranteed to exist in all implementations of Python.

.. data:: last_type last_value last_traceback

These three variables are deprecated; use :data:sys.last_exc instead. They hold the legacy representation of sys.last_exc, as returned from :func:exc_info above.

.. data:: maxsize

An integer giving the maximum value a variable of type :c:type:Py_ssize_t can take. It's usually 2**31 - 1 on a 32-bit platform and 2**63 - 1 on a 64-bit platform.

.. data:: maxunicode

An integer giving the value of the largest Unicode code point, i.e. 1114111 (0x10FFFF in hexadecimal).

.. versionchanged:: 3.3 Before :pep:393, sys.maxunicode used to be either 0xFFFF or 0x10FFFF, depending on the configuration option that specified whether Unicode characters were stored as UCS-2 or UCS-4.

.. data:: meta_path

A list of :term:`meta path finder` objects that have their
:meth:`~importlib.abc.MetaPathFinder.find_spec` methods called to see if one
of the objects can find the module to be imported. By default, it holds entries
that implement Python's default import semantics. The
:meth:`~importlib.abc.MetaPathFinder.find_spec` method is called with at
least the absolute name of the module being imported. If the module to be
imported is contained in a package, then the parent package's
:attr:`~module.__path__`
attribute is passed in as a second argument. The method returns a
:term:`module spec`, or ``None`` if the module cannot be found.

.. seealso::

    :class:`importlib.abc.MetaPathFinder`
      The abstract base class defining the interface of finder objects on
      :data:`meta_path`.
    :class:`importlib.machinery.ModuleSpec`
      The concrete class which
      :meth:`~importlib.abc.MetaPathFinder.find_spec` should return
      instances of.

.. versionchanged:: 3.4

    :term:`Module specs <module spec>` were introduced in Python 3.4, by
    :pep:`451`.

.. versionchanged:: 3.12

    Removed the fallback that looked for a :meth:`!find_module` method
    if a :data:`meta_path` entry didn't have a
    :meth:`~importlib.abc.MetaPathFinder.find_spec` method.

.. data:: modules

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. However, replacing the dictionary will not necessarily work as expected and deleting essential items from the dictionary may cause Python to fail. If you want to iterate over this global dictionary always use sys.modules.copy() or tuple(sys.modules) to avoid exceptions as its size may change during iteration as a side effect of code or activity in other threads.

.. data:: orig_argv

The list of the original command line arguments passed to the Python executable.

The elements of :data:sys.orig_argv are the arguments to the Python interpreter, while the elements of :data:sys.argv are the arguments to the user's program. Arguments consumed by the interpreter itself will be present in :data:sys.orig_argv and missing from :data:sys.argv.

.. versionadded:: 3.10

.. 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.

By default, as initialized upon program startup, a potentially unsafe path is prepended to :data:sys.path (before the entries inserted as a result of :envvar:PYTHONPATH):

python -m module command line: prepend the current working directory.
python script.py command line: prepend the script's directory. If it's a symbolic link, resolve symbolic links.
python -c code and python (REPL) command lines: prepend an empty string, which means the current working directory.

To not prepend this potentially unsafe path, use the :option:-P command line option or the :envvar:PYTHONSAFEPATH environment variable.

A program is free to modify this list for its own purposes. Only strings should be added to :data:sys.path; all other data types are ignored during import.

.. 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
finder is found on :data:`sys.path_hooks` then ``None`` is
stored.

Originally specified in :pep:`302`.

.. data:: platform

A string containing a platform identifier. Known values are:

================ =========================== System platform value ================ =========================== AIX 'aix' Android 'android' Emscripten 'emscripten' FreeBSD 'freebsd' iOS 'ios' Linux 'linux' macOS 'darwin' Windows 'win32' Windows/Cygwin 'cygwin' WASI 'wasi' ================ ===========================

On Unix systems not listed in the table, the value 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', 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('sunos'):
      # SunOS-specific code here...

.. versionchanged:: 3.3 On Linux, :data:sys.platform doesn't contain the major version anymore. It is always 'linux', instead of 'linux2' or 'linux3'.

.. versionchanged:: 3.8 On AIX, :data:sys.platform doesn't contain the major version anymore. It is always 'aix', instead of 'aix5' or 'aix7'.

.. versionchanged:: 3.13 On Android, :data:sys.platform now returns 'android' rather than 'linux'.

.. versionchanged:: 3.14 On FreeBSD, :data:sys.platform doesn't contain the major version anymore. It is always 'freebsd', instead of 'freebsd13' or 'freebsd14'.

.. seealso::

  :data:`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:: platlibdir

Name of the platform-specific library directory. It is used to build the path of standard library and the paths of installed extension modules.

It is equal to "lib" on most platforms. On Fedora and SuSE, it is equal to "lib64" on 64-bit platforms which gives the following sys.path paths (where X.Y is the Python major.minor version):

/usr/lib64/pythonX.Y/: Standard library (like os.py of the :mod:os module)
/usr/lib64/pythonX.Y/lib-dynload/: C extension modules of the standard library (like the :mod:errno module, the exact filename is platform specific)
/usr/lib/pythonX.Y/site-packages/ (always use lib, not :data:sys.platlibdir): Third-party modules
/usr/lib64/pythonX.Y/site-packages/: C extension modules of third-party packages

.. versionadded:: 3.9

.. data:: prefix

A string giving the site-specific directory prefix where the platform independent Python files are installed; on Unix, the default is :file:/usr/local. This can be set at build time with the :option:--prefix argument to the :program:configure script. See :ref:installation_paths for derived paths.

.. note::

  If a :ref:`virtual environment <venv-def>` is in effect, this :data:`prefix`
  will point to the virtual environment. The value for the Python installation
  will still be available, via :data:`base_prefix`.
  Refer to :ref:`sys-path-init-virtual-environments` for more information.

.. versionchanged:: 3.14

  When running under a :ref:`virtual environment <venv-def>`,
  :data:`prefix` and :data:`exec_prefix` are now set to the virtual
  environment prefix by the :ref:`path initialization <sys-path-init>`,
  instead of :mod:`site`. This means that :data:`prefix` and
  :data:`exec_prefix` always point to the virtual environment, even when
  :mod:`site` is disabled (:option:`-S`).

.. data:: ps1 ps2

.. index:: single: interpreter prompts single: prompts, interpreter single: >>>; interpreter prompt single: ...; interpreter prompt

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.

.. function:: setdlopenflags(n)

Set the flags used by the interpreter for :c:func: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(os.RTLD_GLOBAL). Symbolic names for the flag values can be found in the :mod:os module (:samp:RTLD_{xxx} constants, e.g. :const:os.RTLD_LAZY).

.. availability:: Unix.

.. function:: set_int_max_str_digits(maxdigits)

Set the :ref:integer string conversion length limitation <int_max_str_digits> used by this interpreter. See also :func:get_int_max_str_digits.

.. versionadded:: 3.11

.. function:: set_lazy_imports(mode)

Sets the global lazy imports mode. The mode parameter must be one of the following strings:

"normal": Only imports explicitly marked with the lazy keyword are lazy
"all": All top-level imports become potentially lazy
"none": All lazy imports are suppressed (even explicitly marked ones)

This function is intended for advanced users who need to control lazy imports across their entire application. Library developers should generally not use this function as it affects the runtime execution of applications.

In addition to the mode, lazy imports can be controlled via the filter provided by :func:set_lazy_imports_filter.

See also :func:get_lazy_imports and :pep:810.

.. versionadded:: 3.15

.. function:: set_lazy_imports_filter(filter)

Sets the lazy imports filter callback. The filter parameter must be a callable or None to clear the filter.

The filter function is called for every potentially lazy import to determine whether it should actually be lazy. It must have the following signature::

  def filter(importing_module: str, imported_module: str,
             fromlist: tuple[str, ...] | None) -> bool

Where:

importing_module is the name of the module doing the import
imported_module is the resolved name of the module being imported (for example, lazy from .spam import eggs passes package.spam)
fromlist is the tuple of names being imported (for from ... import statements), or None for regular imports

The filter should return True to allow the import to be lazy, or False to force an eager import.

This is an advanced feature intended for specialized users who need fine-grained control over lazy import behavior.

See also :func:get_lazy_imports_filter and :pep:810.

.. versionadded:: 3.15

.. 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 is called with different events, for example 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. Error in the profile function will cause itself unset.

.. note:: The same tracing mechanism is used for :func:!setprofile as :func:settrace. To trace calls with :func:!setprofile inside a tracing function (e.g. in a debugger breakpoint), see :func:call_tracing.

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

The events have the following meaning:

'call' A function is called (or some other code block entered). The profile function is called; arg is None.

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

'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.

.. audit-event:: sys.setprofile "" sys.setprofile

.. 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 they have 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.

If the new limit is too low at the current recursion depth, a :exc:RecursionError exception is raised.

.. versionchanged:: 3.5.1 A :exc:RecursionError exception is now raised if the new limit is too low at the current recursion depth.

.. function:: setswitchinterval(interval)

Set the interpreter's thread switch interval (in seconds). This floating-point value determines the ideal duration of the "timeslices" allocated to concurrently running Python threads. Please note that the actual value can be higher, especially if long-running internal functions or methods are used. Also, which thread becomes scheduled at the end of the interval is the operating system's decision. The interpreter doesn't have its own scheduler.

.. versionadded:: 3.2

.. 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 register a trace function using :func:settrace for each thread being debugged or use :func:threading.settrace.

Trace functions should have three arguments: frame, event, and arg. frame is the :ref:current stack frame <frame-objects>. event is a string: 'call', 'line', 'return', 'exception' or 'opcode'. 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 for the new scope, or None if the scope shouldn't be traced.

The local trace function should return a reference to itself, or to another function which would then be used as the local trace function for the scope.

If there is any error occurred in the trace function, it will be unset, just like settrace(None) is called.

.. note:: Tracing is disabled while calling the trace function (e.g. a function set by :func:!settrace). For recursive tracing see :func:call_tracing.

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. Per-line events may be disabled for a frame by setting :attr:~frame.f_trace_lines to :const:False on that :ref:frame <frame-objects>.

'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.

'opcode' The interpreter is about to execute a new opcode (see :mod:dis for opcode details). The local trace function is called; arg is None; the return value specifies the new local trace function. Per-opcode events are not emitted by default: they must be explicitly requested by setting :attr:~frame.f_trace_opcodes to :const:True on the :ref:frame <frame-objects>.

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

For more fine-grained usage, it's possible to set a trace function by assigning frame.f_trace = tracefunc explicitly, rather than relying on it being set indirectly via the return value from an already installed trace function. This is also required for activating the trace function on the current frame, which :func:settrace doesn't do. Note that in order for this to work, a global tracing function must have been installed with :func:settrace in order to enable the runtime tracing machinery, but it doesn't need to be the same tracing function (e.g. it could be a low overhead tracing function that simply returns None to disable itself immediately on each frame).

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

.. audit-event:: sys.settrace "" sys.settrace

.. 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.

.. versionchanged:: 3.7

  ``'opcode'`` event type added; :attr:`~frame.f_trace_lines` and
  :attr:`~frame.f_trace_opcodes` attributes added to frames

.. function:: set_asyncgen_hooks([firstiter] [, finalizer])

Accepts two optional keyword arguments which are callables that accept an :term:asynchronous generator iterator as an argument. The firstiter callable will be called when an asynchronous generator is iterated for the first time. The finalizer will be called when an asynchronous generator is about to be garbage collected.

.. audit-event:: sys.set_asyncgen_hooks_firstiter "" sys.set_asyncgen_hooks

.. audit-event:: sys.set_asyncgen_hooks_finalizer "" sys.set_asyncgen_hooks

Two auditing events are raised because the underlying API consists of two calls, each of which must raise its own event.

.. versionadded:: 3.6 See :pep:525 for more details, and for a reference example of a finalizer method see the implementation of asyncio.Loop.shutdown_asyncgens in :source:Lib/asyncio/base_events.py

.. note:: This function has been added on a provisional basis (see :pep:411 for details.)

.. function:: set_coroutine_origin_tracking_depth(depth)

Allows enabling or disabling coroutine origin tracking. When enabled, the cr_origin attribute on coroutine objects will contain a tuple of (filename, line number, function name) tuples describing the traceback where the coroutine object was created, with the most recent call first. When disabled, cr_origin will be None.

To enable, pass a depth value greater than zero; this sets the number of frames whose information will be captured. To disable, set depth to zero.

This setting is thread-specific.

.. versionadded:: 3.7

.. note:: This function has been added on a provisional basis (see :pep:411 for details.) Use it only for debugging purposes.

.. function:: activate_stack_trampoline(backend, /)

Activate the stack profiler trampoline backend. The only supported backend is "perf".

Stack trampolines cannot be activated if the JIT is active.

.. availability:: Linux.

.. versionadded:: 3.12

.. seealso::

  * :ref:`perf_profiling`
  * https://perf.wiki.kernel.org

.. function:: deactivate_stack_trampoline()

Deactivate the current stack profiler trampoline backend.

If no stack profiler is activated, this function has no effect.

.. availability:: Linux.

.. versionadded:: 3.12

.. function:: is_stack_trampoline_active()

Return True if a stack profiler trampoline is active.

.. availability:: Linux.

.. versionadded:: 3.12

.. function:: remote_exec(pid, script)

Executes script, a file containing Python code in the remote process with the given pid.

This function returns immediately, and the code will be executed by the target process's main thread at the next available opportunity, similarly to how signals are handled. There is no interface to determine when the code has been executed. The caller is responsible for making sure that the file still exists whenever the remote process tries to read it and that it hasn't been overwritten.

The remote process must be running a CPython interpreter of the same major and minor version as the local process. If either the local or remote interpreter is pre-release (alpha, beta, or release candidate) then the local and remote interpreters must be the same exact version.

See :ref:remote-debugging for more information about the remote debugging mechanism.

.. audit-event:: sys.remote_exec pid script_path

  When the code is executed in the remote process, an
  :ref:`auditing event <auditing>` ``sys.remote_exec`` is raised with
  the *pid* and the path to the script file.
  This event is raised in the process that called :func:`sys.remote_exec`.

.. audit-event:: cpython.remote_debugger_script script_path

  When the script is executed in the remote process, an
  :ref:`auditing event <auditing>`
  ``cpython.remote_debugger_script`` is raised
  with the path in the remote process.
  This event is raised in the remote process, not the one
  that called :func:`sys.remote_exec`.

.. availability:: Unix, Windows. .. versionadded:: 3.14 See :pep:768 for more details.

.. function:: _enablelegacywindowsfsencoding()

Changes the :term:filesystem encoding and error handler to 'mbcs' and 'replace' respectively, for consistency with versions of Python prior to 3.6.

This is equivalent to defining the :envvar:PYTHONLEGACYWINDOWSFSENCODING environment variable before launching Python.

See also :func:sys.getfilesystemencoding and :func:sys.getfilesystemencodeerrors.

.. availability:: Windows.

.. note:: Changing the filesystem encoding after Python startup is risky because the old fsencoding or paths encoded by the old fsencoding may be cached somewhere. Use :envvar:PYTHONLEGACYWINDOWSFSENCODING instead.

.. versionadded:: 3.6 See :pep:529 for more details.

.. deprecated-removed:: 3.13 3.16 Use :envvar:PYTHONLEGACYWINDOWSFSENCODING instead.

.. data:: stdin stdout stderr

:term:File objects <file object> used by the interpreter for standard input, output and errors:

stdin is used for all interactive input (including calls to :func:input);
stdout is used for the output of :func:print and :term:expression statements and for the prompts of :func:input;
The interpreter's own prompts and its error messages go to stderr.

These streams are regular :term:text files <text file> like those returned by the :func:open function. Their parameters are chosen as follows:

The encoding and error handling are is initialized from :c:member:PyConfig.stdio_encoding and :c:member:PyConfig.stdio_errors.

On Windows, UTF-8 is used for the console device. Non-character devices such as disk files and pipes use the system locale encoding (i.e. the ANSI codepage). Non-console character devices such as NUL (i.e. where isatty() returns True) use the value of the console input and output codepages at startup, respectively for stdin and stdout/stderr. This defaults to the system :term:locale encoding if the process is not initially attached to a console.

The special behaviour of the console can be overridden by setting the environment variable PYTHONLEGACYWINDOWSSTDIO before starting Python. In that case, the console codepages are used as for any other character device.

Under all platforms, you can override the character encoding by setting the :envvar:PYTHONIOENCODING environment variable before starting Python or by using the new :option:-X utf8 command line option and :envvar:PYTHONUTF8 environment variable. However, for the Windows console, this only applies when :envvar:PYTHONLEGACYWINDOWSSTDIO is also set.
When interactive, the stdout stream is line-buffered. Otherwise, it is block-buffered like regular text files. The stderr stream is line-buffered in both cases. You can make both streams unbuffered by passing the :option:-u command-line option or setting the :envvar:PYTHONUNBUFFERED environment variable.

.. versionchanged:: 3.9 Non-interactive stderr is now line-buffered instead of fully buffered.

.. note::

  To write or read binary data from/to the standard streams, use the
  underlying binary :data:`~io.TextIOBase.buffer` object.  For example, to
  write bytes to :data:`stdout`, use ``sys.stdout.buffer.write(b'abc')``.

  However, if you are writing a library (and do not control in which
  context its code will be executed), be aware that the standard streams
  may be replaced with file-like objects like :class:`io.StringIO` which
  do not support the :attr:`!buffer` attribute.

.. 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.

.. note:: Under some conditions stdin, stdout and stderr as well as the original values __stdin__, __stdout__ and __stderr__ can be None. It is usually the case for Windows GUI apps that aren't connected to a console and Python apps started with :program:pythonw.

.. data:: stdlib_module_names

A frozenset of strings containing the names of standard library modules.

It is the same on all platforms. Modules which are not available on some platforms and modules disabled at Python build are also listed. All module kinds are listed: pure Python, built-in, frozen and extension modules. Test modules are excluded.

For packages, only the main package is listed: sub-packages and sub-modules are not listed. For example, the email package is listed, but the email.mime sub-package and the email.message sub-module are not listed.

See also the :data:sys.builtin_module_names list.

.. versionadded:: 3.10

.. data:: thread_info

A :term:named tuple holding information about the thread implementation.

.. attribute:: thread_info.name

  The name of the thread implementation:

  * ``"nt"``: Windows threads
  * ``"pthread"``: POSIX threads
  * ``"pthread-stubs"``: stub POSIX threads
    (on WebAssembly platforms without threading support)
  * ``"solaris"``: Solaris threads

.. attribute:: thread_info.lock

  The name of the lock implementation:

  * ``"semaphore"``: a lock uses a semaphore
  * ``"mutex+cond"``: a lock uses a mutex and a condition variable
  * ``None`` if this information is unknown

.. attribute:: thread_info.version

  The name and version of the thread library.
  It is a string, or ``None`` if this information is unknown.

.. versionadded:: 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.

.. function:: unraisablehook(unraisable, /)

Handle an unraisable exception.

Called when an exception has occurred but there is no way for Python to handle it. For example, when a destructor raises an exception or during garbage collection (:func:gc.collect).

The unraisable argument has the following attributes:

:attr:!exc_type: Exception type.
:attr:!exc_value: Exception value, can be None.
:attr:!exc_traceback: Exception traceback, can be None.
:attr:!err_msg: Error message, can be None.
:attr:!object: Object causing the exception, can be None.

The default hook formats :attr:!err_msg and :attr:!object as: f'{err_msg}: {object!r}'; use "Exception ignored in" error message if :attr:!err_msg is None. Similar to the :mod:traceback module, this adds color to exceptions by default. This can be disabled using :ref:environment variables <using-on-controlling-color>.

:func:sys.unraisablehook can be overridden to control how unraisable exceptions are handled.

.. versionchanged:: 3.15 Exceptions are now printed with colorful text.

.. seealso::

  :func:`excepthook` which handles uncaught exceptions.

.. warning::

  Storing :attr:`!exc_value` using a custom hook can create a reference cycle.
  It should be cleared explicitly to break the reference cycle when the
  exception is no longer needed.

  Storing :attr:`!object` using a custom hook can resurrect it if it is set to an
  object which is being finalized. Avoid storing :attr:`!object` after the custom
  hook completes to avoid resurrecting objects.

.. audit-event:: sys.unraisablehook hook,unraisable sys.unraisablehook

  Raise an auditing event ``sys.unraisablehook`` with arguments
  *hook*, *unraisable* when an exception that cannot be handled occurs.
  The *unraisable* object is the same as what will be passed to the hook.
  If no hook has been set, *hook* may be ``None``.

.. versionadded:: 3.8

.. 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, equivalent to the C macro :c:macro:PYTHON_API_VERSION. Defined for backwards compatibility.

Currently, this constant is not updated in new Python versions, and is not useful for versioning. This may change in the future.

.. 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.

.. versionchanged:: 3.1 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 major and minor versions of the running Python interpreter. 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.

.. data:: monitoring :noindex:

Namespace containing functions and constants for register callbacks and controlling monitoring events. See :mod:sys.monitoring for details.

.. data:: _xoptions

A dictionary of the various implementation-specific flags passed through the :option:-X command-line option. Option names are either mapped to their values, if given explicitly, or to :const:True. Example:

.. code-block:: shell-session

  $ ./python -Xa=b -Xc
  Python 3.2a3+ (py3k, Oct 16 2010, 20:14:50)
  [GCC 4.4.3] on linux2
  Type "help", "copyright", "credits" or "license" for more information.
  >>> import sys
  >>> sys._xoptions
  {'a': 'b', 'c': True}

.. impl-detail::

  This is a CPython-specific way of accessing options passed through
  :option:`-X`.  Other implementations may export them through other
  means, or not at all.

.. versionadded:: 3.2

.. rubric:: Citations

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

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

:mod:!sys --- System-specific parameters and functions

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