: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:: 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

.. 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:: base_exec_prefix

Set during Python startup, before site.py is run, to the same value as :data:exec_prefix. If not running in a :ref:virtual environment <venv-def>, the values will stay the same; if site.py finds that a virtual environment is in use, the values of :data:prefix and :data:exec_prefix will be changed to point to the virtual environment, whereas :data:base_prefix and :data:base_exec_prefix will remain pointing to the base Python installation (the one which the virtual environment was created from).

.. versionadded:: 3.3

.. data:: base_prefix

Set during Python startup, before site.py is run, to the same value as :data:prefix. If not running in a :ref:virtual environment <venv-def>, the values will stay the same; if site.py finds that a virtual environment is in use, the values of :data:prefix and :data:exec_prefix will be changed to point to the virtual environment, whereas :data:base_prefix and :data:base_exec_prefix will remain pointing to the base Python installation (the one which the virtual environment was created from).

.. 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 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.

.. 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.

.. 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 configured --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.

.. 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:: breakpointhook displayhook excepthook

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

.. versionadded:: 3.7 breakpointhook

.. 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 an except clause." For any stack frame, only information about the exception being currently handled 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 type of the exception being handled (a subclass of :exc:BaseException); value gets the exception instance (an instance of the exception type); traceback gets a traceback object (see the Reference Manual) which encapsulates the call stack at the point where the exception originally occurred.

.. 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
  value will be changed in ``site.py`` to point to the virtual environment.
  The value for the Python installation will still be available, via
  :data:`base_exec_prefix`.

.. 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])

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.

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

============================= ============================= attribute flag ============================= ============================= :const:debug :option:-d :const:inspect :option:-i :const:interactive :option:-i :const:isolated :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:verbose :option:-v :const:bytes_warning :option:-b :const:quiet :option:-q :const:hash_randomization :option:-R :const:dev_mode :option:-X dev :const:utf8_mode :option:-X utf8 ============================= =============================

.. 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 dev_mode attribute for the new :option:-X dev flag and utf8_mode attribute for the new :option:-X utf8 flag.

.. data:: float_info

A :term:struct sequence 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.

.. tabularcolumns:: |l|l|L|

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_type_cache() 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:: getandroidapilevel()

Return the build time API version of Android as an integer.

.. availability:: Android.

.. versionadded:: 3.7

.. function:: getcheckinterval()

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

.. deprecated:: 3.2 Use :func:getswitchinterval instead.

.. function:: getdefaultencoding()

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

.. 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 (RTLD_xxx constants, e.g. :data:os.RTLD_LAZY).

.. availability:: Unix.

.. function:: getfilesystemencoding()

Return the name of the encoding used to convert between Unicode filenames and bytes filenames. 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.

This encoding is always ASCII-compatible.

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

In the UTF-8 mode, the encoding is utf-8 on any platform.
On Mac OS X, the encoding is 'utf-8'.
On Unix, the encoding is the locale encoding.
On Windows, the encoding may be 'utf-8' or 'mbcs', depending on user configuration.

.. 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' in the UTF-8 mode.

.. function:: getfilesystemencodeerrors()

Return the name of the error mode used to convert between Unicode filenames and bytes filenames. The encoding name 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:: 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.

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>_ 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"; 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.

.. 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.

.. 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 :const: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 accurate 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.

.. 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.

.. function:: get_coroutine_wrapper()

Returns None, or a wrapper set by :func:set_coroutine_wrapper.

.. versionadded:: 3.5 See :pep:492 for more details.

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

.. deprecated:: 3.7 The coroutine wrapper functionality has been deprecated, and will be removed in 3.8. See :issue:32591 for details.

.. data:: hash_info

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

+---------------------+--------------------------------------------------+ | attribute | explanation | +=====================+==================================================+ | :const:width | width in bits used for hash values | +---------------------+--------------------------------------------------+ | :const:modulus | prime modulus P used for numeric hash scheme | +---------------------+--------------------------------------------------+ | :const:inf | hash value returned for a positive infinity | +---------------------+--------------------------------------------------+ | :const:nan | hash value returned for a nan | +---------------------+--------------------------------------------------+ | :const:imag | multiplier used for the imaginary part of a | | | complex number | +---------------------+--------------------------------------------------+ | :const:algorithm | name of the algorithm for hashing of str, bytes, | | | and memoryview | +---------------------+--------------------------------------------------+ | :const:hash_bits | internal output size of the hash algorithm | +---------------------+--------------------------------------------------+ | :const:seed_bits | size of the seed key of the hash algorithm | +---------------------+--------------------------------------------------+

.. versionadded:: 3.2

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

.. 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:struct sequence :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.

: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

.. data:: int_info

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

.. tabularcolumns:: |l|L|

.. versionadded:: 3.1

.. 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>.

.. 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 immortal; you must keep a reference to the return value of :func:intern around to benefit from it.

.. function:: is_finalizing()

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

.. versionadded:: 3.5

.. 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 :mod:pdb module for more information.)

The meaning of the variables is the same as that of the return values 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. 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:`__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`. Earlier versions of Python looked for a method called
    :meth:`~importlib.abc.MetaPathFinder.find_module`.
    This is still called as a fallback if a :data:`meta_path` entry doesn'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.

.. 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. Only strings and bytes 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`.

.. versionchanged:: 3.3
   ``None`` is stored instead of :class:`imp.NullImporter` when no finder
   is found.

.. 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, except on Linux, 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 'freebsd8', 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('freebsd'):
      # FreeBSD-specific code here...
  elif sys.platform.startswith('linux'):
      # Linux-specific code here...

For other systems, the values are:

================ =========================== System platform value ================ =========================== Linux 'linux' Windows 'win32' Windows/Cygwin 'cygwin' Mac OS X 'darwin' ================ ===========================

.. versionchanged:: 3.3 On Linux, :attr:sys.platform doesn't contain the major version anymore. It is always 'linux', instead of 'linux2' or 'linux3'. Since older Python versions include the version number, it is recommended to always use the startswith idiom presented above.

.. 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 :file:{prefix}/lib/python{X.Y} while the platform independent header files (all except :file:pyconfig.h) are stored in :file:{prefix}/include/python{X.Y}, 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 value will be changed in site.py to point to the virtual environment. The value for the Python installation will still be available, via :data:base_prefix.

.. 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:: 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.

.. deprecated:: 3.2 This function doesn't have an effect anymore, as the internal logic for thread switching and asynchronous tasks has been rewritten. Use :func:setswitchinterval instead.

.. 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 (RTLD_xxx constants, e.g. :data:os.RTLD_LAZY).

.. availability:: Unix.

.. 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.

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.

.. 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 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' 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 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.

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

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:f_trace_lines to :const:False on that frame.

'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:f_trace_opcodes to :const:True on the frame.

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.

.. versionchanged:: 3.7

  ``'opcode'`` event type added; :attr:`f_trace_lines` and
  :attr:`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.

.. 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, pass 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:: set_coroutine_wrapper(wrapper)

Allows intercepting creation of :term:coroutine objects (only ones that are created by an :keyword:async def function; generators decorated with :func:types.coroutine or :func:asyncio.coroutine will not be intercepted).

The wrapper argument must be either:

a callable that accepts one argument (a coroutine object);
None, to reset the wrapper.

If called twice, the new wrapper replaces the previous one. The function is thread-specific.

The wrapper callable cannot define new coroutines directly or indirectly::

    def wrapper(coro):
        async def wrap(coro):
            return await coro
        return wrap(coro)
    sys.set_coroutine_wrapper(wrapper)

    async def foo():
        pass

    # The following line will fail with a RuntimeError, because
    # ``wrapper`` creates a ``wrap(coro)`` coroutine:
    foo()

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

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

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