.. highlight:: sh

.. ATTENTION: You probably should update Misc/python.man, too, if you modify
   this file.

.. _using-on-general:

Command line and environment
============================

The CPython interpreter scans the command line and the environment for various
settings.

.. impl-detail::

   Other implementations' command line schemes may differ.  See
   :ref:`implementations` for further resources.


.. _using-on-cmdline:

Command line
------------

When invoking Python, you may specify any of these options::

    python [-bBdEhiIOPqRsSuvVWx?] [-c command | -m module-name | script | - ] [args]

The most common use case is, of course, a simple invocation of a script::

    python myscript.py


.. _using-on-interface-options:

Interface options
~~~~~~~~~~~~~~~~~

The interpreter interface resembles that of the UNIX shell, but provides some
additional methods of invocation:

* When called with standard input connected to a tty device, it prompts for
  commands and executes them until an EOF (an end-of-file character, you can
  produce that with :kbd:`Ctrl-D` on UNIX or :kbd:`Ctrl-Z, Enter` on Windows) is read.
  For more on interactive mode, see :ref:`tut-interac`.
* When called with a file name argument or with a file as standard input, it
  reads and executes a script from that file.
* When called with a directory name argument, it reads and executes an
  appropriately named script from that directory.
* When called with ``-c command``, it executes the Python statement(s) given as
  *command*.  Here *command* may contain multiple statements separated by
  newlines.
* When called with ``-m module-name``, the given module is located using the standard
  import mechanism and executed as a script.

In non-interactive mode, the entire input is parsed before it is executed.

An interface option terminates the list of options consumed by the interpreter,
all consecutive arguments will end up in :data:`sys.argv` -- note that the first
element, subscript zero (``sys.argv[0]``), is a string reflecting the program's
source.

.. option:: -c <command>

   Execute the Python code in *command*.  *command* can be one or more
   statements separated by newlines, with significant leading whitespace as in
   normal module code.

   If this option is given, the first element of :data:`sys.argv` will be
   ``"-c"`` and the current directory will be added to the start of
   :data:`sys.path` (allowing modules in that directory to be imported as top
   level modules).

   .. audit-event:: cpython.run_command command cmdoption-c

   .. versionchanged:: 3.14
      *command* is automatically dedented before execution.

.. option:: -m <module-name>

   Locate the module using the standard import mechanism and execute its contents
   as the :mod:`__main__` module.

   Since the argument is a *module* name, you must not give a file extension
   (``.py``).  The module name should be a valid absolute Python module name, but
   the implementation may not always enforce this (e.g. it may allow you to
   use a name that includes a hyphen).

   Package names (including namespace packages) are also permitted. When a
   package name is supplied instead
   of a normal module, the interpreter will execute ``<pkg>.__main__`` as
   the main module. This behaviour is deliberately similar to the handling
   of directories and zipfiles that are passed to the interpreter as the
   script argument.

   .. note::

      This option cannot be used with built-in modules and extension modules
      written in C, since they do not have Python module files. However, it
      can still be used for precompiled modules, even if the original source
      file is not available.

   If this option is given, the first element of :data:`sys.argv` will be the
   full path to the module file (while the module file is being located, the
   first element will be set to ``"-m"``). As with the :option:`-c` option,
   the current directory will be added to the start of :data:`sys.path`.

   :option:`-I` option can  be used to run the script in isolated mode where
   :data:`sys.path` contains neither the current directory nor the user's
   site-packages directory. All ``PYTHON*`` environment variables are
   ignored, too.

   Many standard library modules contain code that is invoked on their execution
   as a script.  An example is the :mod:`timeit` module::

       python -m timeit -s "setup here" "benchmarked code here"
       python -m timeit -h # for details

   .. audit-event:: cpython.run_module module-name cmdoption-m

   .. seealso::
      :func:`runpy.run_module`
         Equivalent functionality directly available to Python code

      :pep:`338` -- Executing modules as scripts

   .. versionchanged:: 3.1
      Supply the package name to run a ``__main__`` submodule.

   .. versionchanged:: 3.4
      namespace packages are also supported

.. _cmdarg-dash:

.. describe:: -

   Read commands from standard input (:data:`sys.stdin`).  If standard input is
   a terminal, :option:`-i` is implied.

   If this option is given, the first element of :data:`sys.argv` will be
   ``"-"`` and the current directory will be added to the start of
   :data:`sys.path`.

   .. audit-event:: cpython.run_stdin "" ""

.. _cmdarg-script:

.. describe:: <script>

   Execute the Python code contained in *script*, which must be a filesystem
   path (absolute or relative) referring to either a Python file, a directory
   containing a ``__main__.py`` file, or a zipfile containing a
   ``__main__.py`` file.

   If this option is given, the first element of :data:`sys.argv` will be the
   script name as given on the command line.

   If the script name refers directly to a Python file, the directory
   containing that file is added to the start of :data:`sys.path`, and the
   file is executed as the :mod:`__main__` module.

   If the script name refers to a directory or zipfile, the script name is
   added to the start of :data:`sys.path` and the ``__main__.py`` file in
   that location is executed as the :mod:`__main__` module.

   :option:`-I` option can  be used to run the script in isolated mode where
   :data:`sys.path` contains neither the script's directory nor the user's
   site-packages directory. All ``PYTHON*`` environment variables are
   ignored, too.

   .. audit-event:: cpython.run_file filename

   .. seealso::
      :func:`runpy.run_path`
         Equivalent functionality directly available to Python code


If no interface option is given, :option:`-i` is implied, ``sys.argv[0]`` is
an empty string (``""``) and the current directory will be added to the
start of :data:`sys.path`.  Also, tab-completion and history editing is
automatically enabled, if available on your platform (see
:ref:`rlcompleter-config`).

.. seealso::  :ref:`tut-invoking`

.. versionchanged:: 3.4
   Automatic enabling of tab-completion and history editing.


.. _using-on-generic-options:

Generic options
~~~~~~~~~~~~~~~

.. option:: -?
            -h
            --help

   Print a short description of all command line options and corresponding
   environment variables and exit.

.. option:: --help-env

   Print a short description of Python-specific environment variables
   and exit.

   .. versionadded:: 3.11

.. option:: --help-xoptions

   Print a description of implementation-specific :option:`-X` options
   and exit.

   .. versionadded:: 3.11

.. option:: --help-all

   Print complete usage information and exit.

   .. versionadded:: 3.11

.. option:: -V
            --version

   Print the Python version number and exit.  Example output could be:

   .. code-block:: none

       Python 3.8.0b2+

   When given twice, print more information about the build, like:

   .. code-block:: none

       Python 3.8.0b2+ (3.8:0c076caaa8, Apr 20 2019, 21:55:00)
       [GCC 6.2.0 20161005]

   .. versionadded:: 3.6
      The ``-VV`` option.


.. _using-on-misc-options:

Miscellaneous options
~~~~~~~~~~~~~~~~~~~~~

.. option:: -b

   Issue a warning when converting :class:`bytes` or :class:`bytearray` to
   :class:`str` without specifying encoding or comparing :class:`!bytes` or
   :class:`!bytearray` with :class:`!str` or :class:`!bytes` with :class:`int`.
   Issue an error when the option is given twice (:option:`!-bb`).

   .. versionchanged:: 3.5
      Affects also comparisons of :class:`bytes` with :class:`int`.

   .. deprecated:: 3.15

      Deprecate :option:`-b` and :option:`!-bb` command line options
      and schedule them to become no-op in Python 3.17.
      These were primarily helpers for the Python 2 -> 3 transition.
      Starting with Python 3.17, no :exc:`BytesWarning` will be raised
      for these cases; use a type checker instead.


.. option:: -B

   If given, Python won't try to write ``.pyc`` files on the
   import of source modules.  See also :envvar:`PYTHONDONTWRITEBYTECODE`.


.. option:: --check-hash-based-pycs default|always|never

   Control the validation behavior of hash-based ``.pyc`` files. See
   :ref:`pyc-invalidation`. When set to ``default``, checked and unchecked
   hash-based bytecode cache files are validated according to their default
   semantics. When set to ``always``, all hash-based ``.pyc`` files, whether
   checked or unchecked, are validated against their corresponding source
   file. When set to ``never``, hash-based ``.pyc`` files are not validated
   against their corresponding source files.

   The semantics of timestamp-based ``.pyc`` files are unaffected by this
   option.


.. option:: -d

   Turn on parser debugging output (for expert only).
   See also the :envvar:`PYTHONDEBUG` environment variable.

   This option requires a :ref:`debug build of Python <debug-build>`, otherwise
   it's ignored.


.. option:: -E

   Ignore all ``PYTHON*`` environment variables, e.g.
   :envvar:`PYTHONPATH` and :envvar:`PYTHONHOME`, that might be set.

   See also the :option:`-P` and :option:`-I` (isolated) options.


.. option:: -i

   Enter interactive mode after execution.

   Using the :option:`-i` option will enter interactive mode in any of the following circumstances\:

   * When a script is passed as first argument
   * When the :option:`-c` option is used
   * When the :option:`-m` option is used

   Interactive mode will start even when :data:`sys.stdin` does not appear to be a terminal. The
   :envvar:`PYTHONSTARTUP` file is not read.

   This can be useful to inspect global variables or a stack trace when a script
   raises an exception.  See also :envvar:`PYTHONINSPECT`.


.. option:: -I

   Run Python in isolated mode. This also implies :option:`-E`, :option:`-P`
   and :option:`-s` options.

   In isolated mode :data:`sys.path` contains neither the script's directory nor
   the user's site-packages directory. All ``PYTHON*`` environment
   variables are ignored, too. Further restrictions may be imposed to prevent
   the user from injecting malicious code.

   .. versionadded:: 3.4


.. option:: -O

   Remove assert statements and any code conditional on the value of
   :const:`__debug__`.  Augment the filename for compiled
   (:term:`bytecode`) files by adding ``.opt-1`` before the ``.pyc``
   extension (see :pep:`488`).  See also :envvar:`PYTHONOPTIMIZE`.

   .. versionchanged:: 3.5
      Modify ``.pyc`` filenames according to :pep:`488`.


.. option:: -OO

   Do :option:`-O` and also discard docstrings.  Augment the filename
   for compiled (:term:`bytecode`) files by adding ``.opt-2`` before the
   ``.pyc`` extension (see :pep:`488`).

   .. versionchanged:: 3.5
      Modify ``.pyc`` filenames according to :pep:`488`.


.. option:: -P

   Don't prepend a potentially unsafe path to :data:`sys.path`:

   * ``python -m module`` command line: Don't prepend the current working
     directory.
   * ``python script.py`` command line: Don't prepend the script's directory.
     If it's a symbolic link, resolve symbolic links.
   * ``python -c code`` and ``python`` (REPL) command lines: Don't prepend an
     empty string, which means the current working directory.

   See also the :envvar:`PYTHONSAFEPATH` environment variable, and :option:`-E`
   and :option:`-I` (isolated) options.

   .. versionadded:: 3.11


.. option:: -q

   Don't display the copyright and version messages even in interactive mode.

   .. versionadded:: 3.2


.. option:: -R

   Turn on hash randomization. This option only has an effect if the
   :envvar:`PYTHONHASHSEED` environment variable is set to anything other
   than ``random``, since hash randomization is enabled by default.

   On previous versions of Python, this option turns on hash randomization,
   so that the :meth:`~object.__hash__` values of str and bytes objects
   are "salted" with an unpredictable random value.  Although they remain
   constant within an individual Python process, they are not predictable
   between repeated invocations of Python.

   Hash randomization is intended to provide protection against a
   denial-of-service caused by carefully chosen inputs that exploit the worst
   case performance of a dict construction, *O*\ (*n*\ :sup:`2`) complexity.  See
   https://ocert.org/advisories/ocert-2011-003.html for details.

   :envvar:`PYTHONHASHSEED` allows you to set a fixed value for the hash
   seed secret.

   .. versionadded:: 3.2.3

   .. versionchanged:: 3.7
      The option is no longer ignored.


.. option:: -s

   Don't add the :data:`user site-packages directory <site.USER_SITE>` to
   :data:`sys.path`.

   See also :envvar:`PYTHONNOUSERSITE`.

   .. seealso::

      :pep:`370` -- Per user site-packages directory


.. option:: -S

   Disable the import of the module :mod:`site` and the site-dependent
   manipulations of :data:`sys.path` that it entails.  Also disable these
   manipulations if :mod:`site` is explicitly imported later (call
   :func:`site.main` if you want them to be triggered).


.. option:: -u

   Force the stdout and stderr streams to be unbuffered.  This option has no
   effect on the stdin stream.

   See also :envvar:`PYTHONUNBUFFERED`.

   .. versionchanged:: 3.7
      The text layer of the stdout and stderr streams now is unbuffered.


.. option:: -v

   Print a message each time a module is initialized, showing the place
   (filename or built-in module) from which it is loaded.  When given twice
   (:option:`!-vv`), print a message for each file that is checked for when
   searching for a module.  Also provides information on module cleanup at exit.

   .. versionchanged:: 3.10
      The :mod:`site` module reports the site-specific paths
      and :file:`.pth` files being processed.

   See also :envvar:`PYTHONVERBOSE`.


.. _using-on-warnings:
.. option:: -W arg

   Warning control. Python's warning machinery by default prints warning
   messages to :data:`sys.stderr`.

   The simplest settings apply a particular action unconditionally to all
   warnings emitted by a process (even those that are otherwise ignored by
   default)::

       -Wdefault  # Warn once per call location
       -Werror    # Convert to exceptions
       -Walways   # Warn every time
       -Wall      # Same as -Walways
       -Wmodule   # Warn once per calling module
       -Wonce     # Warn once per Python process
       -Wignore   # Never warn

   The action names can be abbreviated as desired and the interpreter will
   resolve them to the appropriate action name. For example, ``-Wi`` is the
   same as ``-Wignore``.

   The full form of argument is::

       action:message:category:module:lineno

   Empty fields match all values; trailing empty fields may be omitted. For
   example ``-W ignore::DeprecationWarning`` ignores all DeprecationWarning
   warnings.

   The *action* field is as explained above but only applies to warnings that
   match the remaining fields.

   The *message* field must match the start of the warning message;
   this match is case-insensitive.
   If it starts and ends with a forward slash (``/``), it specifies
   a regular expression, otherwise it specifies a literal string.

   The *category* field matches the warning category
   (ex: ``DeprecationWarning``). This must be a class name; the match test
   whether the actual warning category of the message is a subclass of the
   specified warning category.

   The *module* field matches the (fully qualified) module name; this match is
   case-sensitive.
   If it starts and ends with a forward slash (``/``), it specifies
   a regular expression that the start of the fully qualified module name
   must match, otherwise it specifies a literal string that the fully
   qualified module name must be equal to.

   The *lineno* field matches the line number, where zero matches all line
   numbers and is thus equivalent to an omitted line number.

   Multiple :option:`-W` options can be given; when a warning matches more than
   one option, the action for the last matching option is performed. Invalid
   :option:`-W` options are ignored (though, a warning message is printed about
   invalid options when the first warning is issued).

   Warnings can also be controlled using the :envvar:`PYTHONWARNINGS`
   environment variable and from within a Python program using the
   :mod:`warnings` module. For example, the :func:`warnings.filterwarnings`
   function can be used to use a regular expression on the warning message.

   See :ref:`warning-filter` and :ref:`describing-warning-filters` for more
   details.

   .. versionchanged:: 3.15
      Added regular expression support for *message* and *module*.


.. option:: -x

   Skip the first line of the source, allowing use of non-Unix forms of
   ``#!cmd``.  This is intended for a DOS specific hack only.


.. option:: -X

   Reserved for various implementation-specific options.  CPython currently
   defines the following possible values:

   * ``-X faulthandler`` to enable :mod:`faulthandler`.
     See also :envvar:`PYTHONFAULTHANDLER`.

     .. versionadded:: 3.3

   * ``-X showrefcount`` to output the total reference count and number of used
     memory blocks when the program finishes or after each statement in the
     interactive interpreter. This only works on :ref:`debug builds
     <debug-build>`.

     .. versionadded:: 3.4

   * ``-X tracemalloc`` to start tracing Python memory allocations using the
     :mod:`tracemalloc` module. By default, only the most recent frame is
     stored in a traceback of a trace. Use ``-X tracemalloc=NFRAME`` to start
     tracing with a traceback limit of *NFRAME* frames.
     See :func:`tracemalloc.start` and :envvar:`PYTHONTRACEMALLOC`
     for more information.

     .. versionadded:: 3.4

   * ``-X int_max_str_digits`` configures the :ref:`integer string conversion
     length limitation <int_max_str_digits>`.  See also
     :envvar:`PYTHONINTMAXSTRDIGITS`.

     .. versionadded:: 3.11

   * ``-X importtime`` to show how long each import takes. It shows module
     name, cumulative time (including nested imports) and self time (excluding
     nested imports).  Note that its output may be broken in multi-threaded
     application.  Typical usage is ``python -X importtime -c 'import asyncio'``.

     ``-X importtime=2`` enables additional output that indicates when an
     imported module has already been loaded.  In such cases, the string
     ``cached`` will be printed in both time columns.

     See also :envvar:`PYTHONPROFILEIMPORTTIME`.

     .. versionadded:: 3.7

     .. versionchanged:: 3.14

         Added ``-X importtime=2`` to also trace imports of loaded modules,
         and reserved values other than ``1`` and ``2`` for future use.

   * ``-X dev``: enable :ref:`Python Development Mode <devmode>`, introducing
     additional runtime checks that are too expensive to be enabled by
     default.  See also :envvar:`PYTHONDEVMODE`.

     .. versionadded:: 3.7

   * ``-X utf8`` enables the :ref:`Python UTF-8 Mode <utf8-mode>`.
     ``-X utf8=0`` explicitly disables :ref:`Python UTF-8 Mode <utf8-mode>`
     (even when it would otherwise activate automatically).
     See also :envvar:`PYTHONUTF8`.

     .. versionadded:: 3.7

   * ``-X pycache_prefix=PATH`` enables writing ``.pyc`` files to a parallel
     tree rooted at the given directory instead of to the code tree. See also
     :envvar:`PYTHONPYCACHEPREFIX`.

     .. versionadded:: 3.8

   * ``-X warn_default_encoding`` issues a :class:`EncodingWarning` when the
     locale-specific default encoding is used for opening files.
     See also :envvar:`PYTHONWARNDEFAULTENCODING`.

     .. versionadded:: 3.10

   * ``-X no_debug_ranges`` disables the inclusion of the tables mapping extra
     location information (end line, start column offset and end column offset)
     to every instruction in code objects. This is useful when smaller code
     objects and pyc files are desired as well as suppressing the extra visual
     location indicators when the interpreter displays tracebacks. See also
     :envvar:`PYTHONNODEBUGRANGES`.

     .. versionadded:: 3.11

   * ``-X frozen_modules`` determines whether or not frozen modules are
     ignored by the import machinery.  A value of ``on`` means they get
     imported and ``off`` means they are ignored.  The default is ``on``
     if this is an installed Python (the normal case).  If it's under
     development (running from the source tree) then the default is ``off``.
     Note that the :mod:`!importlib_bootstrap` and
     :mod:`!importlib_bootstrap_external` frozen modules are always used, even
     if this flag is set to ``off``. See also :envvar:`PYTHON_FROZEN_MODULES`.

     .. versionadded:: 3.11

   * ``-X perf`` enables support for the Linux ``perf`` profiler.
     When this option is provided, the ``perf`` profiler will be able to
     report Python calls. This option is only available on some platforms and
     will do nothing if is not supported on the current system. The default value
     is "off". See also :envvar:`PYTHONPERFSUPPORT` and :ref:`perf_profiling`.

     .. versionadded:: 3.12

   * ``-X perf_jit`` enables support for the Linux ``perf`` profiler with DWARF
     support. When this option is provided, the ``perf`` profiler will be able
     to report Python calls using DWARF information. This option is only available on
     some platforms and will do nothing if is not supported on the current
     system. The default value is "off". See also :envvar:`PYTHON_PERF_JIT_SUPPORT`
     and :ref:`perf_profiling`.

     .. versionadded:: 3.13

   * ``-X disable_remote_debug`` disables the remote debugging support as described
     in :pep:`768`.  This includes both the functionality to schedule code for
     execution in another process and the functionality to receive code for
     execution in the current process.

     This option is only available on some platforms and will do nothing
     if is not supported on the current system. See also
     :envvar:`PYTHON_DISABLE_REMOTE_DEBUG` and :pep:`768`.

     .. versionadded:: 3.14

   * :samp:`-X cpu_count={n}` overrides :func:`os.cpu_count`,
     :func:`os.process_cpu_count`, and :func:`multiprocessing.cpu_count`.
     *n* must be greater than or equal to 1.
     This option may be useful for users who need to limit CPU resources of a
     container system. See also :envvar:`PYTHON_CPU_COUNT`.
     If *n* is ``default``, nothing is overridden.

     .. versionadded:: 3.13

   * :samp:`-X presite={module}` or :samp:`-X presite={module:func}` specifies
     an entry point that should be executed before the :mod:`site` module is
     executed and before the
     :mod:`__main__` module exists.  Therefore, the imported module isn't
     :mod:`__main__`. This can be used to execute code early during Python
     initialization. Python needs to be :ref:`built in debug mode <debug-build>`
     for this option to exist.  See also :envvar:`PYTHON_PRESITE`.

     .. versionchanged:: 3.15
        Accept also ``module:func`` entry point format.

     .. versionadded:: 3.13

   * :samp:`-X gil={0,1}` forces the GIL to be disabled or enabled,
     respectively. Setting to ``0`` is only available in builds configured with
     :option:`--disable-gil`. See also :envvar:`PYTHON_GIL` and
     :ref:`whatsnew313-free-threaded-cpython`.

     .. versionadded:: 3.13

   * :samp:`-X thread_inherit_context={0,1}` causes :class:`~threading.Thread`
     to, by default, use a copy of context of the caller of
     ``Thread.start()`` when starting.  Otherwise, threads will start
     with an empty context.  If unset, the value of this option defaults
     to ``1`` on free-threaded builds and to ``0`` otherwise.  See also
     :envvar:`PYTHON_THREAD_INHERIT_CONTEXT`.

     .. versionadded:: 3.14

   * :samp:`-X context_aware_warnings={0,1}` causes the
     :class:`warnings.catch_warnings` context manager to use a
     :class:`~contextvars.ContextVar` to store warnings filter state.  If
     unset, the value of this option defaults to ``1`` on free-threaded builds
     and to ``0`` otherwise.  See also :envvar:`PYTHON_CONTEXT_AWARE_WARNINGS`.

     .. versionadded:: 3.14

   * :samp:`-X pathconfig_warnings={0,1}` if true (``1``) then
     :ref:`sys-path-init` is allowed to log warnings into stderr.
     If false (``0``) suppress these warnings. Set to true by default.
     See also :envvar:`PYTHON_PATHCONFIG_WARNINGS`.

     .. versionadded:: 3.15

   * :samp:`-X tlbc={0,1}` enables (1, the default) or disables (0) thread-local
     bytecode in builds configured with :option:`--disable-gil`.  When disabled,
     this also disables the specializing interpreter.  See also
     :envvar:`PYTHON_TLBC`.

     .. versionadded:: 3.14

   * :samp:`-X lazy_imports={all,normal}` controls lazy import behavior.
     ``all`` makes all imports lazy by default, and ``normal`` (the default)
     respects the ``lazy`` keyword in source code.
     See also :envvar:`PYTHON_LAZY_IMPORTS`.

     .. versionadded:: 3.15

   It also allows passing arbitrary values and retrieving them through the
   :data:`sys._xoptions` dictionary.

   .. versionadded:: 3.2

   .. versionchanged:: 3.9
      Removed the ``-X showalloccount`` option.

   .. versionchanged:: 3.10
      Removed the ``-X oldparser`` option.

.. versionremoved:: 3.14

   :option:`!-J` is no longer reserved for use by Jython_,
   and now has no special meaning.

   .. _Jython: https://www.jython.org/

.. _using-on-controlling-color:

Controlling color
~~~~~~~~~~~~~~~~~

The Python interpreter is configured by default to use colors to highlight
output in certain situations such as when displaying tracebacks. This
behavior can be controlled by setting different environment variables.

Setting the environment variable ``TERM`` to ``dumb`` will disable color.

If the |FORCE_COLOR|_ environment variable is set, then color will be
enabled regardless of the value of TERM. This is useful on CI systems which
aren’t terminals but can still display ANSI escape sequences.

If the |NO_COLOR|_ environment variable is set, Python will disable all color
in the output. This takes precedence over ``FORCE_COLOR``.

All these environment variables are used also by other tools to control color
output. To control the color output only in the Python interpreter, the
:envvar:`PYTHON_COLORS` environment variable can be used. This variable takes
precedence over ``NO_COLOR``, which in turn takes precedence over
``FORCE_COLOR``.


.. _using-on-envvars:

Environment variables
---------------------

These environment variables influence Python's behavior, they are processed
before the command-line switches other than -E or -I.  It is customary that
command-line switches override environmental variables where there is a
conflict.

.. envvar:: PYTHONHOME

   Change the location of the standard Python libraries.  By default, the
   libraries are searched in :file:`{prefix}/lib/python{version}` and
   :file:`{exec_prefix}/lib/python{version}`, where :file:`{prefix}` and
   :file:`{exec_prefix}` are installation-dependent directories, both defaulting
   to :file:`/usr/local`.

   When :envvar:`PYTHONHOME` is set to a single directory, its value replaces
   both :file:`{prefix}` and :file:`{exec_prefix}`.  To specify different values
   for these, set :envvar:`PYTHONHOME` to :file:`{prefix}:{exec_prefix}`.


.. envvar:: PYTHONPATH

   Augment the default search path for module files.  The format is the same as
   the shell's :envvar:`PATH`: one or more directory pathnames separated by
   :data:`os.pathsep` (e.g. colons on Unix or semicolons on Windows).
   Non-existent directories are silently ignored.

   In addition to normal directories, individual :envvar:`PYTHONPATH` entries
   may refer to zipfiles containing pure Python modules (in either source or
   compiled form). Extension modules cannot be imported from zipfiles.

   The default search path is installation dependent, but generally begins with
   :file:`{prefix}/lib/python{version}` (see :envvar:`PYTHONHOME` above).  It
   is *always* appended to :envvar:`PYTHONPATH`.

   An additional directory will be inserted in the search path in front of
   :envvar:`PYTHONPATH` as described above under
   :ref:`using-on-interface-options`. The search path can be manipulated from
   within a Python program as the variable :data:`sys.path`.


.. envvar:: PYTHONSAFEPATH

   If this is set to a non-empty string, don't prepend a potentially unsafe
   path to :data:`sys.path`: see the :option:`-P` option for details.

   .. versionadded:: 3.11


.. envvar:: PYTHONPLATLIBDIR

   If this is set to a non-empty string, it overrides the :data:`sys.platlibdir`
   value.

   .. versionadded:: 3.9


.. envvar:: PYTHONSTARTUP

   If this is the name of a readable file, the Python commands in that file are
   executed before the first prompt is displayed in interactive mode.  The file
   is executed in the same namespace where interactive commands are executed so
   that objects defined or imported in it can be used without qualification in
   the interactive session.  You can also change the prompts :data:`sys.ps1` and
   :data:`sys.ps2` and the hook :data:`sys.__interactivehook__` in this file.

   .. audit-event:: cpython.run_startup filename envvar-PYTHONSTARTUP

      Raises an :ref:`auditing event <auditing>` ``cpython.run_startup`` with
      the filename as the argument when called on startup.


.. envvar:: PYTHONOPTIMIZE

   If this is set to a non-empty string it is equivalent to specifying the
   :option:`-O` option.  If set to an integer, it is equivalent to specifying
   :option:`-O` multiple times.


.. envvar:: PYTHONBREAKPOINT

   If this is set, it names a callable using dotted-path notation.  The module
   containing the callable will be imported and then the callable will be run
   by the default implementation of :func:`sys.breakpointhook` which itself is
   called by built-in :func:`breakpoint`.  If not set, or set to the empty
   string, it is equivalent to the value "pdb.set_trace".  Setting this to the
   string "0" causes the default implementation of :func:`sys.breakpointhook`
   to do nothing but return immediately.

   .. versionadded:: 3.7

.. envvar:: PYTHONDEBUG

   If this is set to a non-empty string it is equivalent to specifying the
   :option:`-d` option.  If set to an integer, it is equivalent to specifying
   :option:`-d` multiple times.

   This environment variable requires a :ref:`debug build of Python
   <debug-build>`, otherwise it's ignored.


.. envvar:: PYTHONINSPECT

   If this is set to a non-empty string it is equivalent to specifying the
   :option:`-i` option.

   This variable can also be modified by Python code using :data:`os.environ`
   to force inspect mode on program termination.

   .. audit-event:: cpython.run_stdin "" ""

   .. versionchanged:: 3.12.5 (also 3.11.10, 3.10.15, 3.9.20, and 3.8.20)
      Emits audit events.

   .. versionchanged:: 3.13
      Uses PyREPL if possible, in which case :envvar:`PYTHONSTARTUP` is
      also executed. Emits audit events.


.. envvar:: PYTHONUNBUFFERED

   If this is set to a non-empty string it is equivalent to specifying the
   :option:`-u` option.


.. envvar:: PYTHONVERBOSE

   If this is set to a non-empty string it is equivalent to specifying the
   :option:`-v` option.  If set to an integer, it is equivalent to specifying
   :option:`-v` multiple times.


.. envvar:: PYTHONCASEOK

   If this is set, Python ignores case in :keyword:`import` statements.  This
   only works on Windows and macOS.


.. envvar:: PYTHONDONTWRITEBYTECODE

   If this is set to a non-empty string, Python won't try to write ``.pyc``
   files on the import of source modules.  This is equivalent to
   specifying the :option:`-B` option.


.. envvar:: PYTHONPYCACHEPREFIX

   If this is set, Python will write ``.pyc`` files in a mirror directory tree
   at this path, instead of in ``__pycache__`` directories within the source
   tree. This is equivalent to specifying the :option:`-X`
   ``pycache_prefix=PATH`` option.

   .. versionadded:: 3.8


.. envvar:: PYTHONHASHSEED

   If this variable is not set or set to ``random``, a random value is used
   to seed the hashes of str and bytes objects.

   If :envvar:`PYTHONHASHSEED` is set to an integer value, it is used as a fixed
   seed for generating the hash() of the types covered by the hash
   randomization.

   Its purpose is to allow repeatable hashing, such as for selftests for the
   interpreter itself, or to allow a cluster of python processes to share hash
   values.

   The integer must be a decimal number in the range [0,4294967295].  Specifying
   the value 0 will disable hash randomization.

   .. versionadded:: 3.2.3

.. envvar:: PYTHONINTMAXSTRDIGITS

   If this variable is set to an integer, it is used to configure the
   interpreter's global :ref:`integer string conversion length limitation
   <int_max_str_digits>`.

   .. versionadded:: 3.11

.. envvar:: PYTHONIOENCODING

   If this is set before running the interpreter, it overrides the encoding used
   for stdin/stdout/stderr, in the syntax ``encodingname:errorhandler``.  Both
   the ``encodingname`` and the ``:errorhandler`` parts are optional and have
   the same meaning as in :func:`str.encode`.

   For stderr, the ``:errorhandler`` part is ignored; the handler will always be
   ``'backslashreplace'``.

   .. versionchanged:: 3.4
      The ``encodingname`` part is now optional.

   .. versionchanged:: 3.6
      On Windows, the encoding specified by this variable is ignored for interactive
      console buffers unless :envvar:`PYTHONLEGACYWINDOWSSTDIO` is also specified.
      Files and pipes redirected through the standard streams are not affected.

.. envvar:: PYTHONNOUSERSITE

   This is equivalent to the :option:`-s` option.  If this is set, Python won't
   add the :data:`user site-packages directory <site.USER_SITE>` to
   :data:`sys.path`.

   .. seealso::

      :pep:`370` -- Per user site-packages directory


.. envvar:: PYTHONUSERBASE

   Defines the :data:`user base directory <site.USER_BASE>`, which is used to
   compute the path of the :data:`user site-packages directory <site.USER_SITE>`
   and :ref:`installation paths <sysconfig-user-scheme>` for
   ``python -m pip install --user``.

   To disable the user site-packages, see :envvar:`PYTHONNOUSERSITE` or the :option:`-s`
   option.

   .. seealso::

      :pep:`370` -- Per user site-packages directory


.. envvar:: PYTHONEXECUTABLE

   If this environment variable is set, ``sys.argv[0]`` will be set to its
   value instead of the value got through the C runtime.  Only works on
   macOS.

.. envvar:: PYTHONWARNINGS

   This is equivalent to the :option:`-W` option. If set to a comma
   separated string, it is equivalent to specifying :option:`-W` multiple
   times, with filters later in the list taking precedence over those earlier
   in the list.

   The simplest settings apply a particular action unconditionally to all
   warnings emitted by a process (even those that are otherwise ignored by
   default)::

       PYTHONWARNINGS=default  # Warn once per call location
       PYTHONWARNINGS=error    # Convert to exceptions
       PYTHONWARNINGS=always   # Warn every time
       PYTHONWARNINGS=all      # Same as PYTHONWARNINGS=always
       PYTHONWARNINGS=module   # Warn once per calling module
       PYTHONWARNINGS=once     # Warn once per Python process
       PYTHONWARNINGS=ignore   # Never warn

   See :ref:`warning-filter` and :ref:`describing-warning-filters` for more
   details.

   .. versionchanged:: 3.15
      Added regular expression support for *message* and *module*.


.. envvar:: PYTHONFAULTHANDLER

   If this environment variable is set to a non-empty string,
   :func:`faulthandler.enable` is called at startup: install a handler for
   :const:`~signal.SIGSEGV`, :const:`~signal.SIGFPE`,
   :const:`~signal.SIGABRT`, :const:`~signal.SIGBUS` and
   :const:`~signal.SIGILL` signals to dump the Python traceback.
   This is equivalent to :option:`-X` ``faulthandler`` option.

   .. versionadded:: 3.3


.. envvar:: PYTHONTRACEMALLOC

   If this environment variable is set to a non-empty string, start tracing
   Python memory allocations using the :mod:`tracemalloc` module. The value of
   the variable is the maximum number of frames stored in a traceback of a
   trace. For example, ``PYTHONTRACEMALLOC=1`` stores only the most recent
   frame.
   See the :func:`tracemalloc.start` function for more information.
   This is equivalent to setting the :option:`-X` ``tracemalloc`` option.

   .. versionadded:: 3.4


.. envvar:: PYTHONPROFILEIMPORTTIME

   If this environment variable is set to ``1``, Python will show
   how long each import takes. If set to ``2``, Python will include output for
   imported modules that have already been loaded.
   This is equivalent to setting the :option:`-X` ``importtime`` option.

   .. versionadded:: 3.7

   .. versionchanged:: 3.14

      Added ``PYTHONPROFILEIMPORTTIME=2`` to also trace imports of loaded modules.


.. envvar:: PYTHONASYNCIODEBUG

   If this environment variable is set to a non-empty string, enable the
   :ref:`debug mode <asyncio-debug-mode>` of the :mod:`asyncio` module.

   .. versionadded:: 3.4


.. envvar:: PYTHONMALLOC

   Set the Python memory allocators and/or install debug hooks.

   Set the family of memory allocators used by Python:

   * ``default``: use the :ref:`default memory allocators
     <default-memory-allocators>`.
   * ``malloc``: use the :c:func:`malloc` function of the C library
     for all domains (:c:macro:`PYMEM_DOMAIN_RAW`, :c:macro:`PYMEM_DOMAIN_MEM`,
     :c:macro:`PYMEM_DOMAIN_OBJ`).
   * ``pymalloc``: use the :ref:`pymalloc allocator <pymalloc>` for
     :c:macro:`PYMEM_DOMAIN_MEM` and :c:macro:`PYMEM_DOMAIN_OBJ` domains and use
     the :c:func:`malloc` function for the :c:macro:`PYMEM_DOMAIN_RAW` domain.
   * ``mimalloc``: use the :ref:`mimalloc allocator <mimalloc>` for
     :c:macro:`PYMEM_DOMAIN_MEM` and :c:macro:`PYMEM_DOMAIN_OBJ` domains and use
     the :c:func:`malloc` function for the :c:macro:`PYMEM_DOMAIN_RAW` domain.

   Install :ref:`debug hooks <pymem-debug-hooks>`:

   * ``debug``: install debug hooks on top of the :ref:`default memory
     allocators <default-memory-allocators>`.
   * ``malloc_debug``: same as ``malloc`` but also install debug hooks.
   * ``pymalloc_debug``: same as ``pymalloc`` but also install debug hooks.
   * ``mimalloc_debug``: same as ``mimalloc`` but also install debug hooks.

   .. note::

      In the :term:`free-threaded <free threading>` build, the ``malloc``,
      ``malloc_debug``, ``pymalloc``, and ``pymalloc_debug`` values are not
      supported.  Only ``default``, ``debug``, ``mimalloc``, and
      ``mimalloc_debug`` are accepted.

   .. versionadded:: 3.6

   .. versionchanged:: 3.7
      Added the ``"default"`` allocator.


.. envvar:: PYTHONMALLOCSTATS

   If set to a non-empty string, Python will print statistics of the
   :ref:`pymalloc memory allocator <pymalloc>` or the
   :ref:`mimalloc memory allocator <mimalloc>` (whichever is in use)
   every time a new object arena is created, and on shutdown.

   This variable is ignored if the :envvar:`PYTHONMALLOC` environment variable
   is used to force the :c:func:`malloc` allocator of the C library, or if
   Python is configured without both ``pymalloc`` and ``mimalloc`` support.

   .. versionchanged:: 3.6
      This variable can now also be used on Python compiled in release mode.
      It now has no effect if set to an empty string.


.. envvar:: PYTHON_PYMALLOC_HUGEPAGES

   If set to a non-zero integer, enable huge page support for
   :ref:`pymalloc <pymalloc>` arenas.  Set to ``0`` or unset to disable.
   Python must be compiled with :option:`--with-pymalloc-hugepages` for this
   variable to have any effect.

   When enabled, arena allocation uses ``MAP_HUGETLB`` (Linux) or
   ``MEM_LARGE_PAGES`` (Windows) with automatic fallback to regular pages if
   huge pages are not available.

   .. warning::

      On Linux, if the huge-page pool is exhausted, page faults — including
      copy-on-write faults triggered by :func:`os.fork` — deliver ``SIGBUS``
      and kill the process.  Only enable this in environments where the
      huge-page pool is properly sized and fork-safety is not a concern.

      On Windows you need a special privilege. See the
      `Windows documentation for large pages
      <https://learn.microsoft.com/windows/win32/memory/large-page-support>`_
      for details. Python will fail on startup if the required privilege
      `SeLockMemoryPrivilege
      <https://learn.microsoft.com/previous-versions/windows/it-pro/windows-10/security/threat-protection/security-policy-settings/lock-pages-in-memory>`_
      is not held by the user.

   .. versionadded:: 3.15


.. envvar:: PYTHONLEGACYWINDOWSFSENCODING

   If set to a non-empty string, the default :term:`filesystem encoding and
   error handler` mode will revert to their pre-3.6 values of 'mbcs' and
   'replace', respectively.  Otherwise, the new defaults 'utf-8' and
   'surrogatepass' are used.

   This may also be enabled at runtime with
   :func:`sys._enablelegacywindowsfsencoding`.

   .. availability:: Windows.

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

.. envvar:: PYTHONLEGACYWINDOWSSTDIO

   If set to a non-empty string, does not use the new console reader and
   writer. This means that Unicode characters will be encoded according to
   the active console code page, rather than using utf-8.

   This variable is ignored if the standard streams are redirected (to files
   or pipes) rather than referring to console buffers.

   .. availability:: Windows.

   .. versionadded:: 3.6


.. envvar:: PYTHONCOERCECLOCALE

   If set to the value ``0``, causes the main Python command line application
   to skip coercing the legacy ASCII-based C and POSIX locales to a more
   capable UTF-8 based alternative.

   If this variable is *not* set (or is set to a value other than ``0``), the
   ``LC_ALL`` locale override environment variable is also not set, and the
   current locale reported for the ``LC_CTYPE`` category is either the default
   ``C`` locale, or else the explicitly ASCII-based ``POSIX`` locale, then the
   Python CLI will attempt to configure the following locales for the
   ``LC_CTYPE`` category in the order listed before loading the interpreter
   runtime:

   * ``C.UTF-8``
   * ``C.utf8``
   * ``UTF-8``

   If setting one of these locale categories succeeds, then the ``LC_CTYPE``
   environment variable will also be set accordingly in the current process
   environment before the Python runtime is initialized. This ensures that in
   addition to being seen by both the interpreter itself and other locale-aware
   components running in the same process (such as the GNU ``readline``
   library), the updated setting is also seen in subprocesses (regardless of
   whether or not those processes are running a Python interpreter), as well as
   in operations that query the environment rather than the current C locale
   (such as Python's own :func:`locale.getdefaultlocale`).

   Configuring one of these locales (either explicitly or via the above
   implicit locale coercion) automatically enables the ``surrogateescape``
   :ref:`error handler <error-handlers>` for :data:`sys.stdin` and
   :data:`sys.stdout` (:data:`sys.stderr` continues to use ``backslashreplace``
   as it does in any other locale). This stream handling behavior can be
   overridden using :envvar:`PYTHONIOENCODING` as usual.

   For debugging purposes, setting ``PYTHONCOERCECLOCALE=warn`` will cause
   Python to emit warning messages on ``stderr`` if either the locale coercion
   activates, or else if a locale that *would* have triggered coercion is
   still active when the Python runtime is initialized.

   Also note that even when locale coercion is disabled, or when it fails to
   find a suitable target locale, :envvar:`PYTHONUTF8` will still activate by
   default in legacy ASCII-based locales. Both features must be disabled in
   order to force the interpreter to use ``ASCII`` instead of ``UTF-8`` for
   system interfaces.

   .. availability:: Unix.

   .. versionadded:: 3.7
      See :pep:`538` for more details.


.. envvar:: PYTHONDEVMODE

   If this environment variable is set to a non-empty string, enable
   :ref:`Python Development Mode <devmode>`, introducing additional runtime
   checks that are too expensive to be enabled by default.
   This is equivalent to setting the :option:`-X` ``dev`` option.

   .. versionadded:: 3.7

.. envvar:: PYTHONUTF8

   If set to ``1``, enable the :ref:`Python UTF-8 Mode <utf8-mode>`.

   If set to ``0``, disable the :ref:`Python UTF-8 Mode <utf8-mode>`.

   Setting any other non-empty string causes an error during interpreter
   initialisation.

   .. versionadded:: 3.7

.. envvar:: PYTHONWARNDEFAULTENCODING

   If this environment variable is set to a non-empty string, issue a
   :class:`EncodingWarning` when the locale-specific default encoding is used.

   See :ref:`io-encoding-warning` for details.

   .. versionadded:: 3.10

.. envvar:: PYTHONNODEBUGRANGES

   If this variable is set, it disables the inclusion of the tables mapping
   extra location information (end line, start column offset and end column
   offset) to every instruction in code objects. This is useful when smaller
   code objects and pyc files are desired as well as suppressing the extra visual
   location indicators when the interpreter displays tracebacks.

   .. versionadded:: 3.11

.. envvar:: PYTHONPERFSUPPORT

   If this variable is set to a nonzero value, it enables support for
   the Linux ``perf`` profiler so Python calls can be detected by it.

   If set to ``0``, disable Linux ``perf`` profiler support.

   See also the :option:`-X perf <-X>` command-line option
   and :ref:`perf_profiling`.

   .. versionadded:: 3.12

.. envvar:: PYTHON_PERF_JIT_SUPPORT

   If this variable is set to a nonzero value, it enables support for
   the Linux ``perf`` profiler so Python calls can be detected by it
   using DWARF information.

   If set to ``0``, disable Linux ``perf`` profiler support.

   See also the :option:`-X perf_jit <-X>` command-line option
   and :ref:`perf_profiling`.

   .. versionadded:: 3.13

.. envvar:: PYTHON_DISABLE_REMOTE_DEBUG

   If this variable is set to a non-empty string, it disables the remote
   debugging feature described in :pep:`768`. This includes both the functionality
   to schedule code for execution in another process and the functionality to
   receive code for execution in the current process.

   See also the :option:`-X disable_remote_debug` command-line option.

   .. versionadded:: 3.14

.. envvar:: PYTHON_CPU_COUNT

   If this variable is set to a positive integer, it overrides the return
   values of :func:`os.cpu_count` and :func:`os.process_cpu_count`.

   See also the :option:`-X cpu_count <-X>` command-line option.

   .. versionadded:: 3.13

.. envvar:: PYTHON_FROZEN_MODULES

   If this variable is set to ``on`` or ``off``, it determines whether or not
   frozen modules are ignored by the import machinery.  A value of ``on`` means
   they get imported and ``off`` means they are ignored.  The default is ``on``
   for non-debug builds (the normal case) and ``off`` for debug builds.
   Note that the :mod:`!importlib_bootstrap` and
   :mod:`!importlib_bootstrap_external` frozen modules are always used, even
   if this flag is set to ``off``.

   See also the :option:`-X frozen_modules <-X>` command-line option.

   .. versionadded:: 3.13

.. envvar:: PYTHON_COLORS

   If this variable is set to ``1``, the interpreter will colorize various kinds
   of output. Setting it to ``0`` deactivates this behavior.
   See also :ref:`using-on-controlling-color`.

   .. versionadded:: 3.13

.. envvar:: PYTHON_BASIC_REPL

   If this variable is set to any value, the interpreter will not attempt to
   load the Python-based :term:`REPL` that requires :mod:`readline`, and will
   instead use the traditional parser-based :term:`REPL`.

   .. versionadded:: 3.13

.. envvar:: PYTHON_BASIC_COMPLETER

   If this variable is set to any value, PyREPL will use :mod:`rlcompleter` to
   implement tab completion, instead of the default one which uses colors.

   .. versionadded:: 3.15

.. envvar:: PYTHON_HISTORY

   This environment variable can be used to set the location of a
   ``.python_history`` file (by default, it is ``.python_history`` in the
   user's home directory).

   .. versionadded:: 3.13

.. envvar:: PYTHON_GIL

   If this variable is set to ``1``, the global interpreter lock (GIL) will be
   forced on. Setting it to ``0`` forces the GIL off (needs Python configured with
   the :option:`--disable-gil` build option).

   See also the :option:`-X gil <-X>` command-line option, which takes
   precedence over this variable, and :ref:`whatsnew313-free-threaded-cpython`.

   .. versionadded:: 3.13

.. envvar:: PYTHON_THREAD_INHERIT_CONTEXT

   If this variable is set to ``1`` then :class:`~threading.Thread` will,
   by default, use a copy of context of the caller of ``Thread.start()``
   when starting.  Otherwise, new threads will start with an empty context.
   If unset, this variable defaults to ``1`` on free-threaded builds and to
   ``0`` otherwise.  See also :option:`-X thread_inherit_context<-X>`.

   .. versionadded:: 3.14

.. envvar:: PYTHON_CONTEXT_AWARE_WARNINGS

   If set to ``1`` then the :class:`warnings.catch_warnings` context
   manager will use a :class:`~contextvars.ContextVar` to store warnings
   filter state.  If unset, this variable defaults to ``1`` on
   free-threaded builds and to ``0`` otherwise.  See :option:`-X
   context_aware_warnings<-X>`.

   .. versionadded:: 3.14

.. envvar:: PYTHON_PATHCONFIG_WARNINGS

   If true (``1``) then :ref:`sys-path-init` is allowed to log warnings into
   stderr. If false (``0``) suppress these warnings. Set to true by default.
   See also :option:`-X pathconfig_warnings<-X>`.

   .. versionadded:: 3.15

.. envvar:: PYTHON_JIT

   On builds where experimental just-in-time compilation is available, this
   variable can force the JIT to be disabled (``0``) or enabled (``1``) at
   interpreter startup.

   .. versionadded:: 3.13

.. envvar:: PYTHON_TLBC

   If set to ``1`` enables thread-local bytecode. If set to ``0`` thread-local
   bytecode and the specializing interpreter are disabled.  Only applies to
   builds configured with :option:`--disable-gil`.

   See also the :option:`-X tlbc <-X>` command-line option.

   .. versionadded:: 3.14

.. envvar:: PYTHON_LAZY_IMPORTS

   Controls lazy import behavior. Accepts two values: ``all`` makes all
   imports lazy by default, and ``normal`` (the default) respects the
   ``lazy`` keyword in source code.

   See also the :option:`-X lazy_imports <-X>` command-line option.

   .. versionadded:: 3.15

Debug-mode variables
~~~~~~~~~~~~~~~~~~~~

.. envvar:: PYTHONDUMPREFS

   If set, Python will dump objects and reference counts still alive after
   shutting down the interpreter.

   Needs Python configured with the :option:`--with-trace-refs` build option.

.. envvar:: PYTHONDUMPREFSFILE

   If set, Python will dump objects and reference counts still alive
   after shutting down the interpreter into a file under the path given
   as the value to this environment variable.

   Needs Python configured with the :option:`--with-trace-refs` build option.

   .. versionadded:: 3.11

.. envvar:: PYTHON_PRESITE

   If this variable is set to a module, that module will be imported
   early in the interpreter lifecycle, before the :mod:`site` module is
   executed, and before the :mod:`__main__` module is created.
   Therefore, the imported module is not treated as :mod:`__main__`.

   This can be used to execute code early during Python initialization.

   To import a submodule, use ``package.module`` as the value, like in
   an import statement.

   See also the :option:`-X presite <-X>` command-line option,
   which takes precedence over this variable.

   Needs Python configured with the :option:`--with-pydebug` build option.

   .. versionchanged:: 3.15
      Accept also ``module:func`` entry point format.

   .. versionadded:: 3.13
