.. 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 can be used to turn a Python script into a Windows batch file.

Similarly to adding a shebang line and setting the executable bit on Unix,

the extension of the Python script can be changed to ``.bat`` and the

following line can be added at the start of the script:

.. code-block:: batch

@py -x "%~f0" %* & exit /b

Or, to specify the path to the Python interpreter explicitly:

.. code-block:: batch

@"C:\Path\to\python.exe" -x "%~f0" %* & exit /b

Unlike a shebang line which is a Python comment, this line is not valid

Python syntax, and the :option:`-x` option is needed to skip it.

.. 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::