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Command line and environment

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

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

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

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 Ctrl-D on UNIX or Ctrl-Z, Enter on Windows) is read. For more on interactive mode, see 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 sys.argv -- note that the first element, subscript zero (sys.argv[0]), is a string reflecting the program's source.

-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 sys.argv will be "-c" and the current directory will be added to the start of sys.path (allowing modules in that directory to be imported as top level modules).

cpython.run_command command cmdoption-c

3.14 command is automatically dedented before execution.

-m <module-name>

Locate the module using the standard import mechanism and execute its contents as the __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 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 -c option, the current directory will be added to the start of sys.path.

-I option can be used to run the script in isolated mode where 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 timeit module:

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

cpython.run_module module-name cmdoption-m

runpy.run_module Equivalent functionality directly available to Python code

338 -- Executing modules as scripts

3.1 Supply the package name to run a __main__ submodule.

3.4 namespace packages are also supported

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

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

cpython.run_stdin "" ""

<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 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 sys.path, and the file is executed as the __main__ module.

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

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

cpython.run_file filename

runpy.run_path Equivalent functionality directly available to Python code

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

tut-invoking

3.4 Automatic enabling of tab-completion and history editing.

Generic options

-? -h --help

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

--help-env

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

3.11

--help-xoptions

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

3.11

--help-all

Print complete usage information and exit.

3.11

-V

--version

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

Python 3.8.0b2+

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

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

3.6 The -VV option.

Miscellaneous options

-b

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

3.5 Affects also comparisons of bytes with int.

3.15

Deprecate -b and !-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 BytesWarning will be raised for these cases; use a type checker instead.

-B

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

--check-hash-based-pycs default|always|never

Control the validation behavior of hash-based .pyc files. See 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.

-d

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

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

-E

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

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

-i

Enter interactive mode after execution.

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

  • When a script is passed as first argument
  • When the -c option is used
  • When the -m option is used

Interactive mode will start even when sys.stdin does not appear to be a terminal. The 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 PYTHONINSPECT.

-I

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

In isolated mode 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.

3.4

-O

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

3.5 Modify .pyc filenames according to 488.

-OO

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

3.5 Modify .pyc filenames according to 488.

-P

Don't prepend a potentially unsafe path to 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 PYTHONSAFEPATH environment variable, and -E and -I (isolated) options.

3.11

-q

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

3.2

-R

Turn on hash randomization. This option only has an effect if the 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 ~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(n2) complexity. See https://ocert.org/advisories/ocert-2011-003.html for details.

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

3.2.3

3.7 The option is no longer ignored.

-s

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

See also PYTHONNOUSERSITE.

370 -- Per user site-packages directory

-S

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

-u

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

See also PYTHONUNBUFFERED.

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

-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 (!-vv), print a message for each file that is checked for when searching for a module. Also provides information on module cleanup at exit.

3.10 The site module reports the site-specific paths and .pth files being processed.

See also PYTHONVERBOSE.

Warning control. Python's warning machinery by default prints warning messages to 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 -W options can be given; when a warning matches more than one option, the action for the last matching option is performed. Invalid -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 PYTHONWARNINGS environment variable and from within a Python program using the warnings module. For example, the warnings.filterwarnings function can be used to use a regular expression on the warning message.

See warning-filter and describing-warning-filters for more details.

3.15 Added regular expression support for message and module.

-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:

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

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

@"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 -x option is needed to skip it.

-X

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

  • -X faulthandler to enable faulthandler. See also PYTHONFAULTHANDLER.

    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 debug builds <debug-build>.

    3.4

  • -X tracemalloc to start tracing Python memory allocations using the 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 tracemalloc.start and PYTHONTRACEMALLOC for more information.

    3.4

  • -X int_max_str_digits configures the integer string conversion length limitation <int_max_str_digits>. See also PYTHONINTMAXSTRDIGITS.

    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 PYTHONPROFILEIMPORTTIME.

    3.7

    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 Python Development Mode <devmode>, introducing additional runtime checks that are too expensive to be enabled by default. See also PYTHONDEVMODE.

    3.7

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

    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 PYTHONPYCACHEPREFIX.

    3.8

  • -X warn_default_encoding issues a EncodingWarning when the locale-specific default encoding is used for opening files. See also PYTHONWARNDEFAULTENCODING.

    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 PYTHONNODEBUGRANGES.

    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 !importlib_bootstrap and !importlib_bootstrap_external frozen modules are always used, even if this flag is set to off. See also PYTHON_FROZEN_MODULES.

    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 PYTHONPERFSUPPORT and perf_profiling.

    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 PYTHON_PERF_JIT_SUPPORT and perf_profiling.

    3.13

  • -X disable-remote-debug disables the remote debugging support as described in 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 PYTHON_DISABLE_REMOTE_DEBUG and 768.

    3.14

  • -X cpu_count={n} overrides os.cpu_count, os.process_cpu_count, and 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 PYTHON_CPU_COUNT. If n is default, nothing is overridden.

    3.13

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

    3.15 Accept also module:func entry point format.

    3.13

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

    3.13

  • -X thread_inherit_context={0,1} causes ~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 PYTHON_THREAD_INHERIT_CONTEXT.

    3.14

  • -X context_aware_warnings={0,1} causes the warnings.catch_warnings context manager to use a ~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 PYTHON_CONTEXT_AWARE_WARNINGS.

    3.14

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

    3.15

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

    3.14

  • -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 PYTHON_LAZY_IMPORTS.

    3.15

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

3.2

3.9 Removed the -X showalloccount option.

3.10 Removed the -X oldparser option.

3.14

!-J is no longer reserved for use by Jython, and now has no special meaning.

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 PYTHON_COLORS environment variable can be used. This variable takes precedence over NO_COLOR, which in turn takes precedence over FORCE_COLOR.

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.

PYTHONHOME

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

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

PYTHONPATH

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

In addition to normal directories, individual 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 {prefix}/lib/python{version} (see PYTHONHOME above). It is always appended to PYTHONPATH.

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

PYTHONSAFEPATH

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

3.11

PYTHONPLATLIBDIR

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

3.9

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 sys.ps1 and sys.ps2 and the hook sys.__interactivehook__ in this file.

cpython.run_startup filename envvar-PYTHONSTARTUP

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

PYTHONOPTIMIZE

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

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 sys.breakpointhook which itself is called by built-in 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 sys.breakpointhook to do nothing but return immediately.

3.7

PYTHONDEBUG

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

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

PYTHONINSPECT

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

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

cpython.run_stdin "" ""

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

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

PYTHONUNBUFFERED

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

PYTHONVERBOSE

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

PYTHONCASEOK

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

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 -B option.

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 -X pycache_prefix=PATH option.

3.8

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 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.

3.2.3

PYTHONINTMAXSTRDIGITS

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

3.11

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 str.encode.

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

3.4 The encodingname part is now optional.

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

PYTHONNOUSERSITE

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

370 -- Per user site-packages directory

PYTHONUSERBASE

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

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

370 -- Per user site-packages directory

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.

PYTHONWARNINGS

This is equivalent to the -W option. If set to a comma separated string, it is equivalent to specifying -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 warning-filter and describing-warning-filters for more details.

3.15 Added regular expression support for message and module.

PYTHONFAULTHANDLER

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

3.3

PYTHONTRACEMALLOC

If this environment variable is set to a non-empty string, start tracing Python memory allocations using the 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 tracemalloc.start function for more information. This is equivalent to setting the -X tracemalloc option.

3.4

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 -X importtime option.

3.7

3.14

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

PYTHONASYNCIODEBUG

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

3.4

PYTHONMALLOC

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

Set the family of memory allocators used by Python:

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

Install debug hooks <pymem-debug-hooks>:

  • debug: install debug hooks on top of the 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 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.

3.6

3.7 Added the "default" allocator.

PYTHONMALLOCSTATS

If set to a non-empty string, Python will print statistics of the pymalloc memory allocator <pymalloc> or the 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 PYTHONMALLOC environment variable is used to force the malloc allocator of the C library, or if Python is configured without both pymalloc and mimalloc support.

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.

PYTHON_PYMALLOC_HUGEPAGES

If set to a non-zero integer, enable huge page support for pymalloc <pymalloc> arenas. Set to 0 or unset to disable. Python must be compiled with --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 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 for details. Python will fail on startup if the required privilege SeLockMemoryPrivilege is not held by the user.

3.15

PYTHONLEGACYWINDOWSFSENCODING

If set to a non-empty string, the default 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.

Windows.

3.6 See 529 for more details.

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.

Windows.

3.6

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 locale.getdefaultlocale).

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

Unix.

3.7 See 538 for more details.

PYTHONDEVMODE

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

3.7

PYTHONUTF8

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

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

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

3.7

PYTHONWARNDEFAULTENCODING

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

See io-encoding-warning for details.

3.10

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.

3.11

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 -X perf <-X> command-line option and perf_profiling.

3.12

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 -X perf_jit <-X> command-line option and perf_profiling.

3.13

PYTHON_DISABLE_REMOTE_DEBUG

If this variable is set to a non-empty string, it disables the remote debugging feature described in 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 -X disable-remote-debug command-line option.

3.14

PYTHON_CPU_COUNT

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

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

3.13

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 !importlib_bootstrap and !importlib_bootstrap_external frozen modules are always used, even if this flag is set to off.

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

3.13

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 using-on-controlling-color.

3.13

PYTHON_BASIC_REPL

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

3.13

PYTHON_BASIC_COMPLETER

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

3.15

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).

3.13

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 --disable-gil build option).

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

3.13

PYTHON_THREAD_INHERIT_CONTEXT

If this variable is set to 1 then ~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 -X thread_inherit_context<-X>.

3.14

PYTHON_CONTEXT_AWARE_WARNINGS

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

3.14

PYTHON_PATHCONFIG_WARNINGS

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

3.15

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.

3.13

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 --disable-gil.

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

3.14

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 -X lazy_imports <-X> command-line option.

3.15

Debug-mode variables

PYTHONDUMPREFS

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

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

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 --with-trace-refs build option.

3.11

PYTHON_PRESITE

If this variable is set to a module, that module will be imported early in the interpreter lifecycle, before the site module is executed, and before the __main__ module is created. Therefore, the imported module is not treated as __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 -X presite <-X> command-line option, which takes precedence over this variable.

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

3.15 Accept also module:func entry point format.

3.13

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