1. Command line and environment¶

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

CPython implementation detail: Other implementations’ command line schemes may differ. See Alternate Implementations for further resources.

1.1. Command line¶

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

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

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

python myscript.py

1.1.1. 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.
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. Leading whitespace is significant in Python statements!
When called with -m module-name, the given module is located on the Python module path 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).

Raises an auditing event cpython.run_command with argument command.

-m <module-name>¶

Search sys.path for the named module 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

Raises an auditing event cpython.run_module with argument module-name.

1.1.2. 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.

New in version 3.11.

--help-xoptions¶: Print a description of implementation-specific -X options and exit.

New in version 3.11.

--help-all¶: Print complete usage information and exit.

New in version 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]

New in version 3.6: The -VV option.

1.1.3. Miscellaneous options¶

-b¶: Issue a warning when comparing bytes or bytearray with str or bytes with int. Issue an error when the option is given twice (-bb).

Changed in version 3.5: Affects comparisons of bytes with int.

-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 Cached bytecode 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, 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¶

When a script is passed as first argument or the -c option is used, enter interactive mode after executing the script or the command, 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.

New in version 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 PEP 488). See also PYTHONOPTIMIZE.

Changed in version 3.5: Modify .pyc filenames according to PEP 488.

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

Changed in version 3.5: Modify .pyc filenames according to PEP 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.

New in version 3.11.

-q¶: Don’t display the copyright and version messages even in interactive mode.

New in version 3.2.

-R¶

Turn on hash randomization. This option only has an effect if the PYTHONHASHSEED environment variable is set to 0, since hash randomization is enabled by default.

On previous versions of Python, this option turns on hash randomization, so that the __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 http://ocert.org/advisories/ocert-2011-003.html for details.

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

Changed in version 3.7: The option is no longer ignored.

New in version 3.2.3.

-s¶: Don’t add the user site-packages directory to sys.path.

See also

PEP 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.

1.1.4. 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 environment variable FORCE_COLOR 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 environment variable NO_COLOR 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.

1.1.5. Options you shouldn’t use¶

-J¶: Reserved for use by Jython.

1.2. 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/pythonversion and exec_prefix/lib/pythonversion, 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/pythonversion (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 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.

New in version 3.11.

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

New in version 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.

Raises an auditing event 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.

New in version 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, 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.

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.

New in version 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.

New in version 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.

New in version 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'.

Changed in version 3.4: The encodingname part is now optional.

Changed in version 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¶: If this is set, Python won’t add the user site-packages directory to sys.path.

See also

PEP 370 – Per user site-packages directory

PYTHONUSERBASE¶: Defines the user base directory, which is used to compute the path of the user site-packages directory and installation paths for python -m pip install --user.

See also

PEP 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=module   # Warn once per calling module
PYTHONWARNINGS=once     # Warn once per Python process
PYTHONWARNINGS=ignore   # Never warn

See The Warnings Filter and Describing Warning Filters for more details.

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

New in version 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.

New in version 3.4.

PYTHONPROFILEIMPORTTIME¶: If this environment variable is set to a non-empty string, Python will show how long each import takes. This is equivalent to setting the -X importtime option.

New in version 3.7.

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

New in version 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.
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 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 for PYMEM_DOMAIN_MEM and PYMEM_DOMAIN_OBJ domains and use the malloc() function for the PYMEM_DOMAIN_RAW domain.

Install debug hooks:

debug: install debug hooks on top of the 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.

Changed in version 3.7: Added the "default" allocator.

New in version 3.6.

PYTHONMALLOCSTATS¶

If set to a non-empty string, Python will print statistics of the pymalloc memory allocator every time a new pymalloc 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 pymalloc support.

Changed in version 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.

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.

This may also be enabled at runtime with sys._enablelegacywindowsfsencoding().

Availability: Windows.

New in version 3.6: See PEP 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.

Availability: Windows.

New in version 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 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.

Availability: Unix.

New in version 3.7: See PEP 538 for more details.

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

New in version 3.7.

PYTHONUTF8¶

If set to 1, enable the Python UTF-8 Mode.

If set to 0, disable the Python UTF-8 Mode.

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

New in version 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 Opt-in EncodingWarning for details.

New in version 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.

New in version 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 command-line option and Python support for the Linux perf profiler.

New in version 3.12.

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 command-line option.

New in version 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 command-line option.

New in version 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 Controlling color.

New in version 3.13.

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).

New in version 3.13.

1.2.1. 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.

New in version 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 command-line option, which takes precedence over this variable.

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

New in version 3.13.

1. Command line and environment¶

1.1. Command line¶

1.1.1. Interface options¶

1.1.2. Generic options¶

1.1.3. Miscellaneous options¶

1.1.4. Controlling color¶

1.1.5. Options you shouldn’t use¶

1.2. Environment variables¶

1.2.1. Debug-mode variables¶

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