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 [-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

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. For more on interactive mode, see Interactive Mode.
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).

Raises an auditing event cpython.run_command with argument command.

Changed in version 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

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

See also

runpy.run_module(): Equivalent functionality directly available to Python code

PEP 338 – Executing modules as scripts

Changed in version 3.1: Supply the package name to run a __main__ submodule.

Changed in version 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.

Raises an auditing event cpython.run_stdin with no arguments.

<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.

Raises an auditing event cpython.run_file with argument filename.

See also

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 Readline configuration).

See also

Invoking the Interpreter

Changed in version 3.4: Automatic enabling of tab-completion and history editing.

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.

Added in version 3.11.

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

Added in version 3.11.

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

Added 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]

Added in version 3.6: The -VV option.

1.1.3. 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).

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

Deprecated since version 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 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¶

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, Python does not prepend the potentially unsafe path sys._path0 to sys.path, and ignores all PYTHON* environment variables, e.g. PYTHONPATH and PYTHONHOME, that might be set. Further restrictions may be imposed to prevent the user from injecting malicious code.

Added 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 the potentially unsafe path sys._path0 to sys.path.

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

Added in version 3.11.

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

Added in version 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 __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.

Added in version 3.2.3.

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

-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.

See also PYTHONVERBOSE.

-W arg¶

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 The Warnings Filter and Describing Warning Filters for more details.

Changed in version 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.

Added in version 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.

Added in version 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.

Added in version 3.4.
-X int_max_str_digits configures the integer string conversion length limitation. See also PYTHONINTMAXSTRDIGITS.

Added in version 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.

Added in version 3.7.

Changed in version 3.14: Added -X importtime=2 to also trace imports of loaded modules, and reserved values other than 1 and 2 for future use.