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.. ATTENTION: You probably should update Misc/python.man, too, if you modify
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Command line and environment
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============================
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The CPython interpreter scans the command line and the environment for various
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Other implementations' command line schemes may differ. See
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:ref:`implementations` for further resources.
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When invoking Python, you may specify any of these options::
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python [-bBdEhiIOqsSuvVWx?] [-c command | -m module-name | script | - ] [args]
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The most common use case is, of course, a simple invocation of a script::
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.. _using-on-interface-options:
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The interpreter interface resembles that of the UNIX shell, but provides some
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additional methods of invocation:
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* When called with standard input connected to a tty device, it prompts for
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commands and executes them until an EOF (an end-of-file character, you can
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produce that with :kbd:`Ctrl-D` on UNIX or :kbd:`Ctrl-Z, Enter` on Windows) is read.
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* When called with a file name argument or with a file as standard input, it
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reads and executes a script from that file.
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* When called with a directory name argument, it reads and executes an
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appropriately named script from that directory.
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* When called with ``-c command``, it executes the Python statement(s) given as
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*command*. Here *command* may contain multiple statements separated by
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newlines. Leading whitespace is significant in Python statements!
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* When called with ``-m module-name``, the given module is located on the
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Python module path and executed as a script.
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In non-interactive mode, the entire input is parsed before it is executed.
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An interface option terminates the list of options consumed by the interpreter,
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all consecutive arguments will end up in :data:`sys.argv` -- note that the first
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element, subscript zero (``sys.argv[0]``), is a string reflecting the program's
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.. cmdoption:: -c <command>
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Execute the Python code in *command*. *command* can be one or more
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statements separated by newlines, with significant leading whitespace as in
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If this option is given, the first element of :data:`sys.argv` will be
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``"-c"`` and the current directory will be added to the start of
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:data:`sys.path` (allowing modules in that directory to be imported as top
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.. cmdoption:: -m <module-name>
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Search :data:`sys.path` for the named module and execute its contents as
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the :mod:`__main__` module.
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Since the argument is a *module* name, you must not give a file extension
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(``.py``). The module name should be a valid absolute Python module name, but
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the implementation may not always enforce this (e.g. it may allow you to
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use a name that includes a hyphen).
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Package names (including namespace packages) are also permitted. When a
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package name is supplied instead
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of a normal module, the interpreter will execute ``<pkg>.__main__`` as
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the main module. This behaviour is deliberately similar to the handling
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of directories and zipfiles that are passed to the interpreter as the
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This option cannot be used with built-in modules and extension modules
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written in C, since they do not have Python module files. However, it
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can still be used for precompiled modules, even if the original source
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file is not available.
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If this option is given, the first element of :data:`sys.argv` will be the
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full path to the module file (while the module file is being located, the
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first element will be set to ``"-m"``). As with the :option:`-c` option,
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the current directory will be added to the start of :data:`sys.path`.
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Many standard library modules contain code that is invoked on their execution
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as a script. An example is the :mod:`timeit` module::
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python -mtimeit -s 'setup here' 'benchmarked code here'
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python -mtimeit -h # for details
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:func:`runpy.run_module`
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Equivalent functionality directly available to Python code
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:pep:`338` -- Executing modules as scripts
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.. versionchanged:: 3.1
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Supply the package name to run a ``__main__`` submodule.
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.. versionchanged:: 3.4
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namespace packages are also supported
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Read commands from standard input (:data:`sys.stdin`). If standard input is
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a terminal, :option:`-i` is implied.
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If this option is given, the first element of :data:`sys.argv` will be
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``"-"`` and the current directory will be added to the start of
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.. describe:: <script>
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Execute the Python code contained in *script*, which must be a filesystem
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path (absolute or relative) referring to either a Python file, a directory
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containing a ``__main__.py`` file, or a zipfile containing a
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``__main__.py`` file.
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If this option is given, the first element of :data:`sys.argv` will be the
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script name as given on the command line.
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If the script name refers directly to a Python file, the directory
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containing that file is added to the start of :data:`sys.path`, and the
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file is executed as the :mod:`__main__` module.
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If the script name refers to a directory or zipfile, the script name is
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added to the start of :data:`sys.path` and the ``__main__.py`` file in
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that location is executed as the :mod:`__main__` module.
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:func:`runpy.run_path`
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Equivalent functionality directly available to Python code
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If no interface option is given, :option:`-i` is implied, ``sys.argv[0]`` is
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an empty string (``""``) and the current directory will be added to the
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start of :data:`sys.path`. Also, tab-completion and history editing is
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automatically enabled, if available on your platform (see
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:ref:`rlcompleter-config`).
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.. seealso:: :ref:`tut-invoking`
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.. versionchanged:: 3.4
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Automatic enabling of tab-completion and history editing.
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Print a short description of all command line options.
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Print the Python version number and exit. Example output could be::
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.. _using-on-misc-options:
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Miscellaneous options
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~~~~~~~~~~~~~~~~~~~~~
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Issue a warning when comparing :class:`bytes` or :class:`bytearray` with
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:class:`str` or :class:`bytes` with :class:`int`. Issue an error when the
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option is given twice (:option:`-bb`).
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.. versionchanged: 3.5
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Affects comparisons of :class:`bytes` with :class:`int`.
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If given, Python won't try to write ``.pyc`` files on the
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import of source modules. See also :envvar:`PYTHONDONTWRITEBYTECODE`.
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Turn on parser debugging output (for wizards only, depending on compilation
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options). See also :envvar:`PYTHONDEBUG`.
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Ignore all :envvar:`PYTHON*` environment variables, e.g.
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:envvar:`PYTHONPATH` and :envvar:`PYTHONHOME`, that might be set.
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When a script is passed as first argument or the :option:`-c` option is used,
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enter interactive mode after executing the script or the command, even when
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:data:`sys.stdin` does not appear to be a terminal. The
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:envvar:`PYTHONSTARTUP` file is not read.
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This can be useful to inspect global variables or a stack trace when a script
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raises an exception. See also :envvar:`PYTHONINSPECT`.
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Run Python in isolated mode. This also implies -E and -s.
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In isolated mode :data:`sys.path` contains neither the script's directory nor
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the user's site-packages directory. All :envvar:`PYTHON*` environment
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variables are ignored, too. Further restrictions may be imposed to prevent
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the user from injecting malicious code.
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.. versionadded:: 3.4
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Turn on basic optimizations. This changes the filename extension for
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compiled (:term:`bytecode`) files from ``.pyc`` to ``.pyo``. See also
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:envvar:`PYTHONOPTIMIZE`.
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Discard docstrings in addition to the :option:`-O` optimizations.
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Don't display the copyright and version messages even in interactive mode.
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.. versionadded:: 3.2
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Kept for compatibility. On Python 3.3 and greater, hash randomization is
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turned on by default.
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On previous versions of Python, this option turns on hash randomization,
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so that the :meth:`__hash__` values of str, bytes and datetime
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are "salted" with an unpredictable random value. Although they remain
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constant within an individual Python process, they are not predictable
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between repeated invocations of Python.
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Hash randomization is intended to provide protection against a
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denial-of-service caused by carefully-chosen inputs that exploit the worst
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case performance of a dict construction, O(n^2) complexity. See
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http://www.ocert.org/advisories/ocert-2011-003.html for details.
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:envvar:`PYTHONHASHSEED` allows you to set a fixed value for the hash
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.. versionadded:: 3.2.3
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Don't add the :data:`user site-packages directory <site.USER_SITE>` to
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:pep:`370` -- Per user site-packages directory
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Disable the import of the module :mod:`site` and the site-dependent
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manipulations of :data:`sys.path` that it entails. Also disable these
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manipulations if :mod:`site` is explicitly imported later (call
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:func:`site.main` if you want them to be triggered).
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Force the binary layer of the stdout and stderr streams (which is
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available as their ``buffer`` attribute) to be unbuffered. The text I/O
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layer will still be line-buffered if writing to the console, or
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block-buffered if redirected to a non-interactive file.
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See also :envvar:`PYTHONUNBUFFERED`.
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Print a message each time a module is initialized, showing the place
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(filename or built-in module) from which it is loaded. When given twice
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(:option:`-vv`), print a message for each file that is checked for when
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searching for a module. Also provides information on module cleanup at exit.
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See also :envvar:`PYTHONVERBOSE`.
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.. cmdoption:: -W arg
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Warning control. Python's warning machinery by default prints warning
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messages to :data:`sys.stderr`. A typical warning message has the following
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file:line: category: message
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By default, each warning is printed once for each source line where it
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occurs. This option controls how often warnings are printed.
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Multiple :option:`-W` options may be given; when a warning matches more than
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one option, the action for the last matching option is performed. Invalid
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:option:`-W` options are ignored (though, a warning message is printed about
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invalid options when the first warning is issued).
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Warnings can also be controlled from within a Python program using the
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:mod:`warnings` module.
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The simplest form of argument is one of the following action strings (or a
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unique abbreviation):
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Explicitly request the default behavior (printing each warning once per
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Print a warning each time it occurs (this may generate many messages if a
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warning is triggered repeatedly for the same source line, such as inside a
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Print each warning only the first time it occurs in each module.
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Print each warning only the first time it occurs in the program.
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Raise an exception instead of printing a warning message.
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The full form of argument is::
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action:message:category:module:line
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Here, *action* is as explained above but only applies to messages that match
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the remaining fields. Empty fields match all values; trailing empty fields
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may be omitted. The *message* field matches the start of the warning message
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printed; this match is case-insensitive. The *category* field matches the
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warning category. This must be a class name; the match tests whether the
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actual warning category of the message is a subclass of the specified warning
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category. The full class name must be given. The *module* field matches the
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(fully-qualified) module name; this match is case-sensitive. The *line*
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field matches the line number, where zero matches all line numbers and is
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thus equivalent to an omitted line number.
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:mod:`warnings` -- the warnings module
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:pep:`230` -- Warning framework
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:envvar:`PYTHONWARNINGS`
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Skip the first line of the source, allowing use of non-Unix forms of
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``#!cmd``. This is intended for a DOS specific hack only.
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.. note:: The line numbers in error messages will be off by one.
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Reserved for various implementation-specific options. CPython currently
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defines the following possible values:
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* ``-X faulthandler`` to enable :mod:`faulthandler`;
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* ``-X showrefcount`` to enable the output of the total reference count
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and memory blocks (only works on debug builds);
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* ``-X tracemalloc`` to start tracing Python memory allocations using the
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:mod:`tracemalloc` module. By default, only the most recent frame is
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stored in a traceback of a trace. Use ``-X tracemalloc=NFRAME`` to start
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tracing with a traceback limit of *NFRAME* frames. See the
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:func:`tracemalloc.start` for more information.
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It also allows passing arbitrary values and retrieving them through the
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:data:`sys._xoptions` dictionary.
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.. versionchanged:: 3.2
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It is now allowed to pass :option:`-X` with CPython.
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.. versionadded:: 3.3
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The ``-X faulthandler`` option.
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.. versionadded:: 3.4
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The ``-X showrefcount`` and ``-X tracemalloc`` options.
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Options you shouldn't use
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~~~~~~~~~~~~~~~~~~~~~~~~~
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Reserved for use by Jython_.
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.. _Jython: http://www.jython.org/
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.. _using-on-envvars:
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Environment variables
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---------------------
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These environment variables influence Python's behavior, they are processed
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before the command-line switches other than -E or -I. It is customary that
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command-line switches override environmental variables where there is a
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.. envvar:: PYTHONHOME
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Change the location of the standard Python libraries. By default, the
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libraries are searched in :file:`{prefix}/lib/python{version}` and
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:file:`{exec_prefix}/lib/python{version}`, where :file:`{prefix}` and
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:file:`{exec_prefix}` are installation-dependent directories, both defaulting
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to :file:`/usr/local`.
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When :envvar:`PYTHONHOME` is set to a single directory, its value replaces
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both :file:`{prefix}` and :file:`{exec_prefix}`. To specify different values
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for these, set :envvar:`PYTHONHOME` to :file:`{prefix}:{exec_prefix}`.
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.. envvar:: PYTHONPATH
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Augment the default search path for module files. The format is the same as
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the shell's :envvar:`PATH`: one or more directory pathnames separated by
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:data:`os.pathsep` (e.g. colons on Unix or semicolons on Windows).
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Non-existent directories are silently ignored.
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In addition to normal directories, individual :envvar:`PYTHONPATH` entries
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may refer to zipfiles containing pure Python modules (in either source or
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compiled form). Extension modules cannot be imported from zipfiles.
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The default search path is installation dependent, but generally begins with
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:file:`{prefix}/lib/python{version}` (see :envvar:`PYTHONHOME` above). It
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is *always* appended to :envvar:`PYTHONPATH`.
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An additional directory will be inserted in the search path in front of
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:envvar:`PYTHONPATH` as described above under
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:ref:`using-on-interface-options`. The search path can be manipulated from
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within a Python program as the variable :data:`sys.path`.
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.. envvar:: PYTHONSTARTUP
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If this is the name of a readable file, the Python commands in that file are
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executed before the first prompt is displayed in interactive mode. The file
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is executed in the same namespace where interactive commands are executed so
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that objects defined or imported in it can be used without qualification in
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the interactive session. You can also change the prompts :data:`sys.ps1` and
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:data:`sys.ps2` and the hook :data:`sys.__interactivehook__` in this file.
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.. envvar:: PYTHONOPTIMIZE
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If this is set to a non-empty string it is equivalent to specifying the
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:option:`-O` option. If set to an integer, it is equivalent to specifying
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:option:`-O` multiple times.
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.. envvar:: PYTHONDEBUG
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If this is set to a non-empty string it is equivalent to specifying the
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:option:`-d` option. If set to an integer, it is equivalent to specifying
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:option:`-d` multiple times.
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.. envvar:: PYTHONINSPECT
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If this is set to a non-empty string it is equivalent to specifying the
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This variable can also be modified by Python code using :data:`os.environ`
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to force inspect mode on program termination.
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.. envvar:: PYTHONUNBUFFERED
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If this is set to a non-empty string it is equivalent to specifying the
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.. envvar:: PYTHONVERBOSE
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If this is set to a non-empty string it is equivalent to specifying the
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:option:`-v` option. If set to an integer, it is equivalent to specifying
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:option:`-v` multiple times.
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.. envvar:: PYTHONCASEOK
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If this is set, Python ignores case in :keyword:`import` statements. This
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only works on Windows and OS X.
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.. envvar:: PYTHONDONTWRITEBYTECODE
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If this is set to a non-empty string, Python won't try to write ``.pyc`` or
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``.pyo`` files on the import of source modules. This is equivalent to
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specifying the :option:`-B` option.
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.. envvar:: PYTHONHASHSEED
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If this variable is not set or set to ``random``, a random value is used
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to seed the hashes of str, bytes and datetime objects.
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If :envvar:`PYTHONHASHSEED` is set to an integer value, it is used as a fixed
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seed for generating the hash() of the types covered by the hash
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Its purpose is to allow repeatable hashing, such as for selftests for the
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interpreter itself, or to allow a cluster of python processes to share hash
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The integer must be a decimal number in the range [0,4294967295]. Specifying
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the value 0 will disable hash randomization.
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.. versionadded:: 3.2.3
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.. envvar:: PYTHONIOENCODING
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If this is set before running the interpreter, it overrides the encoding used
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for stdin/stdout/stderr, in the syntax ``encodingname:errorhandler``. Both
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the ``encodingname`` and the ``:errorhandler`` parts are optional and have
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the same meaning as in :func:`str.encode`.
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For stderr, the ``:errorhandler`` part is ignored; the handler will always be
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``'backslashreplace'``.
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.. versionchanged:: 3.4
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The ``encodingname`` part is now optional.
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.. envvar:: PYTHONNOUSERSITE
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If this is set, Python won't add the :data:`user site-packages directory
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<site.USER_SITE>` to :data:`sys.path`.
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:pep:`370` -- Per user site-packages directory
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.. envvar:: PYTHONUSERBASE
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Defines the :data:`user base directory <site.USER_BASE>`, which is used to
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compute the path of the :data:`user site-packages directory <site.USER_SITE>`
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and :ref:`Distutils installation paths <inst-alt-install-user>` for
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``python setup.py install --user``.
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:pep:`370` -- Per user site-packages directory
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.. envvar:: PYTHONEXECUTABLE
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If this environment variable is set, ``sys.argv[0]`` will be set to its
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value instead of the value got through the C runtime. Only works on
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.. envvar:: PYTHONWARNINGS
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This is equivalent to the :option:`-W` option. If set to a comma
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separated string, it is equivalent to specifying :option:`-W` multiple
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.. envvar:: PYTHONFAULTHANDLER
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If this environment variable is set to a non-empty string,
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:func:`faulthandler.enable` is called at startup: install a handler for
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:const:`SIGSEGV`, :const:`SIGFPE`, :const:`SIGABRT`, :const:`SIGBUS` and
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:const:`SIGILL` signals to dump the Python traceback. This is equivalent to
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:option:`-X` ``faulthandler`` option.
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.. versionadded:: 3.3
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.. envvar:: PYTHONTRACEMALLOC
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If this environment variable is set to a non-empty string, start tracing
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Python memory allocations using the :mod:`tracemalloc` module. The value of
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the variable is the maximum number of frames stored in a traceback of a
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trace. For example, ``PYTHONTRACEMALLOC=1`` stores only the most recent
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frame. See the :func:`tracemalloc.start` for more information.
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.. versionadded:: 3.4
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.. envvar:: PYTHONASYNCIODEBUG
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If this environment variable is set to a non-empty string, enable the
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:ref:`debug mode <asyncio-debug-mode>` of the :mod:`asyncio` module.
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.. versionadded:: 3.4
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Setting these variables only has an effect in a debug build of Python, that is,
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if Python was configured with the ``--with-pydebug`` build option.
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.. envvar:: PYTHONTHREADDEBUG
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If set, Python will print threading debug info.
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.. envvar:: PYTHONDUMPREFS
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If set, Python will dump objects and reference counts still alive after
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shutting down the interpreter.
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.. envvar:: PYTHONMALLOCSTATS
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If set, Python will print memory allocation statistics every time a new
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object arena is created, and on shutdown.
added
removed
python-3.5.1-docs-html/_sources/using/cmdline.txt
