.. highlightlang:: none

.. _using-on-general:

Command line and environment

============================

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

settings.

.. note:: 

   Other implementations' command line schemes may differ.  See

   :ref:`implementations` for further resources.

.. _using-on-cmdline:

Command line

------------

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

    python [-dEiOQsStuUvxX3?] [-c command | -m module-name | script | - ] [args]

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

    python myscript.py

.. _using-on-interface-options:

Interface options

~~~~~~~~~~~~~~~~~

The interpreter interface resembles that of the UNIX shell, but provides some

additional methods of invocation:

* When called with standard input connected to a tty device, it prompts for

  commands and executes them until an EOF (an end-of-file character, you can

  produce that with *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 :data:`sys.argv` -- note that the first

element, subscript zero (``sys.argv[0]``), is a string reflecting the program's

source.

.. cmdoption:: -c <command>

   Execute the Python code in *command*.  *command* can be one ore more

   statements separated by newlines, with significant leading whitespace as in

   normal module code.

   If this option is given, the first element of :data:`sys.argv` will be

   ``"-c"`` and the current directory will be added to the start of

   :data:`sys.path` (allowing modules in that directory to be imported as top

   level modules).

.. cmdoption:: -m <module-name>

   Search :data:`sys.path` for the named module and execute its contents as

   the :mod:`__main__` module.

   Since the argument is a *module* name, you must not give a file extension

   (``.py``).  The ``module-name`` should be a valid 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).

   .. note::

      This option cannot be used with builtin modules and extension modules

      written in C, since they do not have Python module files. However, it

      can still be used for precompiled modules, even if the original source

      file is not available.

   If this option is given, the first element of :data:`sys.argv` will be the

   full path to the module file. As with the :option:`-c` option, the current

   directory will be added to the start of :data:`sys.path`.

   Many standard library modules contain code that is invoked on their execution

   as a script.  An example is the :mod:`timeit` module::

       python -mtimeit -s 'setup here' 'benchmarked code here'

       python -mtimeit -h # for details

   .. seealso:: 

      :func:`runpy.run_module`

         The actual implementation of this feature.

      :pep:`338` -- Executing modules as scripts

   .. versionadded:: 2.4

   .. versionchanged:: 2.5

      The named module can now be located inside a package.

.. describe:: -

   Read commands from standard input (:data:`sys.stdin`).  If standard input is

   a terminal, :option:`-i` is implied.

   If this option is given, the first element of :data:`sys.argv` will be

   ``"-"`` and the current directory will be added to the start of

   :data:`sys.path`.

.. describe:: <script>

   Execute the Python code contained in *script*, which must be a filesystem

   path (absolute or relative) referring to either a Python file, a directory

   containing a ``__main__.py`` file, or a zipfile containing a

   ``__main__.py`` file.

   If this option is given, the first element of :data:`sys.argv` will be the

   script name as given on the command line.

   If the script name refers directly to a Python file, the directory

   containing that file is added to the start of :data:`sys.path`, and the

   file is executed as the :mod:`__main__` module.

   If the script name refers to a directory or zipfile, the script name is

   added to the start of :data:`sys.path` and the ``__main__.py`` file in

   that location is executed as the :mod:`__main__` module.

   .. versionchanged:: 2.5

      Directories and zipfiles containing a ``__main__.py`` file at the top

      level are now considered valid Python scripts.

If no interface option is given, :option:`-i` is implied, ``sys.argv[0]`` is

an empty string (``""``) and the current directory will be added to the

start of :data:`sys.path`.

.. seealso::  :ref:`tut-invoking`

Generic options

~~~~~~~~~~~~~~~

.. cmdoption:: -?

               -h

               --help

   Print a short description of all command line options.

   .. versionchanged:: 2.5

      The ``--help`` variant.

.. cmdoption:: -V

               --version

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

       Python 2.5.1

   .. versionchanged:: 2.5

      The ``--version`` variant.

Miscellaneous options

~~~~~~~~~~~~~~~~~~~~~

.. cmdoption:: -B

   If given, Python won't try to write ``.pyc`` or ``.pyo`` files on the

   import of source modules.  See also :envvar:`PYTHONDONTWRITEBYTECODE`.

   .. versionadded:: 2.6

.. cmdoption:: -d

   Turn on parser debugging output (for wizards only, depending on compilation

   options).  See also :envvar:`PYTHONDEBUG`.

.. cmdoption:: -E

   Ignore all :envvar:`PYTHON*` environment variables, e.g.

   :envvar:`PYTHONPATH` and :envvar:`PYTHONHOME`, that might be set.

   .. versionadded:: 2.2

.. cmdoption:: -i

   When a script is passed as first argument or the :option:`-c` option is used,

   enter interactive mode after executing the script or the command, even when

   :data:`sys.stdin` does not appear to be a terminal.  The

   :envvar:`PYTHONSTARTUP` file is not read.

   This can be useful to inspect global variables or a stack trace when a script

   raises an exception.  See also :envvar:`PYTHONINSPECT`.

.. cmdoption:: -O

   Turn on basic optimizations.  This changes the filename extension for

   compiled (:term:`bytecode`) files from ``.pyc`` to ``.pyo``.  See also

   :envvar:`PYTHONOPTIMIZE`.

.. cmdoption:: -OO

   Discard docstrings in addition to the :option:`-O` optimizations.

.. cmdoption:: -Q <arg>

   Division control. The argument must be one of the following:

   ``old``

     division of int/int and long/long return an int or long (*default*)

   ``new``

     new division semantics, i.e. division of int/int and long/long returns a

     float

   ``warn``

     old division semantics with a warning for int/int and long/long

   ``warnall``

     old division semantics with a warning for all uses of the division operator

   .. seealso::

      :file:`Tools/scripts/fixdiv.py`

         for a use of ``warnall``

      :pep:`238` -- Changing the division operator

.. cmdoption:: -s

   Don't add user site directory to sys.path

   .. versionadded:: 2.6

   .. seealso::

      :pep:`370` -- Per user site-packages directory

.. cmdoption:: -S

   Disable the import of the module :mod:`site` and the site-dependent

   manipulations of :data:`sys.path` that it entails.

.. cmdoption:: -t

   Issue a warning when a source file mixes tabs and spaces for indentation in a

   way that makes it depend on the worth of a tab expressed in spaces.  Issue an

   error when the option is given twice (:option:`-tt`).

.. cmdoption:: -u

   Force stdin, stdout and stderr to be totally unbuffered.  On systems where it

   matters, also put stdin, stdout and stderr in binary mode.

   Note that there is internal buffering in :meth:`file.readlines` and

   :ref:`bltin-file-objects` (``for line in sys.stdin``) which is not influenced

   by this option.  To work around this, you will want to use

   :meth:`file.readline` inside a ``while 1:`` loop.

   See also :envvar:`PYTHONUNBUFFERED`.

.. XXX should the -U option be documented?

.. cmdoption:: -v

   Print a message each time a module is initialized, showing the place

   (filename or built-in module) from which it is loaded.  When given twice

   (:option:`-vv`), print a message for each file that is checked for when

   searching for a module.  Also provides information on module cleanup at exit.

   See also :envvar:`PYTHONVERBOSE`.

.. cmdoption:: -W arg

   Warning control.  Python's warning machinery by default prints warning

   messages to :data:`sys.stderr`.  A typical warning message has the following

   form::

       file:line: category: message

   By default, each warning is printed once for each source line where it

   occurs.  This option controls how often warnings are printed.

   Multiple :option:`-W` options may be given; when a warning matches more than

   one option, the action for the last matching option is performed.  Invalid

   :option:`-W` options are ignored (though, a warning message is printed about

   invalid options when the first warning is issued).

   Warnings can also be controlled from within a Python program using the

   :mod:`warnings` module.

   The simplest form of argument is one of the following action strings (or a

   unique abbreviation):

   ``ignore``

      Ignore all warnings.

   ``default``

      Explicitly request the default behavior (printing each warning once per

      source line).

   ``all``

      Print a warning each time it occurs (this may generate many messages if a

      warning is triggered repeatedly for the same source line, such as inside a

      loop).

   ``module``

      Print each warning only only the first time it occurs in each module.

   ``once``

      Print each warning only the first time it occurs in the program.

   ``error``

      Raise an exception instead of printing a warning message.

   The full form of argument is:: 

       action:message:category:module:line

   Here, *action* is as explained above but only applies to messages that match

   the remaining fields.  Empty fields match all values; trailing empty fields

   may be omitted.  The *message* field matches the start of the warning message

   printed; this match is case-insensitive.  The *category* field matches the

   warning category.  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 full class name must be given.  The *module* field matches the

   (fully-qualified) module name; this match is case-sensitive.  The *line*

   field matches the line number, where zero matches all line numbers and is

   thus equivalent to an omitted line number.

   .. seealso::

      :mod:`warnings` -- the warnings module

      :pep:`230` -- Warning framework

.. cmdoption:: -x

   Skip the first line of the source, allowing use of non-Unix forms of

   ``#!cmd``.  This is intended for a DOS specific hack only.

   .. warning:: The line numbers in error messages will be off by one!

.. cmdoption:: -3

   Warn about Python 3.x incompatibilities. Among these are:

   * :meth:`dict.has_key`

   * :func:`apply`

   * :func:`callable`

   * :func:`coerce`

   * :func:`execfile`

   * :func:`reduce`

   * :func:`reload`

   Using these will emit a :exc:`DeprecationWarning`.

   .. versionadded:: 2.6

.. _using-on-envvars:

Environment variables

---------------------

These environment variables influence Python's behavior.

.. envvar:: PYTHONHOME

   Change the location of the standard Python libraries.  By default, the

   libraries are searched in :file:`{prefix}/lib/python{version}` and

   :file:`{exec_prefix}/lib/python{version}`, where :file:`{prefix}` and

   :file:`{exec_prefix}` are installation-dependent directories, both defaulting

   to :file:`/usr/local`.

   When :envvar:`PYTHONHOME` is set to a single directory, its value replaces

   both :file:`{prefix}` and :file:`{exec_prefix}`.  To specify different values

   for these, set :envvar:`PYTHONHOME` to :file:`{prefix}:{exec_prefix}`.

.. envvar:: PYTHONPATH

   Augment the default search path for module files.  The format is the same as

   the shell's :envvar:`PATH`: one or more directory pathnames separated by

   :data:`os.pathsep` (e.g. colons on Unix or semicolons on Windows).

   Non-existent directories are silently ignored.

   In addition to normal directories, individual :envvar:`PYTHONPATH` entries

   may refer to zipfiles containing pure Python modules (in either source or

   compiled form). Extension modules cannot be imported from zipfiles.

   The default search path is installation dependent, but generally begins with

   :file:`{prefix}/lib/python{version}`` (see :envvar:`PYTHONHOME` above).  It

   is *always* appended to :envvar:`PYTHONPATH`.

   An additional directory will be inserted in the search path in front of

   :envvar:`PYTHONPATH` as described above under

   :ref:`using-on-interface-options`. The search path can be manipulated from

   within a Python program as the variable :data:`sys.path`.

.. envvar:: PYTHONSTARTUP

   If this is the name of a readable file, the Python commands in that file are

   executed before the first prompt is displayed in interactive mode.  The file

   is executed in the same namespace where interactive commands are executed so

   that objects defined or imported in it can be used without qualification in

   the interactive session.  You can also change the prompts :data:`sys.ps1` and

   :data:`sys.ps2` in this file.

.. envvar:: PYTHONY2K

   Set this to a non-empty string to cause the :mod:`time` module to require

   dates specified as strings to include 4-digit years, otherwise 2-digit years

   are converted based on rules described in the :mod:`time` module

   documentation.

.. envvar:: PYTHONOPTIMIZE

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

   :option:`-O` option.  If set to an integer, it is equivalent to specifying

   :option:`-O` multiple times.

.. envvar:: PYTHONDEBUG

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

   :option:`-d` option.  If set to an integer, it is equivalent to specifying

   :option:`-d` multiple times.

.. envvar:: PYTHONINSPECT

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

   :option:`-i` option.

   This variable can also be modified by Python code using :data:`os.environ`

   to force inspect mode on program termination.

.. envvar:: PYTHONUNBUFFERED

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

   :option:`-u` option.

.. envvar:: PYTHONVERBOSE

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

   :option:`-v` option.  If set to an integer, it is equivalent to specifying

   :option:`-v` multiple times.

.. envvar:: PYTHONCASEOK

   If this is set, Python ignores case in :keyword:`import` statements.  This

   only works on Windows.

.. envvar:: PYTHONDONTWRITEBYTECODE

   If this is set, Python won't try to write ``.pyc`` or ``.pyo`` files on the

   import of source modules.

   .. versionadded:: 2.6

.. envvar:: PYTHONIOENCODING

   Overrides the encoding used for stdin/stdout/stderr, in the syntax

   ``encodingname:errorhandler``.  The ``:errorhandler`` part is optional and

   has the same meaning as in :func:`str.encode`.

   .. versionadded:: 2.6

.. envvar:: PYTHONNOUSERSITE

   If this is set, Python won't add the user site directory to sys.path

   .. versionadded:: 2.6

   .. seealso::

      :pep:`370` -- Per user site-packages directory

.. envvar:: PYTHONUSERBASE

   Sets the base directory for the user site directory

   .. versionadded:: 2.6

   .. seealso::

      :pep:`370` -- Per user site-packages directory

.. envvar:: 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

   Mac OS X.

Debug-mode variables

~~~~~~~~~~~~~~~~~~~~

Setting these variables only has an effect in a debug build of Python, that is,

if Python was configured with the :option:`--with-pydebug` build option.

.. envvar:: PYTHONTHREADDEBUG