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.. highlightlang:: none
Command line and environment
============================
The CPython interpreter scans the command line and the environment for various
settings.
.. note::
Other implementation's command line schemes may differ. See
:ref:`implementations` for further resources.
Command line
------------
When invoking Python, you may specify any of these options::
python [-dEiOQStuUvxX3?] [-c command | -m module-name | script | - ] [args]
The most common use case is, of course, a simple invocation of a script::
python myscript.py
Interface options
~~~~~~~~~~~~~~~~~
The interpreter interface resembles that of the UNIX shell:
* 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 ``-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 searched 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"``.
.. cmdoption:: -m <module-name>
Search :data:`sys.path` for the named module and run the corresponding module
file as if it were executed with ``python modulefile.py`` as a script.
Since the argument is a *module* name, you must not give a file extension
(``.py``). However, the ``module-name`` does not have to be a valid Python
identifer (e.g. you can use a file name including 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.
If this option is given, the first element of :data:`sys.argv` will be the
full path to the module file.
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
.. versionchanged:: 2.5
The module name can now include packages.
.. describe:: <script>
Execute the Python code contained in *script*, which must be an (absolute or
relative) file name.
If this option is given, the first element of :data:`sys.argv` will be the
script file name as given on the command line.
.. 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
``"-"``.
.. seealso::
:ref:`tut-invoking`
If no script name is given, ``sys.argv[0]`` is an empty string (``""``).
Generic options
~~~~~~~~~~~~~~~
.. cmdoption:: -?
-h
--help
Print a short description of all command line options.
.. versionadded:: 2.5
The ``--help`` variant.
.. cmdoption:: -V
--version
Print the Python version number and exit. Example output could be::
Python 2.5.1
.. versionadded:: 2.5
The ``--version`` variant.
Miscellaneous options
~~~~~~~~~~~~~~~~~~~~~
.. 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
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::
: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.
.. versionadded:: 2.6
Related files -- UNIX
---------------------
These are subject to difference depending on local installation conventions;
:envvar:`prefix` (``${prefix}``) and :envvar:`exec_prefix` (``${exec_prefix}``)
are installation-dependent and should be interpreted as for GNU software; they
may be the same.
For example, on most Linux systems, the default for both is :file:`/usr`.
+-----------------------------------------------+------------------------------------------+
| File/directory | Meaning |
+===============================================+==========================================+
| :file:`{exec_prefix}/bin/python` | Recommended location of the interpreter. |
+-----------------------------------------------+------------------------------------------+
| :file:`{prefix}/lib/python{version}`, | Recommended locations of the directories |
| :file:`{exec_prefix}/lib/python{version}` | containing the standard modules. |
+-----------------------------------------------+------------------------------------------+
| :file:`{prefix}/include/python{version}`, | Recommended locations of the directories |
| :file:`{exec_prefix}/include/python{version}` | containing the include files needed for |
| | developing Python extensions and |
| | embedding the interpreter. |
+-----------------------------------------------+------------------------------------------+
| :file:`~/.pythonrc.py` | User-specific initialization file loaded |
| | by the user module; not used by default |
| | or by most applications. |
+-----------------------------------------------+------------------------------------------+
Environment variables
---------------------
.. 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
Augments 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
colons. Non-existent directories are silently ignored.
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`.
If a script argument is given, the directory containing the script is
inserted in the path in front of :envvar:`PYTHONPATH`. 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.
.. 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.
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