:mod:`runpy` --- Locating and executing Python modules

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

.. module:: runpy

   :synopsis: Locate and run Python modules without importing them first.

.. moduleauthor:: Nick Coghlan <ncoghlan@gmail.com>

.. versionadded:: 2.5

The :mod:`runpy` module is used to locate and run Python modules without

importing them first. Its main use is to implement the :option:`-m` command line

switch that allows scripts to be located using the Python module namespace

rather than the filesystem.

When executed as a script, the module effectively operates as follows::

   del sys.argv[0]  # Remove the runpy module from the arguments

   run_module(sys.argv[0], run_name="__main__", alter_sys=True)

The :mod:`runpy` module provides a single function:

.. function:: run_module(mod_name[, init_globals] [, run_name][, alter_sys])

   Execute the code of the specified module and return the resulting module globals

   dictionary. The module's code is first located using the standard import

   mechanism (refer to PEP 302 for details) and then executed in a fresh module

   namespace.

   The optional dictionary argument *init_globals* may be used to pre-populate the

   globals dictionary before the code is executed. The supplied dictionary will not

   be modified. If any of the special global variables below are defined in the

   supplied dictionary, those definitions are overridden by the ``run_module``

   function.

   The special global variables ``__name__``, ``__file__``, ``__loader__`` and

   ``__builtins__`` are set in the globals dictionary before the module code is

   executed.

   ``__name__`` is set to *run_name* if this optional argument is supplied, and the

   *mod_name* argument otherwise.

   ``__loader__`` is set to the PEP 302 module loader used to retrieve the code for

   the module (This loader may be a wrapper around the standard import mechanism).

   ``__file__`` is set to the name provided by the module loader. If the loader

   does not make filename information available, this variable is set to ``None``.

   ``__builtins__`` is automatically initialised with a reference to the top level

   namespace of the :mod:`__builtin__` module.

   If the argument *alter_sys* is supplied and evaluates to ``True``, then

   ``sys.argv[0]`` is updated with the value of ``__file__`` and

   ``sys.modules[__name__]`` is updated with a temporary module object for the

   module being executed. Both ``sys.argv[0]`` and ``sys.modules[__name__]``

   are restored to their original values before the function returns.

   Note that this manipulation of :mod:`sys` is not thread-safe. Other threads may

   see the partially initialised module, as well as the altered list of arguments.

   It is recommended that the :mod:`sys` module be left alone when invoking this

   function from threaded code.

.. seealso::

   :pep:`338` - Executing modules as scripts

      PEP written and  implemented by Nick Coghlan.