: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>


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.

   If the supplied module name refers to a package rather than a normal module,
   then that package is imported and the ``__main__`` submodule within that
   package is then executed and the resulting module globals dictionary returned.

   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__``,
   ``__builtins__`` and ``__package__`` are set in the globals dictionary before
   the module code is executed.

   ``__name__`` is set to *run_name* if this optional argument is supplied, to
   ``mod_name + '.__main__'`` if the named module is a package and to 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:`builtins` module.

   ``__package__`` is set to *mod_name* if the named module is a package and to
   ``mod_name.rpartition('.')[0]`` otherwise.

   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.


   .. versionchanged:: 3.1
         Added ability to execute packages by looking for a ``__main__`` submodule


.. seealso::

   :pep:`338` - Executing modules as scripts
      PEP written and implemented by Nick Coghlan.

   :pep:`366` - Main module explicit relative imports
      PEP written and implemented by Nick Coghlan.