
``runpy`` --- Locating and executing Python modules
***************************************************

The ``runpy`` module is used to locate and run Python modules without
importing them first. Its main use is to implement the *-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 ``runpy`` module provides a single function:

runpy.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 ``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 ``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 ``sys``
   module be left alone when invoking this function from threaded
   code.

   Changed in version 3.1: Added ability to execute packages by
   looking for a ``__main__`` submodule.

See also:

   **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.
