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# 29.5. `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.

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