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 # Remove the runpy module from the arguments run_module(sys.argv, run_name="__main__", alter_sys=True)
The runpy module provides a single function:
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 builtins module.
If the argument alter_sys is supplied and evaluates to True, then sys.argv 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 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.