cpython/Doc/library/__main__.rst

:mod:`__main__` --- Top-level script environment
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.. module:: __main__
   :synopsis: The environment where the top-level script is run.

'__main__' is the name of the scope in which top-level code executes.
A module's __name__ is set equal to '__main__' when read from
standard input, a script, or from an interactive prompt.

A module can discover whether or not it is running in the main scope by
checking its own __name__, which allows a common idiom for conditionally
executing code in a module when it is run as a script or with `python
-m` but not when it is imported:

   if __name__ == "__main__":
       # execute only if run as a script
       main()

For a package, the same effect can be achieved by including a
__main__.py module, the contents of which will be executed when the
module is run with -m.