29.4. __main__ — Top-level script environment
'__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.