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