cpython/Doc/lib/libmain.tex

\section{\module{__main__} ---
         Top-level script environment}

\declaremodule[main]{builtin}{__main__}
\modulesynopsis{The environment where the top-level script is run.}

This module represents the (otherwise anonymous) scope in which the
interpreter's main program executes --- commands read either from
standard input, from a script file, or from an interactive prompt.  It
is this environment in which the idiomatic ``conditional script''
stanza causes a script to run:

\begin{verbatim}
if __name__ == "__main__":
    main()
\end{verbatim}