28.5. "__main__" —— 顶层脚本环境
********************************

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:

   if __name__ == "__main__":
       main()
