__name__ and the if __name__ == "__main__" idiom

Every module has a built-in variable called __name__, a string that Python sets before running the module's code. Its value depends on how the file is being used. When you import a module, __name__ is the module's name, such as 'tools' for tools.py, or the dotted name such as 'pkg.tools' inside a package. When you run a file directly as the main program, for example with python tools.py or Run in IDLE, Python sets its __name__ to the special string '__main__' instead.