When your script is run it as a command to the Python interpreter:
python your_script.py
all of the code that is at indentation level 0 gets executed and functions and classes that are defined but none of their code gets ran.
If will then read :
if __name__ == '__main__'
so it will execute the block standalone.
In other words, when you use the
__main__ this means the module is being run standalone
if __name__ == '__main__':
print '... is being run by itself'
else:
print '... is being run directly'