[Tutor] Tutor FAQ

[Mike Hansen]

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What is if __name__ == "__main__" for?

The 'if __name__ == "__main__":  ..." trick exists in Python so that our
Python files can act as either reusable modules, or as standalone
programs.  As a toy example, let's say that we have two files:

######
mumak:~ dyoo$ cat mymath.py
def square(x):
     return x * x

if __name__ == '__main__':
     print "test: square(42) ==", square(42)


mumak:~ dyoo$ cat mygame.py
import mymath

print "this is mygame."
print mymath.square(17)
######

In this example, we've written mymath.py to be both used as a utility
module, as well as a standalone program.  We can run mymath standalone 
by
doing this:

######
mumak:~ dyoo$ python mymath.py
test: square(42) == 1764
######


But we can also use mymath.py as a module; let's see what happens when 
we
run mygame.py:

######
mumak:~ dyoo$ python mygame.py
this is mygame.
289
######

Notice that here we don't see the 'test' line that mymath.py had near 
the
bottom of its code.  That's because, in this context, mymath is not the
main program.  That's what the 'if __name__ == "__main__": ...' trick is
used for.

[From a post to Python Tutor by Danny Woo]