python

What does __name__==’__main__’ in Python mean ?

All python beginners, once in a while must have stumbled upon the below piece of code.

__name__=='__main__' 

What does it actually mean and what is the use of the above piece of code. We’ll see what the above code does and its relevance with the help of an example.

Lets create a simple python program hello.py with a function called foo.

#!/usr/bin/python
# hello.py

def foo():
    print 'foo is called'

foo()

If you try running the above program, you should have the following output on the console.

foo is called

Now, suppose we need to import this hello.py file into another python program in order to access the foo function.
Let’s create another python program called world.py. In world.py we’ll import our hello.py and try to call foo. Here is howworld.py would look like:

#!/usr/bin/python
# world.py

import hello

def bar():
    hello.foo()

bar()

Now, if you execute world.py, you should have the following output on the terminal console;

foo is called
foo is called

We’ll as you saw the message gets printed twice. The simple solution would be to remove the foo function call fromhello.py. But in that case it won’t print the message when we execute hello.py. Well, this is where

__name__=='__main__'

comes into the picture. When a python program starts to execute, it sets a few special variable, __name__ is one of them. It contains the value __main__ if it’s the main program being executed. But it won’t have the value as __main__ being set, if it’s being imported and executed, since it’s not the main program when imported.
Let’s modify hello.py to include the checking for __name__ .

#!/usr/bin/python
# hello.py

def foo():
    print 'foo is called'

if __name__ == '__main__':
    foo()

Now, if you try running the world.py program it won’t print the message twice. The only message that is printed is of the hello.foo function call.