Python’s Main Function

To conclude, the meaning of if __name__ == "__main__" is declaration and start of the main function. Writing function call code like main below that code to perform function functionality.

# taeng.py

def main():
    # In python 3, use print()
    print "Main Function"

if __name__ == "__main__":
	main()

$ python taeng.py
$ Main Function 

It’s a bit regrettable and unsatisfactory to just pass it off as “Ah~ it’s a main function declaration”. Learning a bit more in detail.

What Happens Without It?

Assuming importing another module. Assuming the module name is taengModule and functionality simply returns addition results as below.

# taengModule.py

def add(x, y):
	return x + y

Running python in terminal and directly importing and using the module.

# run python
>>> import taengModule
>>> print (taengModule.add(2, 3))
5 
>>>

Simple. Expected result value 5 is output normally. To check taengModule separately here, adding a print function as below. And what happens if we directly execute tanegModule only?

# taengModule.py

def add(x, y):
	return x + y
	
print (add(3, 4))

$ python taengModule.py
$ 7

Seeing that execution results output expected values normally. But what happens this time when importing instead of directly executing modules?

# run python
>>> import taengModule
7
>>>

Just by doing import, taengModule.py’s code is executed and result values are output. We only wanted to use taengModule’s add function.

To solve such problems, changing taengModule.py file’s code as below.

# taengModule.py

def add(x, y):
	return x + y

# Add this part.	
if __name__ == "__main__":
    print (add(3, 4))

After modifying code, even if performing import again, the print function isn’t executed as expected. Of course, when directly executing instead of loading modules like python taengModule, the print function executes.

Why Is That?

In Python, the __name__ variable is a special variable name used internally. In the example above, when directly executing taeng.py file like python taeng.py, the __name__ variable is assigned the value __main__.

However, when loading modules through import taengModule like module loading we examined last, the __name__ variable stores the module name (taengModule here).

In conclusion, using conditional statements like if __name__ == "__main__", when directly called and used like in terminal, it performs functionality itself, and simultaneously can provide needed functions, etc. to other modules.