Meaning of if __name__ == "__main__" in Python

20 Jul 2019

Simple answer: if __name__ == "__main__" helps a programme to choose and pick chunks of code from other programmes.

Without this, the called programme will be executed fully, even if only one function is required by the calling programme.

Let’s look at an example. Assume that we already have a programme that has separate functions to find square and cube of a number. Such a programme could be as given given below. Let’s also assume that the programme is stored in the file test.py.

def square(x):
    return x*x

def cube(x):
    return x*x*x

def main():
    from argparse import ArgumentParser

    x = ArgumentParser()
    x.add_argument("p", type = int)
    x.add_argument("q", type = int)
    arguments = x.parse_args()
    print(square(arguments.p) + cube(arguments.q))

if __name__ == "__main__":
    main()

Let’s say we just want to write a programme to compute the cube of a number. Since this functionality is already available in test.py, as smart programmers, we simply want to call that particular chunk of code and use it. Such a programme, say test1.py could be as given below.

import test

def main():
    from argparse import ArgumentParser

    x = ArgumentParser()
    x.add_argument("a", type = int)
    y = x.parse_args()
    print(test.cube(y.a))

if __name__ == "__main__":
    main()

Now, if you run test1.py, you will notice that only the function cube from test.py is executed, but not the entire programme. Without if __name__ == "__main__": condition, running test1.py results in completely running test.py (this actually gives an error msg).

What’s happening?

When executing a programme, the Python interpreter creates the special variable __name__ and assigns a string value to it. In the case of a stand-alone programme, the value assigned is __main__. This is the case when test.py is run.

However, when the programme is executed as module, then the module name is assigned to the __name__ variable. This is what happens when test1.py, which calls a function from test.py, is run. Here, test.py is used as a module and the value test is assigned to the variable __name__ in test.py. Note that this is happening in test.py, but not in test1.py.

The statement if __name__ == "__main__": simply checks if the value assigned to __name__ is __main__. When running test.py, this check yields a positive result and the subsequent statement is executed.

While running test1.py, __name__ has the value test. Therefore, if __name__ == "__main__": returns a negative result, and the subsequent statement is not executed.