What does if __name__ == "__main__" do in Python?

When a code is to be executed, the Python interpreter reads the source file and defines a few special variables. If the Python interpreter is running the source file as the main program, it sets the special variable __name__ to the value "__main__". However, if example.py is run as an imported file, then the interpreter will look at the filename of the module, strip off the .py, and assign that string to the module’s __name__ variable.

A module is a file that contains Python definitions and statements. The file name is the same as the module’s name​​, but appended with the suffix .py.

Understanding the example

Suppose there is a file named example.py that has the following code:

print("before import")
import math

print("before functionA")
def functionA():
    print("Function A")

print("before functionB")
def functionB():
    print("Function B {}".format(math.sqrt(100)))

print("before __name__ guard")
if __name__ == '__main__':
    functionA()
    functionB()
print("after __name__ guard")

If the file example.py runs as a main program, then the interpreter will assign the hard-coded string "__main__" to the __name__ variable:

__name__ = "__main__" 

However, if example.py is run as an imported file, then the interpreter will look at the filename of your module (example.py), strip off the .py, and assign that string to your module’s __name__ variable:

__name__ = "example"

Output as the main file

If the file example.py runs as the main program, the output will be:

before import

before functionA

before functionB

before __name__ guard

Function A

Function B 10.0

after __name__ guard

Output as an imported file

If the file example.py runs when imported, the output will be:

before import

before functionA

before functionB

before __name__ guard

after __name__ guard

Why the outputs are different

If the module is the main program, then the interpreter will set __name__ to "__main__" and call the two functions printing the strings “Function A” and “Function B 10.0”.

If the module is not the main program, then __name__ will be set to “example”, instead of​ "__main__", and the body of the if statement will be skipped over.