Python's if __name__ == '__main_

Python's if name == 'main'

Kiran Koduru • Nov 20, 2017 • 2 minutes to read

When you are new to python, you may have noticed that the variables that are widely used in all projects are __name__ in conjunction with __main__. They might be the most mentioned dunder variables in Python projects on github. To be specific, it is mentioned 6,436,149 times at the time of this writing.

`name`

__name__ is assigned to the module/file that it is associated with. It can also be assigned to module, classes, functions, methods, descriptors or generator instances.

# my_name_is_slim_shady.py
print(__name__) # will be set to `my_name_is_slim_shady`

class MyNameIsSlimShady(object):

    def say_my_name(self):
        print("My name is:")

Calling the my_name_is_slim_shady.py from a file what_is_my_name.py outputs my_name_is_slim_shady and class name MyNameIsSlimShady.

# what_is_my_name.py

print("What's my name?")
import my_name_is_slim_shady

# Outputs:
# What's my name?
# my_name_is_slim_shady

ss = my_name_is_slim_shady.MyNameIsSlimShady()
ss.say_my_name()
print(ss.__class__.__name__)

# Outputs:
# My name is:
# MyNameIsSlimShady

`main`

The __name__ variable for the module that is being executed is set to __main__ for a script, standard output or the interactive terminal. It is used to check if a module is invoked directly or called from a different file, see above description for __name__.

When we run the file my_name_is_slim_shady.py alone we see that main_my_name_is_slim_shady() function is invoked since the __name__ for the file is set to __main__ for this module.

# my_name_is_slim_shady.py
def main_my_name_is_slim_shady():
    print("Standing Up!")

if __name__ == '__main__':
    main_my_name_is_slim_shady()

# Output:
# Standing Up!

When I import the my_name_is_slim_shady.py file in what_is_my_name.py it doesn’t call the main_my_name_is_slim_shady() function but invokes main_what_is_my_name().

# what_is_my_name.py
import my_name_is_slim_shady

def main_what_is_my_name():
    print("Not Standing Up")

if __name__ == '__main__':
    main_what_is_my_name()

# Output:
# Not Standing Up

This idea can be use for larger projects to set a starting point of execution for the whole project. Like in older versions of flask projects where you called app.run() from a python file.

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