Python main function

Filed Under: Python

Python main function is executed only when it’s being executed as a python program. As you know, we can also import a python program as a module, in that case python main method should not execute.

Python main function

Main function is the entry point of any program. But python interpreter executes the source file code sequentially and doesn’t call any method if it’s not part of the code. But if it’s directly part of the code then it will be executed when the file is imported as a module.

That’s why there is a special technique to define main method in python program, so that it gets executed only when the program is run directly and not executed when imported as a module. Let’s see how to define python main function in a simple program.


print("__name__ value: ", __name__)

def main():
    print("python main function")

if __name__ == '__main__':
  • When a python program is executed, python interpreter starts executing code inside it. It also sets few implicit variable values, one of them is __name__ whose value is set as __main__.
  • For python main function, we have to define a function and then use if __name__ == '__main__' condition to execute this function.
  • If the python source file is imported as module, python interpreter sets the __name__ value to module name, so the if condition will return false and main method will not be executed.
  • Python provides us flexibility to keep any name for main method, however it’s best practice to name it as main() method. Below code is perfectly fine, however not recommended.
    def main1():
        print("python main function")
    if __name__ == '__main__':

Below image shows the output when is executed as source file.

python main function, python if name main

Python main function as module

Now let’s use above python source file as a module and import in another program.

import python_main_function


Now when above program is executed, below output is produced.

python main method, python if main

__name__ value:  python_main_function

Notice that first two lines are getting printed from source file. Notice the value of __name__ is different and hence main method is not executed.

Notice that python program statements are executed line by line, so it’s important to define the main() method first before the if condition to execute main method. Otherwise you will get error as NameError: name 'main' is not defined.

That’s all about python main function.

Reference: Python Docs


  1. Nahuel Tamasso says:

    Thanks so much for this breakdown. It was super clear and helpful for a begginer like myself.

  2. Eric Webb says:

    Still relevant. Thanks for the simple but elegant breakdown. I needed that and truly appreciate it.

  3. Kiran says:

    Nice document. It will be helpful for a lot of beginners. Thanks

  4. RandomGoogler says:

    Very well explained .

  5. Hari Haran says:

    Very good explanation , keep it up

  6. Srinivas says:

    Well Done Pankaj, very well explained.
    So far this is the best one from desi !!!!

  7. Sensation Person says:

    The best site with the best information I’ve ever found! Thousands of thanks!

Comments are closed.

Generic selectors
Exact matches only
Search in title
Search in content