leejunkim

main function in Python, and __name__

2 min read

  • main
    • Usually a special function that is automatically executed when an operating system starts to run a program, like java
    • But in Python you don’t need to create a function named main, you can just have a file with just code & it will run it from top to bottom
  • __name__
    • Every Python file has a special variable __name__
    • When you run the file directly (ex. python my_file.py) in the terminal - Python sets __name__ = "__main__"
    • When you import the file as a module, if you have another file importer.py that says import my_file, Python sets __name__ = "my_file" inside my_file.py
  • sources 
def main():
    print("Hello World!")
 
if __name__ == "__main__":
    main()
  • if __name__ == "__main__":
    • When the if statement evaluates to True, the Python interpreter executes main().
    • So basically it’s only set like that when we execute this file directly & not as a module
    • you can use this to distinguish when you run the module as a script or after an import
      • Ex. Only print things when it’s run in a script (add print statements under this condition)
  • This code pattern is quite common in Python files that you want to be executed as a script and imported in another module
  • No matter how you’re running your code, Python defines a special variable named __name__ that contains a string whose value depends on how the code is being used

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