Cave of Python

When you load a Python module, by default all the code in that module will be interpreted by the Python interpreter.

This means that if your module has a ‘main’ function — some function that you use as an entry point to your program — by default it will run, and this is often not what you want to happen.

mymodule.py:

def greet():
    print("Hello")

def main():
    greet()

main()

myprogram.py:

import mymodule as mm

# Nothing else here!

output:

Hello

Conditionally Running ‘Main’

To fix this, we can add some code that will ensure your main function will run only when the module file is run directly.

There is a variable called __name__ which will contain the text __main__ only if the file is run directly from the terminal. Otherwise it will contain the name of the module.

mymodule.py:

def greet():
    print("Hello")

def main():
    greet()

if __name__ == "__main__":
    main()

Note that there is nothing special about the name “main” for a function. It’s just a good name to call the starting point of your program.

Now, main() will only run if mymodule.py is run as a standalone program from the terminal. If it’s loaded as a module, main() will not run.