Python main Function

The Hidden __name__ Variable

Every Python file automatically gets a built-in variable called __name__. When you run a file directly, Python sets __name__ to the string "__main__". When that same file is imported as a module into another file, __name__ is set to the module's filename instead.

print(__name__)

Why This Check Exists

Suppose you write a module full of useful functions, and also add some test code at the bottom to try them out. Without a guard, that test code runs every single time someone else imports your module — which is rarely what you want. Wrapping it in if __name__ == "__main__": ensures it only runs when the file is executed directly.

A Practical Example

def add(a, b):
    return a + b

def subtract(a, b):
    return a - b

if __name__ == "__main__":
    print(add(4, 5))
    print(subtract(10, 3))

Run this file directly and you see the print output. But if another file does import calculator, only the functions become available — the test prints do not run.

import calculator

result = calculator.add(2, 2)
print(result)

Script vs Module Behaviour

This single check is what allows one Python file to work both as a standalone script and as a reusable module elsewhere — a pattern you will see in almost every serious Python project, including packages and command-line tools.

def main():
    print("Program started")

if __name__ == "__main__":
    main()