Creating your own modules

Definition

A module is simply a .py file. Any file can be imported by name, which lets you split a program across files. The check if __name__ == "__main__" runs code only when the file is run directly, and not when it's imported.

In simple words

Instead of one giant notebook, you keep maths notes in one file and chemistry in another, and open the one you need. The __name__ check says: run this part only if I'm the file you opened directly.

Why it matters

Every real project is split into modules. Without the __name__ check, importing a file would also run its demo code.

How it works

# shapes.py
PI = 3.14159

def circle_area(r):
    return PI * r * r

if __name__ == "__main__":
    print(circle_area(2))    # runs only for: python shapes.py

# main.py, in the same folder
import shapes
from shapes import circle_area
print(shapes.PI, circle_area(1))

In code

name_check.py

print("This file's __name__ is:", __name__)

def main():
    print("Running as the main program")

if __name__ == "__main__":
    main()

Output

This file's __name__ is: __main__
Running as the main program

Common mistakes

Calling your file random.py, and then wondering why import random fails.
Putting code that does work at the top level of a module, so it runs on import.

Check yourself

1. What is __name__ when a file is run directly?

2. What is a module in Python?

Summary

A module is a .py file you can import.
Use the __name__ check for code that shouldn't run on import.
Don't name files after libraries.