Modules, Imports and the Main Guard

As programs grow you split them into modules: separate .py files, each holding related functions. A file named geometry.py is a module called geometry, and another file imports it:

# geometry.py
def area(w, h):
    return w * h

# main.py
import geometry
print(geometry.area(3, 4))   # 12

# or pull in just one name
from geometry import area
print(area(3, 4))

This keeps each file focused and lets you reuse code across projects. A package is just a folder of modules.

The __main__ guard

When Python runs a file directly, it sets the special variable __name__ to the string "__main__". When the same file is imported, __name__ is the module's name instead. That difference powers a very common pattern:

def area(w, h):
    return w * h

if __name__ == "__main__":
    # only runs when you execute this file directly,
    # NOT when another file imports it
    print(area(3, 4))

The guard lets a file be both an importable library and a runnable script, without the demo code firing on import. We grade this lesson on a normal function, since the grader runs a single snippet, but the pattern is essential once you have many files.