Create importable modules

Completed

Once your program has more than a handful of functions, keeping everything in a single file becomes hard to navigate. Python solves this with modules. A module is just a .py file whose functions and variables can be reused from other files.

Creating your own module

Any Python file you write can be imported as a module. The module's name is the file name without the .py extension.

Suppose you have this file, math_utils.py:

# math_utils.py

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

def multiply(a, b):
    return a * b

Importing a module

From another file in the same folder, you can bring math_utils in and use its functions:

# app.py
import math_utils

print(math_utils.add(2, 3))        # Output: 5
print(math_utils.multiply(4, 5))   # Output: 20

Importing specific names

If you only need a few things from a module, you can import them by name. This syntax lets you use them without the module prefix:

from math_utils import add, multiply

print(add(2, 3))        # Output: 5
print(multiply(4, 5))   # Output: 20

Using an alias

If a module name is long or clashes with another name, you can give it a shorter alias with as:

import math_utils as mu

print(mu.add(2, 3))   # Output: 5

You'll see this pattern often with third-party libraries—for example, import pandas as pd.

The if __name__ == "__main__": block

When you import a file, Python runs the code in it. If you want to include code that should only run when the file is executed directly, put it in an if __name__ == "__main__": block:

# math_utils.py

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

if __name__ == "__main__":
    # Quick self-test — only runs when this file is executed directly
    print(add(2, 3))

This pattern lets a file work both as a standalone script and as an importable module.

Why modules matter

• They keep each file focused on one responsibility.
• They make code easier to test, since functions live in one place.
• They make it natural to share and reuse code across projects.