Python Creating Modules

How to Create a Module in Python

A module is just a file with a .py extension. We can put any Python code inside.

Let us create a file called calculator.py.

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

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

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

def divide(a, b):
    if b == 0:
        return "Cannot divide by zero"
    return a / b

That is a module. It has four functions.

How to Use Our Module in Another File

Now we create another file called main.py in the same folder.

import calculator

print(calculator.add(5, 3))
print(calculator.subtract(10, 4))
print(calculator.multiply(6, 7))
print(calculator.divide(20, 5))

8
6
42
4.0

We imported the calculator module and used its functions.

Using the name Variable in Python

Every Python module has a special variable called __name__. When a module is run directly, __name__ is "__main__". When it is imported, __name__ is the module’s name.

We can use this to write code that only runs when the module is run directly, not when it is imported.

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

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

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

If we run calculator.py directly, we see:

Testing calculator module
8
6

But if we import it from main.py, the test code does not run.

import calculator

print(calculator.add(5, 3))

8

That is a very common pattern. It lets us test our module without affecting other programs that use it.

Adding Documentation to a Module

We can add a docstring at the top of our module to describe what it does.

"""
This module provides basic arithmetic functions.
It includes add, subtract, multiply, and divide.
"""

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

# rest of the functions...

That helps others understand what the module is for.

A Real-Life Example – String Utilities Module

Let us create a module with common string helpers.

def reverse_string(s):
    return s[::-1]

def is_palindrome(s):
    s = s.lower()
    return s == s[::-1]

def count_vowels(s):
    return sum(1 for c in s.lower() if c in "aeiou")

if __name__ == "__main__":
    print(reverse_string("hello"))
    print(is_palindrome("racecar"))
    print(count_vowels("programming"))

Now we can use this module in our main program.

import string_utils

print(string_utils.reverse_string("Python"))
print(string_utils.is_palindrome("madam"))
print(string_utils.count_vowels("Hello World"))

nohtyP
True
3

That is a useful module.

Common Mistakes When Creating Modules in Python

# my module.py