Creating Your Own Modules

Introduction

Any .py file is a module. Splitting your code across multiple files keeps things organised and makes logic reusable across different scripts.

Creating a Module

Create a file called utils.py:

# utils.py

def calculate_tax(salary, rate=0.2):
    """Return the tax amount for a given salary."""
    return salary * rate

def full_name(firstname, lastname):
    """Return a formatted full name."""
    return f"{firstname} {lastname}"

TAX_RATE = 0.2

Importing Your Module

In another file in the same directory:

# main.py
import utils

name = utils.full_name("Alice", "Smith")
tax = utils.calculate_tax(80000)

print(f"{name} owes £{tax:,.2f} in tax")

Or import specific names:

from utils import full_name, calculate_tax

print(full_name("Alice", "Smith"))
print(calculate_tax(80000, rate=0.3))

The if name == "main" Guard

When Python runs a file directly, it sets __name__ to "__main__". When the file is imported as a module, __name__ is set to the module name instead.

This guard lets you include test code in a module that only runs when the file is executed directly — not when it is imported.

# utils.py

def calculate_tax(salary, rate=0.2):
    return salary * rate

if __name__ == "__main__":
    # This block only runs when utils.py is executed directly
    print(calculate_tax(80000))

python utils.py        # prints 16000.0
python main.py         # does NOT print 16000.0

Organising Modules into Packages

A package is a directory containing an __init__.py file. This signals to Python that the directory should be treated as a package.

project/
├── main.py
└── payroll/
    ├── __init__.py
    ├── tax.py
    └── employees.py

# main.py
from payroll.tax import calculate_tax
from payroll.employees import full_name

The __init__.py can be empty, or it can import names to make them available at the package level.

Practice Exercises

• Create a helpers.py module with two functions: one that formats a salary as a string (e.g. "£80,000") and one that checks whether a salary is above a threshold.
• Import and use both functions in a separate main.py file.
• Add the if __name__ == "__main__" guard to helpers.py with a quick test of both functions.
• Create a data/ package with an __init__.py and a loader.py file. In loader.py write a function that reads a CSV file and returns a list of dictionaries.