Python Modules & Packages: Architecture, Imports & Package Management

As Python software expands to thousands of lines, keeping all code in a single file becomes impossible. Python solves this via Modules and Packages. A module is a single .py file; a package is a directory of modules. Together, they form the foundation of Python's modular architecture.

1. Creating and Importing a Custom Module

Create a file named math_utils.py:

# File: math_utils.py
PI = 3.14159

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

def circle_area(radius):
    return PI * (radius ** 2)

Now import it in another script in the same directory (main.py):

# File: main.py
# 1. Standard module import:
import math_utils
print(math_utils.add(10, 25))

# 2. Specific function import:
from math_utils import circle_area, PI
print("Area of Circle:", circle_area(7))

# 3. Aliasing with 'as':
import math_utils as mu
print(mu.PI)

2. The Famous if __name__ == "__main__": Idiom

Every Python module has a special built-in string variable named __name__:

• When you run the file directly: Python assigns __name__ = "__main__".

• When the file is imported elsewhere: Python assigns __name__ = "math_utils".

# File: calculator.py
def multiply(x, y):
    return x * y

# Unit testing code that runs ONLY when executing calculator.py directly:
if __name__ == "__main__":
    print("Self-test: 5 * 6 =", multiply(5, 6))

3. Creating Python Packages (__init__.py)

A package is structured as a filesystem folder:

my_ecommerce_app/
│
├── __init__.py           # Marks directory as a Python package
├── payment/
│   ├── __init__.py
│   ├── razorpay.py
│   └── stripe.py
└── orders/
    ├── __init__.py
    └── cart.py

You can import from package sub-modules with dot notation:

from my_ecommerce_app.payment.stripe import process_charge

Frequently Asked Questions (FAQ)