Creating Your Own Modules and Packages

Creating a Custom Module

Any .py file is automatically a module. Save the following as calculator.py:

# calculator.py
def add(a, b):
    return a + b

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

PI = 3.14159

Now use it from another file in the same folder:

# main.py
import calculator

print(calculator.add(5, 3))         # 8
print(calculator.subtract(10, 4))    # 6
print(calculator.PI)                  # 3.14159

# or import specific names
from calculator import add
print(add(2, 2))   # 4

name == 'main' inside a module

# calculator.py
def add(a, b):
    return a + b

if __name__ == '__main__':
    # runs ONLY when calculator.py is executed directly,
    # NOT when it is imported by another file
    print("Testing:", add(2, 3))

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Concept of Packages

A package is a folder containing multiple related modules, along with a special init.py file that marks it as a package.

mypackage/
    __init__.py
    shapes.py
    strings_util.py

# mypackage/shapes.py
def area_circle(r):
    return 3.14159 * r * r

# using the package from outside
from mypackage import shapes
print(shapes.area_circle(5))    # 78.53975

# or
from mypackage.shapes import area_circle
print(area_circle(5))

Why init.py?

• Marks a directory as a Python package so it can be imported.
• Can be empty, or can control what gets exposed when the package is imported (via all).

Module vs Package

Real-world example — organizing a project

project/
    main.py
    utils/
        __init__.py
        math_utils.py
        string_utils.py

# main.py
from utils.math_utils import add
from utils.string_utils import reverse_string