__init.py__

What is __init__.py?

__init__.py is a special file that tells Python:

“This folder is a package — it can be imported.”

Without it (in older Python versions), Python would not treat a directory as importable.

Project Layout

mathlib_project/
├── main.py
└── mathlib/
    ├── __init__.py
    ├── calculator.py
    └── geometry.py

📄 mathlib/calculator.py

class Calculator:
    def add(self, a, b):
        return a + b

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

---

📄 mathlib/geometry.py

def area_of_circle(radius):
    pi = 3.14159
    return pi * radius * radius

def perimeter_of_square(side):
    return 4 * side

---

📄 mathlib/__init__.py

# Re-export classes/functions from submodules
from .calculator import Calculator
from .geometry import area_of_circle, perimeter_of_square

Now you can just do from mathlib import Calculator instead of from mathlib.calculator import Calculator.

---

📄 main.py

from mathlib import Calculator, area_of_circle, perimeter_of_square

def main():
    calc = Calculator()
    print("2 + 3 =", calc.add(2, 3))
    print("9 - 4 =", calc.subtract(9, 4))

    print("Area of circle (r=5):", area_of_circle(5))
    print("Perimeter of square (s=3):", perimeter_of_square(3))

if __name__ == "__main__":
    main()

---

▶ How to Run

Make sure you're in the mathlib_project/ directory, then run:

python main.py

✅ Output

2 + 3 = 5
9 - 4 = 5
Area of circle (r=5): 78.53975
Perimeter of square (s=3): 12