TDD Worksheet – Calculator Case (Python)

This worksheet will guide you through the process of applying Test-Driven Development (TDD) to build a simple calculator in Python. You will follow the classic Red-Green-Refactor cycle for each arithmetic operation.

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Case Description

You are asked to build a Calculator module that supports the following operations:

• Addition (add)
• Subtraction (subtract)
• Multiplication (multiply)
• Division (divide)

All operations must be covered by test cases first, before the actual implementation.

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Project Setup

Your project should have the following structure:

tdd_calculator/
├── calculator/
│   └── operations.py
├── tests/
│   └── test_operations.py

Make sure you can run tests using the following command:

python -m unittest discover -s tests

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Step-by-Step Instructions

1. Write a Test for add() – Red Phase

In tests/test_operations.py, write a test case for the addition function:

import unittest
from calculator.operations import add

class TestCalculator(unittest.TestCase):
    def test_add(self):
        self.assertEqual(add(2, 3), 5)
        self.assertEqual(add(-1, 1), 0)
        self.assertEqual(add(0, 0), 0)

if __name__ == '__main__':
    unittest.main()

Run the test, and confirm it fails because the function does not exist yet.

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2. Implement add() – Green Phase

In calculator/operations.py, implement the add() function:

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

Run the test again. It should now pass.

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3. Continue the Cycle for Other Operators

Now repeat the Red-Green-Refactor cycle for the following:

• subtract(a, b) → with test case and implementation
• multiply(a, b) → with test case and implementation
• divide(a, b) → make sure to test division by zero (expect an exception)

Write each test first, make sure it fails, then write the implementation.

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💡 Extra Challenge!
If this calculator case feels too easy for you, maybe basic math is already beneath your greatness.
Feel free to level up with the TDD Bus Fare repo – where real-world logic and chaos await.
👉 Go to the Bus Fare repo