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2025•Engineering Note

Test-Driven Development (TDD): Lessons from Kent Beck’s Classic Book

What is Test-Driven Development (TDD)?

Test-Driven Development (TDD) is a software development practice where tests are written before writing the actual code. The cycle typically follows the mantra:

  1. Red – Write a failing test.
  2. Green – Write the minimal code to make the test pass.
  3. Refactor – Improve the code while keeping all tests passing.

This approach ensures that your codebase remains reliable, maintainable, and flexible as it grows.

About the Book: Test-Driven Development: By Example

Kent Beck, one of the original signatories of the Agile Manifesto, authored Test-Driven Development: By Example to demonstrate TDD in action. The book doesn’t just explain theory—it walks you through concrete coding examples, showing how incremental development and refactoring lead to cleaner design.

The book is split into three main parts:

  • The Money Example – Building a simple currency application to showcase TDD fundamentals.
  • The xUnit Example – Writing your own testing framework to deepen understanding.
  • Patterns for TDD – Sharing lessons, common pitfalls, and practical patterns for applying TDD in real-world projects.

Step 1: Write a Failing Test (Red)

The first step is to describe the behavior you want in a test. For example, if you want to add two numbers, you’d start with:

import unittest

class TestMath(unittest.TestCase):
    def test_addition(self):
        self.assertEqual(add(2, 3), 5)

At this point, the function add doesn’t exist, so the test fails.

Step 2: Make the Test Pass (Green)

Now, write the simplest possible code to pass the test:

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

When you rerun the test, it passes.

Step 3: Refactor

With the test green, you can clean up the code if needed. For small examples, there may not be much to refactor. But in larger systems, this step ensures code quality improves continuously.

Why TDD Matters

  • Confidence – Tests provide safety nets for future changes.
  • Design Feedback – Writing tests first clarifies requirements and design.
  • Maintainability – Codebases with tests are easier to extend and refactor.

Next Steps

If you want to dive deeper:

  • Practice the Red-Green-Refactor cycle on small coding exercises (katas).
  • Explore unit testing frameworks in your preferred language (e.g., JUnit, pytest, Jest).
  • Read Kent Beck’s Test-Driven Development: By Example to see TDD applied in full projects.

Conclusion

TDD is more than just a testing strategy—it’s a mindset for building reliable, adaptable software. Kent Beck’s book remains one of the best starting points to understand and apply TDD in your everyday coding practice.