Test-Driven Development (TDD)

Test-Driven Development is a workflow where you write your tests before you write your functional code.

The Red-Green-Refactor Cycle

1. RED: Write a test for a small bit of functionality and watch it fail. (It fails because the code doesn’t exist yet).
2. GREEN: Write the absolute minimum amount of code to make the test pass. Don’t worry about “clean code” yet.
3. REFACTOR: Now that you have a passing test, clean up your code. The test ensures you don’t break the logic while making it pretty.

Why use TDD?

• No Over-Engineering: You only write the code you actually need to pass the test.
• 100% Coverage: Every line of code you write is guaranteed to have a test.
• Better Design: It forces you to think about the interface (how the function is used) before you think about the implementation.

TDD Example (Python)

Requirement: Create a function is_password_valid that requires at least 8 characters.

Step 1: Red

def test_password_length():
    assert is_password_valid("1234567") == False
    assert is_password_valid("12345678") == True

Result: NameError: name 'is_password_valid' is not defined

Step 2: Green

def is_password_valid(pwd):
    return len(pwd) >= 8

Result: Test Passed!

Step 3: Refactor

Maybe we want to move the “8” to a constant.

MIN_LENGTH = 8
def is_password_valid(pwd):
    return len(pwd) >= MIN_LENGTH

Result: Test still passes!

Summary

TDD feels slow at first, but it prevents the “spaghetti code” and “fear of change” that plagues large projects.