TDD (Test-Driven Development)

TDD (Test-Driven Development) is a software development practice in which you write an automated test before the code it checks. The cycle is called red, green, refactor: write a test that fails, write the simplest code that makes it pass, then clean up the code while keeping the tests passing.

Why it matters

Writing tests first forces you to define the expected behavior before implementing it, which tends to produce smaller functions and clearer interfaces. The growing test suite becomes a safety net that lets you refactor with confidence. TDD takes discipline and can feel slow at first, and it fits some work (logic, libraries, APIs) better than others (exploratory prototypes, visual layout). Evidence on its benefits is mixed, and many teams use it selectively.

Example

# 1. Red: test written first, fails because slugify doesn't exist
def test_slugify():
    assert slugify("Hello World") == "hello-world"

# 2. Green: simplest code that passes
def slugify(s):
    return s.lower().replace(" ", "-")

# 3. Refactor, then add a test for punctuation

Common mistakes

• Writing many tests at once instead of one small step at a time.
• Testing implementation details so tests break on every refactor.
• Skipping the refactor step.

Related

• Unit test
• Refactoring
• Code coverage
• Mocking

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