Test Driven Development

Test-Driven Development (TDD)

> From obra/superpowers

Core Principle

Write tests before implementation. Watch them fail. Write minimal code to pass.

THE IRON LAW: NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

When to Apply TDD

• New features
• Bug fixes
• Refactoring
• Any behavior changes

The Red-Green-Refactor Cycle

RED: Write a Failing Test

• Write ONE minimal test demonstrating desired behavior
• Use clear, descriptive test names
• Use real code, avoid unnecessary mocks
• Test should fail for the RIGHT reason

# Verify RED
- Run the test
- Confirm it fails
- Confirm failure is NOT due to syntax errors
- Confirm you're not testing existing functionality

GREEN: Make It Pass

• Write the SIMPLEST code that makes the test pass
• Don't add extra features
• Don't optimize yet
• Just make it work

# Verify GREEN
- Run all tests
- Confirm new test passes
- Confirm no other tests broke

REFACTOR: Clean Up

• Improve code quality while keeping tests green
• Remove duplication
• Improve naming
• Extract helpers if needed
• Run tests after each change

Why This Order Matters

Tests written AFTER implementation:

• Pass immediately (proves nothing)
• You never see them fail
• Cannot verify they test what matters
• Manual testing is NOT a substitute

Common Rationalizations to REJECT

| Excuse | Reality |

|--------|---------|

| "Too simple to test" | Simple code still breaks |

| "I'll write tests after" | Post-implementation tests pass immediately |

| "Already manually tested" | Manual testing lacks systematic rigor |

| "Deleting work is wasteful" | Unverified code is technical debt |

| "Keep as reference" | You'll adapt it; that's testing-after |

Red Flags Requiring RESTART

If any of these occur, DELETE the code and start over:

• Writing code before tests
• Tests passing immediately on first run
• Rationalizing "just this once"
• Planning to "test later"
• Keeping pre-written code "as reference"

Example Workflow

# 1. RED - Write failing test
def test_user_can_login_with_valid_credentials():
    user = create_user(email="[email protected]", password="secret")
    result = login(email="[email protected]", password="secret")
    assert result.success == True
    assert result.user == user

# 2. Run test - MUST FAIL
# 3. GREEN - Implement minimal code
def login(email, password):
    user = User.find_by_email(email)
    if user and user.check_password(password):
        return LoginResult(success=True, user=user)
    return LoginResult(success=False, user=None)

# 4. Run test - MUST PASS
# 5. REFACTOR - Clean up if needed

Summary

• Write test first
• Watch it fail
• Write minimal code to pass
• Refactor while green
• Repeat