tdd

Test-Driven Development

The Rule

NO PRODUCTION CODE WITHOUT A FAILING TEST FIRST

Red-Green-Refactor Cycle

RED - Write Failing Test

def test_validates_empty_email():
    result = validate_email("")
    assert result.is_valid is False
    assert result.error == "Email required"

Run it. Watch it fail. Confirm it fails for the right reason (missing feature, not typo).

GREEN - Minimal Code

Write the simplest code that makes the test pass:

def validate_email(email: str) -> ValidationResult:
    if not email:
        return ValidationResult(is_valid=False, error="Email required")
    return ValidationResult(is_valid=True, error=None)

Run tests. All pass.

REFACTOR

Clean up only if needed. Keep tests green.

Repeat

Next failing test for next behavior.

Verification

pytest path/to/test.py -v

Must see:

• Test fails first (RED)
• Test passes after implementation (GREEN)
• All other tests still pass

Quick Checklist

• Test written before code
• Watched test fail
• Minimal implementation
• All tests pass
• No skipped steps

Anti-Patterns