/tdd - Test-Driven Development (INTERNAL)

INTERNAL SKILL — Automatically called by /build, not invoked directly by users.

Enforce TDD discipline with Red-Green-Refactor cycle.

Purpose

Called automatically by @build to ensure:

• •Tests written BEFORE implementation
• •Minimal code in Green phase
• •Refactoring doesn't break tests

The TDD Cycle

Phase 1: RED - Write Failing Test

1. •Write test FIRST - before any implementation code
2. •Run test - verify it FAILS with expected error
3. •NO implementation yet - if you wrote code, you cheated

Phase 2: GREEN - Minimal Implementation

1. •Write minimal code - just enough to make test pass
2. •Run test - verify it PASSES
3. •NO refactoring yet - that comes next

Phase 3: REFACTOR - Improve Code

1. •Improve code - clean up, extract, rename
2. •Run test - verify it STILL PASSES
3. •Add more tests if new edge cases discovered

Phase 4: COMMIT - Save Working State

1. •Commit - each cycle ends in a commit
2. •Message - describes what was built

Self-Review Checklist

After each cycle:

• • Test written BEFORE implementation
• • Test verified FAILING in Red phase
• • Only minimal code in Green phase
• • All tests passing after Refactor
• • Commit created with conventional message

Exit When

• •All acceptance criteria met
• •Coverage >= 80%
• •mypy --strict passes

Full Cycle Example

# Phase 1: RED - Write Failing Test

→ Writing test for email validation...

File: tests/unit/test_validators.py
```python
def test_email_validation():
    validator = EmailValidator()
    assert validator.is_valid("user@example.com") is True
    assert validator.is_valid("invalid-email") is False

→ Running test... $ pytest tests/unit/test_validators.py FAILED - ModuleNotFoundError: No module named 'validators' ✓ Test fails as expected (RED phase complete)

Phase 2: GREEN - Minimal Implementation

→ Writing minimal implementation...

File: src/validators.py

import re

class EmailValidator:
    def is_valid(self, email: str) -> bool:
        pattern = r'^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$'
        return bool(re.match(pattern, email))

→ Running test... $ pytest tests/unit/test_validators.py PASSED ✓ Test passes (GREEN phase complete)

Phase 3: REFACTOR - Improve Code

→ Refactoring: Extract pattern as constant...

File: src/validators.py

import re
from typing import Final

EMAIL_PATTERN: Final[str] = r'^[a-zA-Z0-9._%+-]+@[a-zA-Z0-9.-]+\.[a-zA-Z]{2,}$'

class EmailValidator:
    def is_valid(self, email: str) -> bool:
        return bool(re.match(EMAIL_PATTERN, email))

→ Running test... $ pytest tests/unit/test_validators.py PASSED ✓ Tests still pass after refactor

→ Running coverage... $ pytest --cov=src/validators tests/unit/test_validators.py Coverage: 100% ✓ Coverage >= 80%

→ Running type check... $ mypy src/validators.py --strict Success: no issues found ✓ Type checking passed

Phase 4: COMMIT - Save State

→ Creating commit... $ git add src/validators.py tests/unit/test_validators.py $ git commit -m "feat(validators): add email validation

• •Add EmailValidator class with regex pattern
• •Cover basic valid/invalid cases
• •100% test coverage"

✓ TDD cycle complete! Ready for next AC.