tdd

tdd — Test-Driven Development Behavioral Protocol

Operating Invariants

• Public Seam Only: Test observable behaviors strictly through public module interfaces; never assert against private helper methods or internal state variables.
• Single Failing Assertion (RED): Write exactly one failing test representing the acceptance criteria before touching production code. Run the test and verify it fails with the expected assertion error, not a syntax or import error.
• Minimal Implementation (GREEN): Write only the minimal production code necessary to turn the failing test green; forbid speculative features, anticipatory abstractions, or untested code branches.
• Zero Assertion Roulette: Assert on concrete state transitions, return payloads, or domain invariants; forbid shallow existence assertions (expect(res).toBeDefined(), assert len(x) > 0).
• Canonical Output Contract: (Omitted by Design — This is a Model-Invoked Behavioral Protocol. Its output is the production code and test suite delta).

Domain Rubric

1. The Red-Green-Refactor Cycle

1. RED: Write a single test case capturing the concrete acceptance criteria. Execute test runner to verify RED status.
2. GREEN: Implement minimal logic to pass. Run test runner and typechecker to verify GREEN status.
3. REFACTOR: Prune redundant variables, eliminate duplicate logic, and align naming with domain language while preserving passing tests.

2. Invariant & Property Checks

Prefer invariant testing over shallow example tests:

• Boundedness: Range boundaries must hold under arbitrary input (0 <= discount <= total).
• Idempotency: Re-applying the same mutation yields the identical resulting state.
• Round-Trip: Serializing and deserializing a domain entity preserves deep equality.

Smells vs. Remedies

// Smell: Testing private internals (Couples test to implementation)
expect(service['internalCache'].has('item-123')).toBe(true);

// Remedy: Testing through public observable seam
const result = await service.fetchItem('item-123');
expect(result.id).toBe('item-123');

# Smell: Shallow assertion roulette (Passes even if data is corrupt)
assert response.status_code == 200
assert response.json() is not None

# Remedy: Concrete invariant assertion
assert response.status_code == 200
data = response.json()
assert data["order_id"] == expected_id
assert data["status"] == "CONFIRMED"