Testing

TDD Cycle (Mandatory)

1. RED:      Write a failing test
2. VERIFY:   Confirm it fails for the right reason
3. GREEN:    Write minimal code to pass
4. VERIFY:   Confirm ALL tests pass
5. REFACTOR: Improve code quality (keep tests green)
6. REPEAT:   Next test case

Fixture Patterns

Python (pytest)

@pytest.fixture
def db_session():
    """Provide a clean database session for each test."""
    session = TestingSessionLocal()
    yield session
    session.rollback()
    session.close()

@pytest.fixture
def sample_data(db_session):
    """Create sample data for testing."""
    item = Model(name="test", value=42)
    db_session.add(item)
    db_session.commit()
    return item

JavaScript (Jest/Vitest)

beforeEach(() => {
  // Reset mocks and state
  jest.clearAllMocks();
});

const createMockData = () => ({
  id: 1,
  name: 'test',
  value: 42,
});

Test Patterns

Unit Test

def test_calculate_total_with_discount():
    """Test that discount is applied correctly."""
    # Arrange
    items = [Item(price=100), Item(price=200)]
    discount = 0.1

    # Act
    total = calculate_total(items, discount)

    # Assert
    assert total == 270  # (100 + 200) * 0.9

Integration Test

def test_create_item_endpoint(client, db_session):
    """Test creating an item via API."""
    response = client.post("/api/items", json={
        "name": "Test Item",
        "value": 42,
    })
    assert response.status_code == 201
    assert response.json()["name"] == "Test Item"

E2E Test (Playwright)

test('user can create an item', async ({ page }) => {
  await page.goto('/items/new');
  await page.fill('[name="itemName"]', 'Test Item');
  await page.click('button[type="submit"]');
  await expect(page.locator('.success-message')).toBeVisible();
});

Coverage Targets

Running Tests

# Python
pytest tests/                      # All tests
pytest tests/unit/                 # Unit only
pytest -x                          # Stop on first failure
pytest --cov=app --cov-report=html # With coverage

# JavaScript/TypeScript
npm test                           # All tests
npm test -- --watch                # Watch mode
npm test -- --coverage             # With coverage

Best Practices

1. One assertion focus per test: Test one behavior
2. Descriptive names: test_create_item_with_invalid_name_returns_400
3. Independent tests: No test depends on another
4. Fast execution: Mock external services
5. Deterministic: Same result every run
6. No production code changes: Test code only