Testing Patterns

Systematic approach to effective testing. Use this skill when:

• Writing or changing tests (load anti-patterns reference)
• Designing test strategies for new features
• Reviewing test coverage adequacy
• Implementing test frameworks or infrastructure

Test-Driven Development (TDD)

TDD is MANDATORY for new feature code. Write tests before implementation.

The TDD Cycle

┌─────────────────────────────────────────┐
│                                         │
│   1. RED     → Write failing test       │
│   2. GREEN   → Minimal code to pass     │
│   3. REFACTOR → Clean up, tests stay green │
│   4. REPEAT                             │
│                                         │
└─────────────────────────────────────────┘

Why TDD?

When TDD Applies

TDD Violations

If implementation arrives without tests:

1. Reject with "TDD Required"
2. Specify which tests should exist
3. Implementation writes tests first, then code

See references/testing-anti-patterns.md for detailed anti-patterns and gate functions.

Test Pyramid

        /\
       /  \        E2E Tests (10%)
      /----\       Slow, expensive, few
     /      \
    /--------\     Integration Tests (20%)
   /          \    Medium speed, focused
  /------------\
 /              \  Unit Tests (70%)
/________________\ Fast, isolated, many

Unit Tests

What: Test single function/class in isolation When: All business logic, utilities, data transformations Speed: Milliseconds Isolation: Mock external dependencies (DB, network, filesystem)

# Good unit test
def test_calculate_discount():
    order = Order(items=[Item(price=100)])
    assert order.calculate_discount(0.2) == 80

# Bad: tests integration, not unit
def test_order_discount():
    db.create_order(...)  # Touches database
    api.apply_coupon(...)  # External call

Integration Tests

What: Test component interactions When: Database queries, API contracts, service boundaries Speed: Seconds Isolation: Real dependencies for component under test

# Integration: tests DB interaction
def test_user_repository_finds_by_email():
    repo = UserRepository(test_db)
    repo.create(User(email="test@example.com"))
    found = repo.find_by_email("test@example.com")
    assert found.email == "test@example.com"

E2E Tests

What: Test full user workflows When: Critical paths, smoke tests, happy paths Speed: Minutes Isolation: None—tests complete system

# E2E: tests full flow
def test_user_can_checkout():
    browser.goto("/")
    browser.login("user@example.com", "password")
    browser.add_to_cart("product-1")
    browser.checkout()
    assert browser.has_text("Order confirmed")

Coverage Strategy

What to Cover (Priority Order)

1. Business logic — revenue-impacting calculations
2. Security boundaries — auth, validation, access control
3. Error paths — exception handling, edge cases
4. Integration points — API contracts, DB queries
5. Happy paths — standard user workflows

What NOT to Prioritize

• Getters/setters without logic
• Framework code (already tested)
• Third-party libraries
• One-time scripts
• UI layout (unless critical)

Coverage Targets

Edge Case Generation

Systematic Approach

For numeric inputs:

• Zero
• Negative numbers
• Very large numbers (overflow)
• Floating point precision
• Boundary values (n-1, n, n+1)

For string inputs:

• Empty string
• Very long string
• Unicode/emoji
• Special characters
• Whitespace only
• SQL/HTML injection attempts

For collections:

• Empty
• Single element
• Many elements
• Duplicates
• null/undefined elements

For dates:

• Leap years
• Timezone boundaries
• DST transitions
• Far past/future
• Invalid dates

Example Matrix

| Input | Scenario | Expected |
|-------|----------|----------|
| price | 0 | Free item handling |
| price | -5 | Validation error |
| price | 999999.99 | Large number display |
| name | "" | Required field error |
| name | "a"*1000 | Truncation or error |
| email | "test" | Invalid format error |

Mocking Patterns

When to Mock

⚠️ TDD prevents over-mocking: If you write the test first and watch it fail, you know exactly what needs mocking.

Mock vs Stub vs Spy

# Stub: Returns canned response
payment_gateway = Mock()
payment_gateway.charge.return_value = {"status": "success"}

# Mock: Verifies interactions
email_service = Mock()
order.complete()
email_service.send.assert_called_once_with(
    to="user@example.com",
    subject="Order Confirmed"
)

# Spy: Wraps real implementation
real_logger = Logger()
spy_logger = Mock(wraps=real_logger)
# Calls real method but records calls

The Iron Laws of Mocking

1. NEVER test mock behavior — Test real component, not that mock exists
2. NEVER mock without understanding — Know side effects before isolating
3. NEVER create incomplete mocks — Mirror real API structure completely

Anti-Pattern Red Flags

• Mock setup longer than test logic
• Assertions on *-mock test IDs
• Can't explain why mock is needed
• Mocking "just to be safe"

Full anti-pattern details: references/testing-anti-patterns.md

Test Quality Checklist

Test Naming

test_[unit]_[scenario]_[expected]

test_calculateDiscount_withExpiredCoupon_returnsZero
test_userRepository_findByEmail_whenNotFound_returnsNone
test_checkout_withEmptyCart_showsError

See references/testing-frameworks.md for framework-specific guidance.