Testing Patterns

Universal testing strategies and patterns applicable across languages.

The Test Pyramid

    /\
   /  \     E2E Tests (few, slow, expensive)
  /    \    - Full system tests
 /------\   - Real browser/API calls
/        \

/ Integ \ Integration Tests (some) / Tests \ - Service boundaries /--------------\ - Database, APIs /
/ Unit Tests \ Unit Tests (many, fast, cheap) ------------------ - Single function/class - Mocked dependencies

Test Types

Unit Tests

Scope: Single function/method/class Speed: Milliseconds Dependencies: All mocked When: Every code change Coverage: 80%+ of codebase

Integration Tests

Scope: Multiple components together Speed: Seconds Dependencies: Real databases, mocked external APIs When: PR/merge, critical paths Coverage: Key integration points

End-to-End Tests

Scope: Full user journey Speed: Minutes Dependencies: Real system (or staging) When: Pre-deploy, nightly Coverage: Critical user flows only

Test Naming Convention

test_<unit><scenario><expected>

Examples:

• test_calculate_total_with_discount_returns_reduced_price
• test_user_login_with_invalid_password_returns_401
• test_order_submit_when_out_of_stock_raises_error

Arrange-Act-Assert (AAA)

def test_calculate_discount(): # Arrange - Set up test data and dependencies cart = Cart() cart.add_item(Item(price=100)) discount = Discount(percent=10)

# Act - Execute the code under test
total = cart.calculate_total(discount)

# Assert - Verify the results
assert total == 90

Test Doubles

Type Purpose Example

Stub Returns canned data stub.get_user.returns(fake_user)

Mock Verifies interactions mock.send_email.assert_called_once()

Spy Records calls, uses real impl spy.on(service, 'save')

Fake Working simplified impl FakeDatabase() instead of real DB

Dummy Placeholder, never used null object for required param

Test Isolation Strategies

Database Isolation

Option 1: Transaction rollback (fast)

• Start transaction before test
• Rollback after test

Option 2: Truncate tables (medium)

• Clear all data between tests

Option 3: Separate database (slow)

• Each test gets fresh database

External Service Isolation

Option 1: Mock at boundary

• Replace HTTP client with mock

Option 2: Fake server

• WireMock, MSW, VCR cassettes

Option 3: Contract testing

• Pact, consumer-driven contracts

What to Test

MUST Test

• Business logic and calculations

• Input validation and error handling

• Security-sensitive code (auth, permissions)

• Edge cases and boundary conditions

SHOULD Test

• Integration points (DB, APIs)

• State transitions

• Configuration handling

AVOID Testing

• Framework internals

• Third-party library behavior

• Simple getters/setters

• Private implementation details

Test Quality Checklist

• Tests are independent (no order dependency)

• Tests are deterministic (no flaky tests)

• Tests are fast (unit < 100ms, integration < 5s)

• Tests have clear names describing behavior

• Tests cover happy path AND error cases

• Tests don't repeat production logic

• Mocks are minimal (only external boundaries)

Additional Resources

• ./references/tdd-workflow.md

• Test-Driven Development cycle

• ./references/mocking-strategies.md

• When and how to mock

• ./references/test-data-patterns.md

• Fixtures, factories, builders

• ./references/ci-testing.md

• Testing in CI/CD pipelines

Scripts

• ./scripts/coverage-check.sh
• Run coverage and fail if below threshold