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name: testing-patterns description: "Cross-language testing strategies and patterns. Triggers on: test pyramid, unit test, integration test, e2e test, TDD, BDD, test coverage, mocking strategy, test doubles, test isolation." compatibility: "Language-agnostic patterns. Framework-specific details in references." allowed-tools: "Read Write Bash"

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