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

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