Testing Patterns Skill | Agent Skills

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

Agent Skills: Testing Patterns

Skill Files