Unit Testing Patterns

Focused patterns for writing isolated, fast, maintainable unit tests. Covers test structure (AAA), parametrization, fixture management, HTTP mocking (MSW/VCR), and test data generation with factories.

Each category has individual rule files in rules/ loaded on-demand, plus reference material, checklists, and scaffolding scripts.

Core Principles (ALWAYS apply)

1. AAA structure: Every test MUST follow Arrange-Act-Assert. Use // Arrange, // Act, // Assert comments for clarity.
2. Parametrize, don't duplicate: Use test.each (TypeScript) or @pytest.mark.parametrize (Python) when testing multiple inputs. Never copy-paste the same test body with different values.
3. Fixture scoping matters: Use scope="function" (default) for mutable data. Use scope="module" or scope="session" ONLY for expensive read-only resources (DB engines, ML models). Mutable data with shared scope causes flaky tests.
4. Speed target: Each unit test should run under 100ms. If it's slower, you're likely hitting I/O — mock it.
5. Mock at the network level: Use MSW (TypeScript) or VCR.py (Python) to intercept HTTP at the network layer. Never mock fetch/axios/requests directly.

Quick Reference