test-patterns

Use unittest.mock.Mock to replace external dependencies and inject them via constructor parameters. Verify interactions with assert_called_once_with, and use pytest fixtures for shared setup like database sessions and test users.

A stub provides canned return values without verification, a mock verifies it was called with specific arguments, and a spy records all calls for later inspection. Choose based on whether you need to verify interactions or just control return values.

Use Factory Boy to define factory classes with Faker-generated attributes, sequences for unique values, and traits for state variations like admin users. Call UserFactory() for defaults or pass overrides for specific scenarios.

Yes, the patterns cover TypeScript with Jest/Vitest using vi.fn() mocks and faker-based factories, plus Ruby with RSpec and FactoryBot traits. The core principles like AAA structure and test independence apply across all three languages.

High line coverage can hide untested branches and missing edge cases. Use branch coverage, boundary testing, and mutation testing with tools like mutmut or Stryker to verify tests actually catch bugs rather than just executing lines.

Avoid mocking everything, as tests then only verify mock configuration rather than real behavior. Mock only external services like email or payment APIs, and use real objects for domain logic to keep tests meaningful.