Unit Testing

Create maintainable, readable, deterministic, and refactor-resilient unit tests.

Rules

1. Test Behavior, Not Implementation
   • Assert only public API outputs and observable side effects
   • Do not test:
     • Private fields or state
     • Private methods
     • Internal functions or Python dunder methods (test only via public APIs as needed)
     • Internal call sequences
     • Intermediate steps outside the public contract
   • Tests must stay valid after refactoring if behavior is unchanged
2. Apply AAA (Arrange, Act, Assert) Pattern
   • Each test must include:
     • Arrange: set up inputs, fixtures, mocks, or stubs
     • Act: invoke the code under test
     • Assert: verify expected behavior
   • Visually separate these sections with blank lines (or comments, if necessary)
3. Total Isolation
   • Unit tests must be:
     • Order-independent
     • Runnable individually
     • Free from shared mutable state
   • Avoid external systems (database, network, filesystem, environment, system clock)
   • Mock or stub all external dependencies, including time
4. No Logic in Test Code
   • For multiple cases, use parameterized features in your test framework
   • Test code should be declarative. Avoid:
     • if/else statements
     • Loops (use parameterized tests instead)
     • Complex calculations
     • Branching assertions
5. Hardcode Expected Values
   • Do not compute expected values in tests (except trivial helpers)
   • Use explicit literals in assertions for clarity
   • Magic numbers and strings are allowed if they improve readability

General advice

• Never modify the code under test just to adapt to new or changed tests
• Inject clocks, randomness, and I/O boundaries as dependencies. Avoid reliance on real time or external systems. Use explicit seeding and deterministic sources for randomness testing
• Assert only outputs, not domain logic within tests
• Prefer built-in fixtures/utilities from test libraries
• Code under test must not detect it is being tested; tests should not depend on debug/test hooks
• Use descriptive names reflecting test intent
• Each test should verify a single logical behavior. Multiple assertions are fine if for one behavior; otherwise, split tests
• Cover error cases, including null or missing inputs, invalid formats, boundaries, empty collections, negative values, exceptions, and failures (with mocks)
• Unit tests must complete in seconds and avoid dependencies on time or external state; treat flaky tests as bugs