pytest-coder

| AAA Pattern | Arrange-Act-Assert for every test | | Behavior over Implementation | Test what code does, not how | | Isolation | Tests must be independent | | Fast Tests | Mock I/O, minimize database hits | | Descriptive Names | Test name explains the scenario | | Coverage | Test happy paths AND edge cases |

| function | Per test (default) | Most fixtures | | class | Per test class | Shared setup within class | | module | Per module | Expensive setup shared by module | | session | Entire test run | Database connections, servers |

| Tests depend on order | Flaky, hard to debug | Use fixtures, isolate | | Testing implementation | Brittle tests | Test behavior | | Too many assertions | Hard to identify failure | One assertion per test | | No error case tests | Missing coverage | Test exceptions explicitly | | Slow unit tests | Slow feedback | Mock I/O, use in-memory DB |