Testing Python Systems

Testing provides evidence about behavior under stated conditions. It does not prove the absence of bugs, so confidence should come from several layers with different failure-detection strengths.

Map

• Unit Tests — examples, parametrization, fixtures, test doubles, and isolation boundaries.
• Test Strategy — integration, system, regression, performance, CI, and flaky-test policy.
• Errors and Exceptions — defining and testing failure contracts in Python.

Working loop

1. State the observable contract and important failure modes.
2. Put most deterministic domain cases in fast focused tests.
3. Add boundary tests where components, storage, networks, processes, or users interact.
4. Run the smallest relevant set during development and the required suite in a clean CI environment.
5. Treat a discovered bug as a missing model or missing check, then preserve a regression test when it adds lasting value.

Coverage measures which code executed, not whether its behavior was meaningfully checked. Use coverage to find blind spots rather than as a substitute for test design.

Source

• unittest — Unit testing framework
• pytest documentation