Why this chapter matters
Tests protect production workflows by detecting breaks early and preserving bug fixes over time.
What you will learn
- Write unit tests for deterministic transformation functions.
- Use fixtures to share setup and keep tests readable.
- Convert production bugs into regression tests.
Lessons in this chapter
- Core pytest workflowCreate discoverable test files with readable assertions.
- Fixtures and test dataReuse setup logic without hidden coupling.
- Debugging failing testsIsolate root causes with minimal repro cases.
- Guide: testing and debuggingHarden script reliability through tests and diagnostics. Read the full guide →
Study task
Write tests for parse_amount and normalize_channel, then add a regression test for one malformed real-world input.
Chapter checkpoint
Why should you add a test for a bug before or with the fix?
It creates a regression guard so the same failure is caught automatically in future changes.