How do you approach code quality and testing in Python projects?

Give a layered answer, and avoid quoting a coverage percentage as if it were the goal.

Automated tooling first, because conventions that are not enforced decay:

• Formatting — black or ruff format, so style is never a review topic.
• Linting — ruff or flake8 for the errors a human reader misses.
• Type checking — mypy or pyright. Type hints on function boundaries catch a genuine class of bug and double as documentation.
• All of it in CI, so it cannot be skipped.

Testing, in order of value per line:

• Unit tests on business logic — fast, no I/O, and where most real bugs live.
• Integration tests on the seams — the database layer, the external API client.
• A few end-to-end tests on the flows that must never break.

Note: Mention pytest fixtures for setup, parametrize for table-driven cases, and mocking only at the boundary. Over-mocking produces tests that pass while the code is broken, which is worse than having no tests.