**Approach:** pytest; one test per behavior. Mock external deps (DB, API). Fixtures for setup. >80% coverage for critical paths. Edge cases. Run in CI. **Pattern:** AAA—Arrange, Act, Assert. **Why:** Regression prevention; refactor safety. **Production:** Fast tests; isolate I/O....
The complete answer continues with detailed implementation patterns, architectural trade-offs, and production-grade considerations. It covers performance optimization strategies, common pitfalls to avoid, and real-world examples from companies like Thoughtworks. The answer also includes follow-up discussion points that interviewers commonly explore.
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