Phase 4: Mutation Testing & Behavioral Verification · 50 min · Python · pytest · mutmut

Why Coverage Lies

92% coverage. 100% bugs. Coverage measures execution, not verification.

Real-world application: Engineers who can explain why 92% code coverage doesn't mean 92% tested demonstrate understanding that coverage measures execution, not verification.

What you will learn

Explain why code coverage measures execution, not verification
Understand the 92% coverage with deduplication bug case study
Define mutation score: killed mutants / total mutants
Identify the 40% untested behavioral paths hidden behind green coverage

Introduction

Why Coverage Lies

Objective

Run the coverage vs mutation score comparison on the AI-generated deduplication function. Observe how 100% line coverage can coexist with a low mutation score, revealing the gap between executing code and verifying behavior.

92% coverage. 100% bugs. Coverage measures execution, not verification.

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You've read the first 2 sections. The rest of this lesson covers The Problem, The Concept, Build, Check Yourself, Key Terms & Next — plus a hands-on lab, quiz, and project artifact.

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