How does SRP help with testing?

SRP dramatically improves testability by ensuring that each class has a small, focused surface area to test. A class with a single responsibility has fewer states to account for, fewer dependencies to mock, and a simpler test setup. Compare testing a 500-line God Class (complex setup, many dependencies, unclear test boundaries) to testing a 50-line PriceCalculator class (takes inputs, returns a price, pure logic, no mocks needed). When a class follows SRP, its unit tests are precise — a failing test clearly indicates which responsibility is broken. Additionally, SRP-compliant classes tend to be easier to make deterministic (no side effects mixed with pure logic), which is a prerequisite for reliable automated tests.