TDD, Chicago School — also called Classicist TDD or the Detroit School — is a style of test-driven development associated with Kent Beck and Martin Fowler. It verifies the state of objects after operations, uses real objects wherever possible and mocks only external dependencies, and builds inside-out from the core domain logic. Design emerges through refactoring within the Red-Green-Refactor cycle.

TDD, Chicago School

Details
Also known as

Classicist TDD, Detroit School

Core Concepts:

State-based testing

Verify the state of objects after operations

Minimal mocking

Use real objects whenever possible; mock only external dependencies

Inside-out development

Start with core domain logic and build outward

Simplicity focus

Emergent design through refactoring

Red-Green-Refactor

The fundamental TDD cycle

YAGNI

You Aren’t Gonna Need It - avoid premature abstraction

Key Proponents

Kent Beck, Martin Fowler

When to Use:

  • Domain-driven design projects

  • When business logic is central

  • Smaller, cohesive modules