## Metadata * URL: [https://github.com/testdouble/contributing-tests/wiki/London-school-TDD](https://github.com/testdouble/contributing-tests/wiki/London-school-TDD) * Author: [[testdouble|Testdouble]] ## Highlights * This school of TDD [[Mockist TDD (or London School)|Mockist Tdd Or London School]] * It describes an entirely different approach to test-driven development from its predecessor, * the GOOS's authors, Nat Pryce and Steve Freeman, * consistently arrive at clean, minimal designs of small, focused units of code * a London-school unit test suite only provides regression safety of each unit in isolation, it's typically necessary to have a second, minimal suite of end-to-end tests that validate the system works when everything is wired up and integrated. * London-school TDD encourages programmers to use external constraints as a starting point * Typically, systems designed with London-school TDD demonstrate greater emphasis on design patterns, object-oriented design, and domain-driven design. The greatest risk posed by this approach is YAGNI resulting from APIs that wind up being a poor fit for the integrated code that would actually call it (e.g. mismatched parameters, differing return types, etc.) * gateway drug to functional programming, because it encourages separating code with side effects and maximizing pure functions. * An interesting consequence of practicing London-school TDD is that the design of each unit is not only heavily-influenced by its test and the overall testing workflow, but that the shape of each unit will typically resemble one of a handful of archetypes. Incidentally, each of these archetypical object patterns have some favorable design characteristics * mocked-out dependencies (like Collaborator and Signaler) require significantly more effort to refactor significantly.