London school TDD

London-school TDD originated from the Extreme Programming community in London. It differs from Detroit-school TDD by using external constraints as a starting point, producing cleaner designs with small, focused units. It encourages separating side effects from pure functions, leaning toward functional programming. Units often fall into archetypes. The approach influences design by mocking dependencies, exposing awkward APIs early. A criticism is higher refactoring cost for units with test doubles. It typically requires a separate end-to-end test suite.