pydantic

Dependency injection as a pattern, FastAPI's Depends as one concrete implementation. Sub-dependencies and per-request caching, yield-based cleanup, where Pydantic v2 validation runs, lifespan-scoped resources, BackgroundTasks vs real queues, and the override trick that makes the whole thing testable.

Python's type system has two audiences — static checkers and runtime frameworks. Generics, Protocol, TypedDict, ParamSpec, type narrowing, and runtime introspection, framed as the spec that Pydantic, FastAPI, and SQLAlchemy actually execute.