Pydantic

Pydantic v2 brings Rust-powered validation that's 5-50x faster than v1, making it the standard for FastAPI request/response validation and settings management. The BaseModel approach with type hints feels natural in modern Python, and field validators handle everything from email formats to custom business logic without boilerplate. The v2 migration changes method names (dict() becomes model_dump(), parse_obj() becomes model_validate()) but the performance jump and better error messages make it worth updating. If you're building APIs, parsing config files, or just want runtime type safety without writing manual checks, this is the de facto choice. The progressive disclosure here covers field constraints, custom types, and the validator decorators you'll actually use day-to-day.