FastAPI

FastAPI is a modern Python web framework for building HTTP APIs with automatic request validation, OpenAPI documentation generation, and async/await support. It uses Python type hints as the source of truth: declare a function parameter as `user_id: int` and FastAPI parses, validates, and documents that parameter automatically — no separate schema definition.

Under the hood FastAPI is built on Starlette (an ASGI framework) and Pydantic (a runtime type-validation library). The ASGI foundation means it can handle async I/O — slow database queries, external API calls, file uploads — without blocking the event loop, which is why it consistently benchmarks among the fastest Python frameworks. The Pydantic integration means request bodies and responses are validated and serialized by the same models you'd use in business logic.

FastAPI is a good fit for standalone API services, microservices, or any project where you want OpenAPI-driven docs and async I/O. For projects already on Django, the trade-off is steeper — Django + DRF gives you the ORM, admin, and migrations that FastAPI leaves to you to assemble.