FastAPI describes itself in its official documentation as “a modern, fast (high-performance) web framework for building APIs with Python based on standard Python type hints.” It was created by Sebastian Ramirez (tiangolo) and first released in 2018, and it quickly became one of the most popular ways to build web APIs in Python. The project README compresses its pitch into a single line: “FastAPI framework, high performance, easy to learn, fast to code, ready for production.”
The framework’s central idea is to use ordinary Python type hints as the single source of truth. A function parameter annotated as an int is parsed, validated, and documented automatically; a request body declared as a Pydantic model is validated and serialized without extra boilerplate. The documentation credits this approach with concrete productivity claims, including increasing development speed “by about 200% to 300%” and reducing “about 40% of human (developer) induced errors,” while also delivering strong editor autocompletion because the types are real.
FastAPI is built on the shoulders of two libraries: as the docs state, it uses “Starlette for the web parts” and “Pydantic for the data parts.” Starlette provides the underlying ASGI toolkit, routing, and async support, while Pydantic provides the data validation and parsing layer. Unlike the older synchronous WSGI frameworks, FastAPI is built on ASGI and offers first-class support for async and await, allowing high-concurrency I/O-bound workloads; the documentation claims performance “on par with NodeJS and Go.”
A signature feature is automatic, interactive API documentation. Because the type hints and Pydantic models fully describe each endpoint, FastAPI generates an OpenAPI schema at runtime and serves two interactive documentation UIs out of the box: Swagger UI at /docs and ReDoc at /redoc. The framework is, in its own words, “based on (and fully compatible with) the open standards for APIs: OpenAPI (previously known as Swagger) and JSON Schema,” so the documentation, validation, and client tooling all derive from the same machine-readable contract.
FastAPI represents a generational shift in Python web frameworks. Where Flask and Django were designed in the WSGI era around synchronous request handling and manual serialization, FastAPI arrived in the era of type hints, async I/O, and JSON-first REST APIs, and made the type system itself do the work of validation and documentation. It became a default choice for building modern Python backends and machine-learning serving layers, and a model that later frameworks emulated.