FastAPI

FastAPI is a Python web framework for building APIs that turns standard type hints into request validation, serialization, and interactive documentation. Built on Starlette for the ASGI layer and Pydantic for data validation, it lets you declare path operations as plain typed functions and get automatic OpenAPI/JSON Schema generation, editor autocompletion, and Swagger UI/ReDoc docs for free.

Used in production by teams at Microsoft, Uber, and Netflix, FastAPI has grown into one of the most widely adopted Python web frameworks, prized for combining near-Node.js/Go performance with a developer experience built entirely around native Python typing rather than framework-specific decorators or config files.

What You Get

• A FastAPI application class that layers routing, dependency injection, middleware, and exception handling on top of Starlette’s ASGI core
• Automatic request validation and response serialization powered by Pydantic v2 models and Python type hints
• Auto-generated interactive API docs (Swagger UI at /docs, ReDoc at /redoc) derived directly from your route type hints
• A dependency-injection system (Depends) for sharing auth, DB sessions, and other request-scoped resources across endpoints
• First-class async support with async def endpoints, background tasks, and WebSocket routes
• An official fastapi-cli companion (fastapi dev / fastapi run) for local development and production serving via Uvicorn

Common Use Cases

• Building REST/JSON APIs for web and mobile backends with automatic request validation
• Serving machine-learning models behind a typed prediction endpoint with auto-generated docs
• Building internal microservices where OpenAPI schemas need to stay in sync with code automatically
• Rapid prototyping of API-first applications where interactive docs speed up frontend/backend collaboration

Under The Hood

Architecture: FastAPI’s FastAPI class (fastapi/applications.py) subclasses Starlette’s Starlette app, wiring in exception handlers, OpenAPI schema generation (fastapi/openapi/utils.py, fastapi/openapi/docs.py), and a dependency-injection system (fastapi/dependencies/) that resolves at request time. The heavy lifting happens in fastapi/routing.py, which inspects each endpoint function’s signature and Annotated type metadata to auto-generate request parsing, delegate validation to Pydantic, serialize responses, and register interactive docs routes (/docs, /redoc) at app startup — all derived from the same type hints developers already write.

Tech Stack: A Python 3.10-3.14 package with a deliberately small core dependency surface — starlette>=0.46, pydantic>=2.9, typing-extensions, typing-inspection, and annotated-doc. The optional standard extra pulls in fastapi-cli, uvicorn[standard], httpx, jinja2, python-multipart, email-validator, pydantic-settings, and pydantic-extra-types. It builds with pdm-backend and locks dependencies via uv.lock.

Code Quality: The repo ships 584+ test files under tests/ covering nearly every feature combination — parameter styles, response models, dependency overrides, security schemes, WebSockets, background tasks — alongside strict mypy typing (strict = true with the pydantic.mypy plugin), ruff linting, and pre-commit hooks. A CI test workflow and coverage badge back every change.

API Design: The public API is built entirely around plain Python type hints and Annotated[...] metadata (with Doc() annotations for editor tooltips), so declaring a path operation is just a decorated, typed function — FastAPI infers request parsing, validation, serialization, and the OpenAPI schema without extra boilerplate. Getting started is a ~6-line main.py plus fastapi dev, and a 461-file docs_src example corpus backs a docs-driven learning experience widely regarded as best-in-class for Python web frameworks.