FastAPI Fundamentals

FastAPI is a Python framework for building HTTP APIs. You declare the paths your application serves and write a Python function for each operation. Type annotations describe input and output data. FastAPI uses those declarations to parse requests, validate data, and generate an OpenAPI description.

The useful mental model is a contract at the HTTP boundary. A client sends a method, path, headers, query values, and sometimes a body. A path operation selects a function. FastAPI obtains declared inputs, runs dependencies, validates data, and serializes the response. Your function still owns the application decision.

What FastAPI gives you