Learn FastAPI

Install FastAPI, write a five-line app and serve JSON over HTTP with the fastapi dev command.

Map URL paths and HTTP methods to Python functions with @app.get, @app.post and the other method decorators.

Every app serves an OpenAPI schema plus Swagger UI and ReDoc pages generated from your code.

Capture parts of the URL like /items/{item_id} as typed function arguments that FastAPI converts and validates.

Function parameters that aren't in the path become query parameters like ?skip=0&limit=10: typed, optional or required.

Declare a Pydantic BaseModel parameter and FastAPI reads, validates and parses the JSON request body into it.

Add constraints like max_length, ge/le and regex patterns to parameters and model fields with Annotated metadata.

Use return type annotations or response_model to shape, filter and document what an endpoint sends back.

Raise HTTPException for expected failures, and register exception handlers that turn your own exceptions into responses.

Accept uploads with UploadFile, send files with FileResponse, and stream generated data with StreamingResponse.

Declare shared logic such as auth, pagination or DB sessions as functions, and let FastAPI call them and inject the results.

Split a growing app into one router per feature, and include them with prefixes, tags and shared dependencies.

Load configuration from environment variables and .env files into a typed, validated Settings class.

Use yield in dependencies for setup and teardown, handle exceptions in them, and control when teardown runs with scope.

Define tables as SQLModel classes, get a session per request through a dependency, and run CRUD queries.

Issue signed JWT access tokens from a /token endpoint and protect routes with an OAuth2PasswordBearer dependency.

Run code around every request with middleware, and let browser front ends on other origins call your API with CORSMiddleware.

Schedule small jobs, like sending an email, to run after the response is sent, using the BackgroundTasks parameter.

How FastAPI runs async endpoints on the event loop and sync ones in a threadpool, and how to choose between them.

Create shared resources such as connection pools, HTTP clients or ML models once per process with a lifespan context manager.

Hold long-lived two-way connections with @app.websocket, handle disconnects, and know the limits of in-process broadcasting.

Call your app in-process with TestClient, swap dependencies with dependency_overrides, and assert on responses.

Document extra responses, examples and tags, control operation ids for client generation, and post-process the schema.

Run FastAPI in production with fastapi run or Uvicorn, choose worker counts, sit behind a proxy and package it in Docker.

Size workers, threads and connection pools together, set timeouts, and instrument logs, metrics and traces per route.

How an ASGI server calls your app with scope, receive and send, and how FastAPI layers its middleware stack and router on Starlette.

FastAPI builds a dependency tree per route at startup and solves it per request: sequentially, cached by callable, errors aggregated.

How Pydantic v2's Rust core validates and serializes, how FastAPI uses it, and how to keep hot paths fast.