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FastAPI — Modern Async Web Framework

High-performance Python web framework built on Starlette and Pydantic. Automatic OpenAPI docs, type-safe request validation, async-first design, and dependency injection out of the box.

Installation

uv add fastapi
uv add "uvicorn[standard]"     # ASGI server with uvloop & httptools
uv add pydantic-settings        # env-based configuration
uv add python-multipart         # form/file uploads
uv add python-jose[cryptography] # JWT tokens
uv add passlib[bcrypt]          # password hashing

When to Use What

Feature Best for
Path operations (@app.get) Standard REST endpoints
Depends() Shared logic: auth, DB sessions, config
BackgroundTasks Quick fire-and-forget work (emails, logs)
WebSocket Real-time bidirectional communication
APIRouter Splitting routes into modules
Lifespan events App-level startup/shutdown (DB pools, caches)

Section Map

File Topics
01 App & Routing App config, path/query params, request/response models, status codes
02 Dependencies & Middleware Depends, yield deps, middleware, lifespan, CORS
03 Database Integration Async SQLAlchemy, session management, CRUD patterns, repositories
04 Auth & Security OAuth2, JWT, password hashing, role-based access, API keys
05 Testing TestClient, async tests, dependency overrides, fixtures, coverage
06 Production Patterns Project structure, background tasks, WebSocket, error handling, deploy
07 ASGI, Uvicorn, Starlette ASGI vs WSGI, Uvicorn role, FastAPI-Starlette stack, when to consider Litestar

Quick Start

from fastapi import FastAPI

app = FastAPI(title="My API", version="0.1.0")


@app.get("/")
async def root():
    return {"message": "Hello, FastAPI"}


@app.get("/items/{item_id}")
async def read_item(item_id: int, q: str | None = None):
    return {"item_id": item_id, "q": q}

uvicorn main:app --reload
URL Purpose
http://127.0.0.1:8000 API root
http://127.0.0.1:8000/docs Swagger UI (interactive)
http://127.0.0.1:8000/redoc ReDoc (read-only docs)
http://127.0.0.1:8000/openapi.json Raw OpenAPI schema

Response Model Cheat Sheet

Return type What happens
dict / list JSON-serialized as-is
Pydantic model Validated + serialized via model_dump()
Response / JSONResponse Full control over headers, status, body
StreamingResponse Stream large files or SSE
FileResponse Serve static files

Quick Rules

  1. Use async def — for I/O-bound endpoints; sync def runs in a threadpool.
  2. Type everything — path params, query params, body models get auto-validated.
  3. Use Depends() — never instantiate shared resources inside endpoints.
  4. Keep routes thin — move business logic to service layer.
  5. Use APIRouter — one router per domain/module.
  6. Use Pydantic models for I/O — separate request and response schemas.
  7. Lifespan over events — use lifespan context manager, not deprecated on_event.

See also