Modern async API development with FastAPI.
\nfrom fastapi import FastAPI\nfrom contextlib import asynccontextmanager\n\n@asynccontextmanager\nasync def lifespan(app: FastAPI):\n \"\"\"Application lifespan - startup and shutdown.\"\"\"\n # Startup\n app.state.db = await create_db_pool()\n yield\n # Shutdown\n await app.state.db.close()\n\napp = FastAPI(\n title=\"My API\",\n version=\"1.0.0\",\n lifespan=lifespan,\n)\n\n@app.get(\"/\")\nasync def root():\n return {\"message\": \"Hello World\"}\nfrom pydantic import BaseModel, Field, EmailStr\nfrom datetime import datetime\n\nclass UserCreate(BaseModel):\n \"\"\"Request model with validation.\"\"\"\n name: str = Field(..., min_length=1, max_length=100)\n email: EmailStr\n age: int = Field(..., ge=0, le=150)\n\nclass UserResponse(BaseModel):\n \"\"\"Response model.\"\"\"\n id: int\n name: str\n email: EmailStr\n created_at: datetime\n\n model_config = {\"from_attributes\": True} # Enable ORM mode\n\n@app.post(\"/users\", response_model=UserResponse, status_code=201)\nasync def create_user(user: UserCreate):\n db_user = await create_user_in_db(user)\n return db_user\nfrom fastapi import Query, Path\nfrom typing import Annotated\n\n@app.get(\"/users/{user_id}\")\nasync def get_user(\n user_id: Annotated[int, Path(..., ge=1, description=\"User ID\")],\n):\n return await fetch_user(user_id)\n\n@app.get(\"/users\")\nasync def list_users(\n skip: Annotated[int, Query(ge=0)] = 0,\n limit: Annotated[int, Query(ge=1, le=100)] = 10,\n search: str | None = None,\n):\n return await fetch_users(skip=skip, limit=limit, search=search)\nfrom fastapi import Depends\nfrom typing import Annotated\n\nasync def get_db():\n \"\"\"Database session dependency.\"\"\"\n async with async_session() as session:\n yield session\n\nasync def get_current_user(\n token: Annotated[str, Depends(oauth2_scheme)],\n db: Annotated[AsyncSession, Depends(get_db)],\n) -> User:\n \"\"\"Authenticate and return current user.\"\"\"\n user = await authenticate_token(db, token)\n if not user:\n raise HTTPException(status_code=401, detail=\"Invalid token\")\n return user\n\n# Annotated types for reuse\nDB = Annotated[AsyncSession, Depends(get_db)]\nCurrentUser = Annotated[User, Depends(get_current_user)]\n\n@app.get(\"/me\")\nasync def get_me(user: CurrentUser):\n return user\nfrom fastapi import HTTPException\nfrom fastapi.responses import JSONResponse\n\n# Built-in HTTP exceptions\n@app.get(\"/items/{item_id}\")\nasync def get_item(item_id: int):\n item = await fetch_item(item_id)\n if not item:\n raise HTTPException(status_code=404, detail=\"Item not found\")\n return item\n\n# Custom exception handler\nclass ItemNotFoundError(Exception):\n def __init__(self, item_id: int):\n self.item_id = item_id\n\n@app.exception_handler(ItemNotFoundError)\nasync def item_not_found_handler(request, exc: ItemNotFoundError):\n return JSONResponse(\n status_code=404,\n content={\"detail\": f\"Item {exc.item_id} not found\"},\n )\nfrom fastapi import APIRouter\n\n# users.py\nrouter = APIRouter(prefix=\"/users\", tags=[\"users\"])\n\n@router.get(\"/\")\nasync def list_users():\n return []\n\n@router.get(\"/{user_id}\")\nasync def get_user(user_id: int):\n return {\"id\": user_id}\n\n# main.py\nfrom app.routers import users, items\n\napp.include_router(users.router)\napp.include_router(items.router, prefix=\"/api/v1\")\n| Feature | \nUsage | \n
|---|---|
| Path param | \n@app.get(\"/items/{id}\") | \n
| Query param | \ndef f(q: str = None) | \n
| Body | \ndef f(item: ItemCreate) | \n
| Dependency | \nDepends(get_db) | \n
| Auth | \nDepends(get_current_user) | \n
| Response model | \nresponse_model=ItemResponse | \n
| Status code | \nstatus_code=201 | \n
./references/dependency-injection.md - Advanced DI patterns, scopes, caching./references/middleware-patterns.md - Middleware chains, CORS, error handling./references/validation-serialization.md - Pydantic v2 patterns, custom validators./references/background-tasks.md - Background tasks, async workers, scheduling./scripts/scaffold-api.sh - Generate API endpoint boilerplate./assets/fastapi-template.py - Production-ready FastAPI app skeletonPrerequisites:
\npython-typing-ops - Pydantic models and type hintspython-async-ops - Async endpoint patternsRelated Skills:
\npython-database-ops - SQLAlchemy integrationpython-observability-ops - Logging, metrics, tracing middlewarepython-pytest-ops - API testing with TestClient