FastAPI lifespan events — startup and shutdown with async context manager

The old @app.on_event('startup') and @app.on_event('shutdown') decorators are deprecated. Initializing DB connection pools, loading ML models, or starting background services needs a clean async lifecycle.

Use the lifespan context manager parameter on FastAPI(). Everything before yield runs on startup; everything after yield runs on shutdown. Store shared resources in the app.state dict.

from contextlib import asynccontextmanager
from fastapi import FastAPI
import httpx

@asynccontextmanager
async def lifespan(app: FastAPI):
    # Startup
    app.state.http_client = httpx.AsyncClient()
    app.state.db_pool = await create_db_pool()
    print('App started')
    yield
    # Shutdown
    await app.state.http_client.aclose()
    await app.state.db_pool.close()
    print('App stopped')

app = FastAPI(lifespan=lifespan)

@app.get('/status')
async def status(request: Request):
    # Access shared resource
    client = request.app.state.http_client
    resp = await client.get('https://api.example.com/health')
    return resp.json()

The lifespan approach uses a single async context manager, making setup and teardown co-located and easier to reason about. It integrates cleanly with TestClient (the lifespan runs when used as a context manager in tests).

FastAPI apps that need to initialize or tear down resources at startup/shutdown