FastAPI lifespan event for startup and shutdown tasks

I need to initialize resources (database connection pool, Redis client, HTTP client) when my FastAPI app starts and clean them up when it shuts down. The old @app.on_event('startup') pattern is deprecated in newer FastAPI versions.

Use the lifespan context manager pattern (FastAPI 0.93+):

from contextlib import asynccontextmanager
from fastapi import FastAPI
import httpx

@asynccontextmanager
async def lifespan(app: FastAPI):
    # Startup: initialize resources
    app.state.http_client = httpx.AsyncClient(timeout=10.0)
    app.state.redis = await create_redis_connection()
    yield
    # Shutdown: close resources
    await app.state.http_client.aclose()
    await app.state.redis.aclose()

app = FastAPI(lifespan=lifespan)

Access in dependencies:

from fastapi import Request

def get_http_client(request: Request) -> httpx.AsyncClient:
    return request.app.state.http_client

Key points: - Everything before yield runs at startup, after yield at shutdown - Resources on app.state are accessible from any route via request.app.state - Lifespan replaces both @app.on_event('startup') and @app.on_event('shutdown') - FastAPI wraps the lifespan in an asynccontextmanager automatically