What is the scope of a FastAPI dependency, and how do you share state across requests?

By default FastAPI dependencies are request-scoped — a new instance is created per request. For shared state (connection pools, caches, ML models) use module-level variables initialised in the lifespan context manager.

from contextlib import asynccontextmanager
from fastapi import FastAPI, Depends
from typing import Annotated
import asyncpg

# Module-level: shared across all requests
db_pool: asyncpg.Pool | None = None
ml_model = None

@asynccontextmanager
async def lifespan(app: FastAPI):
    global db_pool, ml_model

    # Create once at startup — shared by all workers in same process
    db_pool = await asyncpg.create_pool("postgresql://localhost/db", min_size=5)
    # ml_model = load_model("model.pkl")  # load once

    yield  # requests are served here

    await db_pool.close()

app = FastAPI(lifespan=lifespan)

# Request-scoped dependency — new connection per request from the pool
async def get_conn():
    async with db_pool.acquire() as conn:  # borrows from the shared pool
        yield conn                          # returned to pool after request

Conn = Annotated[asyncpg.Connection, Depends(get_conn)]

@app.get("/users")
async def list_users(conn: Conn):
    rows = await conn.fetch("SELECT id, username FROM users")
    return [dict(r) for r in rows]

# Singleton dependency — same instance for all requests (use_cache=True, default)
class AppState:
    def __init__(self):
        self.request_count = 0  # not thread-safe — example only

app_state = AppState()

def get_app_state() -> AppState:
    return app_state  # same object every request

@app.get("/stats")
def stats(state: Annotated[AppState, Depends(get_app_state)]):
    state.request_count += 1
    return {"total_requests": state.request_count}

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