How to Efficiently Initialize and Reuse a Global Object Across FastAPI Endpoints?

How to Initialize a Global Object or Variable and Reuse It in Every FastAPI Endpoint?

Background

Consider a custom class designed for sending notifications, whose initialization involves establishing a connection to a notification server, a time-consuming process. This class is utilized within a background task in FastAPI to avoid delaying the endpoint response. However, the current approach has limitations:

file1.py:
noticlient = NotificationClient()

@app.post("/{data}")
def send_msg(somemsg: str, background_tasks: BackgroundTasks):
    result = add_some_tasks(data, background_tasks, noticlient)
    return result

file2.py:
def add_some_tasks(data, background_tasks: BackgroundTasks, noticlient):
    background_tasks.add_task(noticlient.send, param1, param2)
    result = some_operation
    return result

The global NotificationClient initialization in file1.py results in multiple redundant initializations every time a request is received, which is inefficient.

Approaches

Option 1: Utilizing app.state

FastAPI allows you to store arbitrary state using app.state. You can initialize the NotificationClient object and add it to app.state during FastAPI startup or shutdown using a dependency lifecycle function such as lifespan.

from fastapi import FastAPI, Request
from contextlib import asynccontextmanager


@asynccontextmanager
async def lifespan(app: FastAPI):
    ''' Run at startup
        Initialise the Client and add it to app.state
    '''
    app.state.n_client = NotificationClient()
    yield
    ''' Run on shutdown
        Close the connection
        Clear variables and release the resources
    '''
    app.state.n_client.close()


app = FastAPI(lifespan=lifespan)


@app.get('/')
async def main(request: Request):
    n_client = request.app.state.n_client
    # ...

Option 2: Utilizing Starlette lifespan

Starlette's lifespan handler allows you to define state objects accessible within endpoints via request.state.

from fastapi import FastAPI, Request
from contextlib import asynccontextmanager


@asynccontextmanager
async def lifespan(app: FastAPI):
    ''' Run at startup
        Initialise the Client and add it to request.state
    '''
    n_client = NotificationClient()
    yield {'n_client': n_client}
    ''' Run on shutdown
        Close the connection
        Clear variables and release the resources
    '''
    n_client.close()


app = FastAPI(lifespan=lifespan)


@app.get('/')
async def main(request: Request):
    n_client = request.state.n_client
    # ...

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