how do you properly reuse an httpx.AsyncClient within a FastAPI application?

I have a FastAPI application which, in several different occasions, needs to call external APIs. I use httpx.AsyncClient for these calls. The point is that I don't fully understand how I shoud use it.

From httpx' documentation I should use context managers,

async def foo():
    """"
    I need to call foo quite often from different 
    parts of my application
    """
    async with httpx.AsyncClient() as aclient:
        # make some http requests, e.g.,
        await aclient.get("http://example.it")

However, I understand that in this way a new client is spawned each time I call foo(), and is precisely what we want to avoid by using a client in the first place.

I suppose an alternative would be to have some global client defined somewhere, and just import it whenever I need it like so

aclient = httpx.AsyncClient()

async def bar():
    # make some http requests using the global aclient, e.g.,
    await aclient.get("http://example.it")

This second option looks somewhat fishy, though, as nobody is taking care of closing the session and the like.

So the question is: how do I properly (re)use httpx.AsyncClient() within a FastAPI application?

You can have a global client that is closed in the FastApi shutdown event.

import logging
from fastapi import FastAPI
import httpx

logging.basicConfig(level=logging.INFO, format="%(levelname)-9s %(asctime)s - %(name)s - %(message)s")
LOGGER = logging.getLogger(__name__)


class HTTPXClientWrapper:

    async_client = None

    def start(self):
        """ Instantiate the client. Call from the FastAPI startup hook."""
        self.async_client = httpx.AsyncClient()
        LOGGER.info(f'httpx AsyncClient instantiated. Id {id(self.async_client)}')

    async def stop(self):
        """ Gracefully shutdown. Call from FastAPI shutdown hook."""
        LOGGER.info(f'httpx async_client.is_closed(): {self.async_client.is_closed} - Now close it. Id (will be unchanged): {id(self.async_client)}')
        await self.async_client.aclose()
        LOGGER.info(f'httpx async_client.is_closed(): {self.async_client.is_closed}. Id (will be unchanged): {id(self.async_client)}')
        self.async_client = None
        LOGGER.info('httpx AsyncClient closed')

    def __call__(self):
        """ Calling the instantiated HTTPXClientWrapper returns the wrapped singleton."""
        # Ensure we don't use it if not started / running
        assert self.async_client is not None
        LOGGER.info(f'httpx async_client.is_closed(): {self.async_client.is_closed}. Id (will be unchanged): {id(self.async_client)}')
        return self.async_client


httpx_client_wrapper = HTTPXClientWrapper()
app = FastAPI()


@app.get('/test-call-external')
async def call_external_api(url: str = 'https://stackoverflow.com'):
    async_client = httpx_client_wrapper()
    res = await async_client.get(url)
    result = res.text
    return {
        'result': result,
        'status': res.status_code
    }


@app.on_event("startup")
async def startup_event():
    httpx_client_wrapper.start()


@app.on_event("shutdown")
async def shutdown_event():
    await httpx_client_wrapper.stop()


if __name__ == '__main__':
    import uvicorn
    LOGGER.info(f'starting...')
    uvicorn.run(f"{__name__}:app", host="127.0.0.1", port=8000)

Note - this answer was inspired by a similar answer I saw elsewhere a long time ago for aiohttp, I can't find the reference but thanks to whoever that was!

EDIT

I've added uvicorn bootstrapping in the example so that it's now fully functional. I've also added logging to show what's going on on startup and shutdown, and you can visit localhost:8000/docs to trigger the endpoint and see what happens (via the logs).

The reason for calling the start() method from the startup hook is that by the time the hook is called the eventloop has already started, so we know we will be instantiating the httpx client in an async context.

Also I was missing the async on the stop() method, and had a self.async_client = None instead of just async_client = None, so I have fixed those errors in the example.