Should I use the httpx async client anyway?

[idan3]

I have a fastapi server and I want to use httpx client when it requests another web server.
I'm not using fastapi with async def but maybe I'll change that in the future.
My question is: Is there a reason not to always use the async httpx client and use the asynchronous client?

[ketpatil77]

For your current def endpoints, use httpx.Client. If you change an endpoint to async def, use httpx.AsyncClient and await its requests.

FastAPI runs ordinary def endpoints in a worker thread. Calling an async HTTPX method there only creates a coroutine unless you arrange to execute it. Conversely, calling a synchronous client directly inside async def blocks that endpoint's event loop while waiting for network I/O.

Reuse the chosen client across requests and close it during application shutdown. Avoid wrapping every request in asyncio.run() or creating a new client per call: that defeats useful connection reuse and complicates event-loop ownership.

So there is no need to migrate working synchronous endpoints solely to use HTTPX. Choose the client that matches the calling code; migrate to async when concurrent outbound I/O makes that useful.

References: HTTPX async support and FastAPI's handling of def versus async def.

I checked both endpoint styles with shared clients and mock upstream responses on FastAPI 0.162.0 / HTTPX 0.28.1. This checks client integration, not a performance comparison.