Websocket Disconnect Potentially Broken

[econeale]

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Example Code

import asyncio
import logging

import fastapi
import uvicorn

APP = fastapi.FastAPI()


@APP.websocket("/ws")
async def ws_telemetry(websocket: fastapi.WebSocket) -> None:
    await websocket.accept()

    try:
        while True:
            print("Still Looping...")
            await asyncio.sleep(1)

    except fastapi.WebSocketDisconnect:
        logging.info("Websocket disconnected")


if __name__ == "__main__":
    uvicorn.run("websocket_duplicator:APP", host="127.0.0.1", port=8080, reload=True)

Description

When running FastAPI with a websocket, the WebSocketDisconnect error isn't raised per the documentation. Using the given reproducer code and Postman's websocket function, I get the following output:

>$ python3 ./websocket_duplicator.py
INFO:     Will watch for changes in these directories: ['/home/neale/btbr-middleware']
INFO:     Uvicorn running on http://127.0.0.1:8080 (Press CTRL+C to quit)
INFO:     Started reloader process [3256733] using WatchFiles
INFO:     Started server process [3256735]
INFO:     Waiting for application startup.
INFO:     Application startup complete.
INFO:     ('127.0.0.1', 42336) - "WebSocket /ws" [accepted]
Still Looping...
INFO:     connection open
Still Looping...
Still Looping...
Still Looping...
INFO:     connection closed
Still Looping...
Still Looping...
Still Looping...
Still Looping...
Still Looping...
Still Looping...

It seems to me that this functionality has broken as similar code worked with FastAPI 0.95.2 however, I found a Starlette discussion which makes me think the behavior is, for some reason, intentional. My question is: is the WebSocketDisconnect exception supposed to be raised when a client disconnects or is the feature broken?

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.109.0

Pydantic Version

2.5.3

Python Version

3.10

Additional Context

starlette==0.35.1
uvicorn==0.26.0
websockets==12.0

[Kludex]

Your snippet is unrealistic. The WebSocketDisconnect only raises on websocket.receive().

Note: the next release of Uvicorn/Starlette will raise WebSocketDisconnect on send() as well.

[econeale]

I'm not sure what you mean by unrealistic. Sending status information from a server to a client seems like a pretty standard use case. For example, here's a function that worked with FastAPI 0.95.2 and no longer works in version 0.109.0. It seems silly to have to implement a separate heartbeat function in the application logic when the keepalive is already implemented in the websocket logic.

@APP.websocket("/get_serial_state")
async def get_serial_state(websocket: fastapi.WebSocket):
    await websocket.accept()
    to_send = {}
    try:
        while True:
            to_send["state"] = SERIAL_PROCESS_OBJ.serial_ok()
            await websocket.send_json(to_send)
            await asyncio.sleep(SOCKET_UPDATE_SECONDS)
    except fastapi.WebSocketDisconnect:
        pass
    except fastapi.WebSocketException:
        pass

[Kludex]

Your while True above only had a sleep...

As I've said, receive() and send() (from the latest Starlette) raise WebSocketDisconnect.

[econeale]

Well then I'll wait for FastAPI to update its dependencies and see if it fixes my issue. Thank you.

[Kludex]

Your while True above only had a sleep...
As I've said, receive() and send() (from the latest Starlette) raise WebSocketDisconnect.

Thanks! It solved my Issue. how do you know that? can you give me the ref?

Because I'm a Starlette maintainer.

Yes, I can: https://github.com/encode/starlette/releases/tag/0.36.0

[vatheara]

I just deleted my previous comment because I don't want to bother you. you had me everything when I go to your profile 🫡.
Anyway thank for your instant notice!
Appreciate your works!