Detecting disconnection

If in your route or websocket handler (or code called from within it) you are awaiting and the client disconnects the await will raise a CancelledError. This can be used to detect when a client disconnects, to allow for cleanup, for example the sse handler from the Tutorial: Broadcast Server Side Events uses this to remove clients on disconnect,

@app.route('/sse')
async def sse():
    queue = asyncio.Queue()
    app.clients.add(queue)
    async def send_events():
        while True:
            try:
                data = await queue.get()
                event = ServerSentEvent(data)
                yield event.encode()
            except asyncio.CancelledError:
                app.clients.remove(queue)

or with only the relevant parts,

@app.route('/sse')
async def sse():
    try:
        await ...
    except asyncio.CancelledError:
        # Has disconnected

The same applies for WebSockets, streaming the request, etc…