Right now broadcast() sends to all clients sequentially. If one client is slow, it delays everything.

[janwilmans]

A safer version:

async def broadcast(message):
    data = json.dumps(message)

    dead = []
    tasks = []

    for ws in clients:
        tasks.append(ws.send_text(data))

    results = await asyncio.gather(*tasks, return_exceptions=True)

    for ws, result in zip(list(clients), results):
        if isinstance(result, Exception):
            dead.append(ws)

    for ws in dead:
        clients.remove(ws)