WebSocket

Tremolo has a built-in, minimal implementation of WebSocket.

To enable WebSocket support for a handler, use the websocket=None placeholder as follows:

@app.route('/ws')
async def ws_handler(websocket=None, **server):
    if websocket is None:
        # show a not found page
        raise NotFound

    # an upgrade request is received.
    # accept it by sending the "101 Switching Protocols"
    await websocket.accept()

    while True:
        message = await websocket.receive()
        # send back the received message
        await websocket.send(f'You said: {message}')

Alternatively, you can use async iterator instead of the while loop. This will also perform websocket.accept() under the hood. Thus calling websocket.accept() separately becomes optional:

    # ...

    async for message in websocket:
        await websocket.send(f'You said: {message}')

Here’s a working example of a WebSocket Chat: https://github.com/nggit/tremolo/blob/main/websocket_chat.py .

Close the connection gracefully

When a client initiates a connection closure or closes a browser tab, Tremolo will close the connection at the transport level without sending the websocket close code.

To close the connection gracefully, you can for example use websocket.close(code=1000):

    from tremolo.exceptions import WebSocketClientClosed

    # ...

    try:
        async for message in websocket:
            await websocket.send(f'You said: {message}')
    except WebSocketClientClosed as exc:
        print(f'the client has closed the connection with code {exc.code}')
        # close the connection accordingly
        await websocket.close()

If you think the built-in WebSocket support does not fulfill the features you expect, or want to use an external WebSocket server, you can disable it with ws=False in the framework configuration.

Or --no-ws in the ASGI server configuration.