Cannot send events from 'disconnect' event handler of Flask-SocketIO server

[ThomasHerondale]

I'm building a simple presence announcement system using Flask-SocketIO for my server.
My idea was to detect every 'connect'/'disconnect' event on the server and broadcast to all online users a 'presence' event that correctly indicates whether User A has just connected or disconnected.
The 'connect' handler, i.e. the handler that dispatches the 'User A has come online' presence works well; the problem has to do with the 'disconnect' one, since trying to send/emit an event from that handler gets me this error:

Traceback (most recent call last):   
  File "...\.venv\Lib\site-packages\werkzeug\serving.py", line 362, in run_wsgi
    execute(self.server.app)   
  File "...\.venv\Lib\site-packages\werkzeug\serving.py", line 328, in execute
    write(b"")    
  File "...\.venv\Lib\site-packages\werkzeug\serving.py", line 253, in write
    assert status_set is not None, "write() before start_response"    
AssertionError: write() before start_response

Here is my code:

app = Flask(__name__)
sio = SocketIO(
    app,
    kwargs={'logger': True, 'engineio_logger': True}
)
logging.basicConfig(level=logging.INFO)


class _User:
    def __init__(self, username):
        self.username = username

    def with_sid(self, sid):
        return User(self.username, sid)

    def __hash__(self):
        return hash(self.username)

    def __eq__(self, other):
        return self.username == other.username

class User(_User):
    def __init__(self, username, sid):
        super().__init__(username)
        self.sid = sid

    @staticmethod
    def get_user(username, sid):
        for user in userStorage:
            if user.username == username:
                return user.with_sid(sid)

        user = User(username, sid)
        print(f'New user {username} created')
        userStorage.add(user)
        return user


userStorage: Set[_User] = set()
onlineUsers: Set[User] = set()


@sio.on('connect', namespace='/chat')
def user_online():
    username = request.headers['username']
    sid = request.sid
    user = User.get_user(username, sid)
    onlineUsers.add(user)
    print(f'User {username} connected')

    # presence broadcating
    sio.emit(
        event='presence',
        data="""{
        "header": "presence", 
        "type": "online", 
        "user_ref": "%s"
        }""" % username,
        namespace='/chat',
        include_self=True,
        to=[u.sid for u in onlineUsers]
    )


@sio.on('disconnect', namespace='/chat')
def user_offline():
    username = request.headers['username']
    sid = request.sid
    user = User.get_user(username, sid)
    onlineUsers.remove(user)
    print(f'User {username} disconnected')
    sio.send(
        event='presence',
        data="""{
        "header": "presence", 
        "type": "offline", 
        "user_ref": "%s"
        }""" % username,
        namespace='/chat',
        include_self=True,
        to=[u.sid for u in onlineUsers]
    )


if __name__ == '__main__':
    sio.run(app)

[miguelgrinberg]

Do you have logs for a complete session leading to this error?

[miguelgrinberg]

Try passing the logger and engineio_logger options as their own keyword arguments, not as a single argument named kwargs. Not sure where you copied this from, but there is no kwargs option in the SocketIO class.

[ThomasHerondale]

Thank you! Don't know why I put those values in a dictionary.
This is the log, which includes the server start, the connection of user "test" and the error following its disconnection:

[2024-03-04 12:30:58,161] INFO p16020 root [init:99] Server initialized for threading.
[2024-03-04 12:30:58,169] INFO p16020 werkzeug [_log:96] �[31m�[1mWARNING: This is a development server. Do not use it in a production deployment. Use a production WSGI server instead.�[0m
Running on http://127.0.0.1:5000
[2024-03-04 12:30:58,169] INFO p16020 werkzeug [_log:96] �[33mPress CTRL+C to quit�[0m
[2024-03-04 12:31:00,785] INFO p16020 root [send:76] fYqUO_odhDs7fEoZAAAA: Sending packet OPEN data {'sid': 'fYqUO_odhDs7fEoZAAAA', 'upgrades': [], 'pingTimeout': 20000, 'pingInterval': 25000}
[2024-03-04 12:31:00,786] INFO p16020 root [handle_get_request:88] fYqUO_odhDs7fEoZAAAA: Received request to upgrade to websocket
[2024-03-04 12:31:00,787] INFO p16020 root [_websocket_handler:214] fYqUO_odhDs7fEoZAAAA: Upgrade to websocket successful
[2024-03-04 12:31:00,790] INFO p16020 root [receive:39] fYqUO_odhDs7fEoZAAAA: Received packet MESSAGE data 0/chat,
[2024-03-04 12:31:00,791] INFO p16020 root [emit:161] emitting event "presence" to ['1BjNakssN6fI1vl7AAAB'] [/chat]
[2024-03-04 12:31:00,792] INFO p16020 root [send:76] fYqUO_odhDs7fEoZAAAA: Sending packet MESSAGE data 2/chat,["presence","{\n "header": "presence", \n "type": "online", \n "user_ref": "test"\n }"]
[2024-03-04 12:31:00,792] INFO p16020 root [send:76] fYqUO_odhDs7fEoZAAAA: Sending packet MESSAGE data 0/chat,{"sid":"1BjNakssN6fI1vl7AAAB"}
[2024-03-04 12:31:01,622] INFO p16020 root [receive:39] fYqUO_odhDs7fEoZAAAA: Received packet MESSAGE data 1/chat,
[2024-03-04 12:31:01,623] INFO p16020 root [emit:161] emitting event "presence" to all [/chat]
[2024-03-04 12:31:01,623] INFO p16020 root [send:76] fYqUO_odhDs7fEoZAAAA: Sending packet MESSAGE data 2/chat,["presence"]
[2024-03-04 12:31:01,628] INFO p16020 werkzeug [_log:96] 127.0.0.1 - - [04/Mar/2024 12:31:01] "�[35m�[1mGET /socket.io/?EIO=4&transport=websocket HTTP/1.1�[0m" 500 -
[2024-03-04 12:31:01,632] ERROR p16020 werkzeug [_log:96] Error on request:
Traceback (most recent call last):
File "....venv\Lib\site-packages\werkzeug\serving.py", line 362, in run_wsgi
execute(self.server.app)
File "....venv\Lib\site-packages\werkzeug\serving.py", line 328, in execute
write(b"")
File "....venv\Lib\site-packages\werkzeug\serving.py", line 253, in write
assert status_set is not None, "write() before start_response"
AssertionError: write() before start_response

[miguelgrinberg]

Okay, thanks.

One possible issue here is that you are using an older version of Werkzeug, which is not designed to support WebSocket connections and requires some hacks to work well. If you are not on the latest version, try to upgrade both Flask and Werkzeug to latest and see if that helps.

In any case, this error is not going to occur when you use a production web server such as Gunicorn or uWSGI.

[ThomasHerondale]

Thanks, I'll keep that in mind! Is there a fix with respect to the Flask development server anyway, since my versions are already updated? It would be very convenient to be able to use it for quick prototyping.

[miguelgrinberg]

I cannot reproduce this error using the example app in this repository, so this must be caused in some way by your own application. If you want to provide a small and complete example I can use to reproduce the issue I can investigate further.