Python - paho.mqtt.client - WebSocket handshake error, connection not upgraded

Question

I'm trying to connect to an existing MQTT service (over websockets) that is not managed by me, using paho.mqtt.client.

This is my code:

import paho.mqtt.client as mqtt

def on_connect(client, userdata, flags, rc):
    print("Connected with result code "+str(rc))
    client.subscribe("$SYS/#")

def on_message(client, userdata, msg):
    print(msg.topic+" "+str(msg.payload))


client = mqtt.Client(client_id="<MY_CLIENT_ID>", transport="websockets")
client.username_pw_set(username="<MY_USERNAME>",password="<MY_PASSWORD>")

client.on_connect = on_connect
client.on_message = on_message

client.tls_set()
client.connect("SERVER_ADDRESS", SERVER_PORT, 55)

client.loop_forever()

When I try to run it I'm getting the following exception

Traceback (most recent call last):
  File "C:\Development\CockpitHomePyScript\test.py", line 24, in <module>
    client.connect("SERVER_ADDRESS", 5222, 55)
  File "C:\Users\Ophir\AppData\Local\Programs\Python\Python310\lib\site-packages\paho\mqtt\client.py", line 914, in connect
    return self.reconnect()
  File "C:\Users\Ophir\AppData\Local\Programs\Python\Python310\lib\site-packages\paho\mqtt\client.py", line 1080, in reconnect
    sock = WebsocketWrapper(sock, self._host, self._port, self._ssl,
  File "C:\Users\Ophir\AppData\Local\Programs\Python\Python310\lib\site-packages\paho\mqtt\client.py", line 3713, in __init__
    self._do_handshake(extra_headers)
  File "C:\Users\Ophir\AppData\Local\Programs\Python\Python310\lib\site-packages\paho\mqtt\client.py", line 3766, in _do_handshake
    raise WebsocketConnectionError(
paho.mqtt.client.WebsocketConnectionError: WebSocket handshake error, connection not upgraded

When I searched online the only thing I found was to have tls_set before connect, but that doesn't seem to help...

To make sure that I'm not missing any client certificate to connect, I tried writing a client that connects to the same server with exactly the same details with .NET (using MQTT.NET library) and without setting up anything else special other than the settings above I successfully connected, consumed events, and sent messages.

What am I doing wrong in Python?

Answer 1

I found the solution, pretty stupid actually. By default paho.mqtt.client uses /mqtt as path. In my case I needed the root as path, so what I needed to do is just set it:

client.ws_set_options(path="/")

The actual response from the service was 404, but the way the library code is written is recognizing it as a handshake issue, so the actual exception is misleading.