Python Socketio not connecting with JavaScript socket.io-client

Question

I want to simply connect socket.io-client to python server, but for some reason it's keep failing. Initially I start my python server and then try to connect JavaScript as my client server after following that process what I see is JavaScript client server is keep trying to connect and failing with following error:

websocket.js:88 WebSocket connection to 'ws://localhost:5100/socket.io/?EIO=4&transport=websocket' failed:

and python server also repeating following error:

(10448) accepted ('127.0.0.1', 61471) 127.0.0.1 - - [01/Feb/2022 15:14:01] "GET /socket.io/?EIO=4&transport=websocket HTTP/1.1" 400 136 0.000000

I am using python3.10 with socketio following Document:

https://python-socketio.readthedocs.io/en/latest/.

Also tried version compatibility:

https://pypi.org/project/python-socketio/

pip3 freeze

I have tried multiple version for python-engineio and python-socketio to match my socket.io-client but no improvements.

eventlet==0.33.0
python-engineio==3.13.0
python-socketio==4.6.1

server.py

import eventlet
import socketio

sio = socketio.Server()
app = socketio.WSGIApp(sio)

@sio.event
def connect(sid, environ):
    print('[INFO] Connect to client', sid)

@sio.event
def disconnect(sid):
    print('disconnect ', sid)

#ESI.local 192.168.2.95
if __name__ == '__main__':
    eventlet.wsgi.server(eventlet.listen(('0.0.0.0', 5100)), app)

HTML/JavaScript

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
<h1>Socket IO Demo</h1>
    <script type="module">
        import { io } from "https://cdn.socket.io/4.3.2/socket.io.esm.min.js";
        
        const sio = io("http://localhost:5100:", {
            forceJSONP: true,
            secure: true,
            jsonp: true,
            transports: [ "websocket","polling"]
        });

        sio.on("connect", () => {
            console.log("connected");
        });

        sio.on("disconnect", () => {
            console.log("Disconnected");
        });
    </script>
</body>
</html>

The version compatibility table that you refer shows that the versions that you are using are incompatible. What am I missing? — Miguel Grinberg
– Miguel Grinberg, Commented Feb 1, 2022 at 23:26

smith-christian · Accepted Answer · 2022-02-04 15:29:20Z

1

After many trial and error. I have finally cracked it. This was happening due to CORS even though I used CORS google extension to handle this situation, but I guess this was supposed to be handled on backend. Simply passing (cors_allowed_origins="*") to socketio.Server() it fixed everything.

sio = socketio.Server(cors_allowed_origins="*")

answered Feb 4, 2022 at 15:29

smith-christian

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2 Comments

Miguel Grinberg Over a year ago

Sorry, but this isn't fixing anything. You are just exposing your server to attacks when you disable the protections of the browser. Instead of "*", set the origin that you need to support, allowing all origins is a bad and insecure solution.

smith-christian Over a year ago

Thank you For detailed advice Miguel! :)

Quentin AM · Accepted Answer · 2022-02-01 22:58:58Z

If you want to perform websocket, i'll recommend using WebSocket package built-in and websocket from gevent.

Here what I have done to achieve a thing similar as what you want to do :

let socket = new WebSocket('ws://localhost:8080'); 
console.log('Created socket');

// Connection opened
socket.addEventListener('open', function (event) {
    socket.send('Hello Server!');
});

// Listen for messages
socket.addEventListener('message', function (event) {
    console.log('Message from server ', event.data);
});

// For example : send array
socket.send(JSON.stringify([1, 2, 3]));

Python server:

from geventwebsocket import WebSocketServer, WebSocketApplication, Resource
from collections import OrderedDict

class Application(WebSocketApplication):
    def on_open(self):
        print("Connection opened")

    def on_message(self, message):
        print("Got message: ", message)
        # self.ws.send(...) to make a response

    def on_close(self, reason):
        print(reason)

WebSocketServer(
    ('localhost', 8080),
    Resource(OrderedDict([('/', Application)]))
).serve_forever()

Gevent (python) : https://pypi.org/project/gevent-websocket/

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