python Flask socketio 404/400 from browser

Question

I'm trying to run a flask-socketio server that connects to a react app in the browser.

python Server code (relevant portions):

from flask import Flask, request, jsonify
from flask_socketio import SocketIO

app = Flask(__name__)
socketio = SocketIO(app)

@socketio.on('message')
def handle_message(message):
    print('received message: ' + message)

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

client js(running in browser):

var ws = new WebSocket("ws://127.0.0.1:5000");
console.log('connecting to log ws')
ws.onopen = function (event) {
  console.log('connection opened')
};
ws.onmessage = function (event) {
  console.log(event.data);
  self.recieve(event.data);
}

I get the following error in the browser:

failed: Error during WebSocket handshake: Unexpected response code: 404

Now I've heard that this is often because I've made a mistake in the url I use. However, when I change

var ws = new WebSocket("ws://127.0.0.1:5000");

to

var ws = new WebSocket("wss://127.0.0.1:5000");

I can see that the server gets the connection request, but it returns the following error on the server side:

127.0.0.1 - - [02/Nov/2017 13:07:59] code 400, message Bad HTTP/0.9 request 
type ('\x16\x03\x01\x00t\x01\x00\x00p\x03\x01Y\xfb{\x1f\x0
5\x8d\xbf|\xdb\xeb\x12\xe8\xf6\xb0\xf9')

What do i need to do to get a successful connection set up here? The browser used here is chrome, but Im having difficulty with others as well.

jzeef · Accepted Answer · 2017-11-02 22:04:36Z

1

I figured out the answer. You cannot use WebSocket in the browser to communicate with flask-socketio, instead I needed to also use regular socket.io in the client side to communicate with the server. Since switching, I'm getting perfect communication between client and server.

answered Nov 2, 2017 at 22:04

jzeef

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1 Comment

Fredrik Jonsén Over a year ago

It can be done using the browser, but if you're using a web based client you need to make sure it uses SocketIO, and not some other implementation. One I have found that works for is amritb.github.io/socketio-client-tool

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