Websocket with Flask and Gevent

Posted on 28 October 2012

Ubik, package manager for Unix

If you follow me on Github you have maybe already seen that I currently develop a package manager for Unix, Ubik.

In order to test my API, I create a WebUI for Ubik and the websockets comes to me. My two prefered web framework are Django and Flask, but I need a lightweight web app, so welcome to Flask.

Node.js, blah, blah

Ok, I have many many times hear good things about Node.js and, no doubt. But my focus is to do a lightweight and full Python implemented application.

So, tools are:

By the way, gevent-socketio seems to be a solid Python implementation.

App presentation

This is my Flask app structure:

├── requirements.txt
└── app
    ├── static
    │   └── ...
    └── templates
        └── index.html



This is the app core, create your app, import every shared object you want to use into your app, like databases, etc.

And don't forget to import views at the end to avoid circular imports.

# coding: utf-8
import os

from flask import Flask
from websocket import handle_websocket

app = Flask(__name__)
app.secret_key = os.urandom(24)
app.debug = True

def my_app(environ, start_response):
    path = environ["PATH_INFO"]
    if path == "/":
        return app(environ, start_response)
    elif path == "/websocket":
        return app(environ, start_response)

import views

All your websocket stuff could be here, this is the websocket handler. Messages from browser arrived here, and you can easily send message throught websocket with send method from ws object.

Use json, please.

# coding: utf-8
import json

def handle_websocket(ws):
    while True:
        message = ws.receive()
        if message is None:
            message = json.loads(message)

            r  = "I have received this message from you : %s" % message
            r += "<br>Glad to be your webserver."
            ws.send(json.dumps({'output': r}))

Simple flask view to index.html template.

# coding: utf-8
from flask import render_template

from app import app

def index():
    return render_template('index.html')

Run your app with this script. It import your app and feed WSGIServer with it.

#!/usr/bin/env python
# coding: utf-8
from gevent.pywsgi import WSGIServer
from geventwebsocket.handler import WebSocketHandler

from app import my_app

if __name__ == '__main__':
    http_server = WSGIServer(('',5000), my_app, handler_class=WebSocketHandler)


And a quick Javascript websocket example.

<!DOCTYPE html>
<html lang="en">
    <meta charset="utf-8">
<p>Websocket with Flask, Gevent and Gevent-websocket</p>
<p id="log"></p>
<button id="send" type="button">Send!</button>
    <script src=""></script>
            if ("WebSocket" in window) {
                ws = new WebSocket("ws://" + document.domain + ":5000/websocket");
                ws.onmessage = function (msg) {
                    var message = JSON.parse(;

            // Bind send button to websocket
            $("button#send").live("click", function() {
                ws.send(JSON.stringify({'output': 'Sent from my browser!'}));

            // Cleanly close websocket when unload window
            window.onbeforeunload = function() {
                ws.onclose = function () {}; // disable onclose handler first

All can find all this stuff in this repository. Thanks to tzangms.

What now ?

Play websockets with Flask and Python is fun !

I will continue to improve Ubik webui with websocket and you have to take a look at the 0.2 branch. Bunch of new features is coming !