@cherrypy.expose
def getUpdate(self):
    #Set the expected headers...
    cherrypy.response.headers["Content-Type"] = "text/event-stream;charset=utf-8"
    def content():
         yield "Hello,"
         yield "world"
    return content()

var sseEvent = new EventSource('http://localhost:8090/getUpdate');
sseEvent.onmessage = function (event) {
    console.log(event);
};
sseEvent.onopen = function (event) {
  //console.log("I have started...");
};

@cherrypy.expose
def getUpdate(self, _=None):
    cherrypy.response.headers["Content-Type"] = "text/event-stream;charset=utf-8"
    if _:
        data = 'retry: 200ndata: ' + str( self.prog_output) + 'nn'
        return data
    else:
        def content():
            data = 'retry: 200ndata: ' + str( self.prog_output) + 'nn'
            return data
        return content()

getUpdate._cp_config = {'response.stream': True, 'tools.encode.encoding':'utf-8'}

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8">
    <title>title</title>
    <link rel="stylesheet" href="style.css">
</head>
<body>
    <div>server-sent events</div>
    <h3>sse_handler.onmessage()</h3>
    <ul id="sse_message"></ul>

<script>

    // crée l'objet qui permettra la connexion avec le serveur
    var sse_handler = new EventSource('http://localhost:8080/getUpdate');

    sse_handler.onmessage = function (event) {
        console.log("-- sse_handler.onmessage()", event);
        /* https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events#Fields
        the onmessage handler catch all generic message, with no event field
        */
        var h_ul = document.getElementById('sse_message');
        var h_li = document.createElement("li");

        h_li.innerHTML = event.data;
        h_ul.appendChild(h_li);
    };

    sse_handler.onopen = function (event) {
        console.log("-- sse_handler.onopen()", event);
    };

</script>
</body>
</html>

#!/usr/bin/env python3

import datetime

import cherrypy

from pathlib import Path

class TestServerSentEvent(object):
    @cherrypy.expose
    def index(self):
        return Path("index.html").read_text()

    @cherrypy.expose
    def getUpdate(self, * pos, ** nam):
        cherrypy.response.headers["Content-Type"] = "text/event-stream;charset=utf-8"
        return 'retry: 1200ndata: {0}nn'.format(self.output())

    def output(self) :
        d = datetime.datetime.now()
        return f"TEST - {d}"

    getUpdate._cp_config = {'response.stream': True, 'tools.encode.encoding':'utf-8'}

if __name__ == '__main__':
    cherrypy.quickstart(TestServerSentEvent())