Python Flask emit() function not send real time data to client

Question

I made a countdown timer and i placed emit function in the for loop. But emit doesn't send my timer data to client.

This is my python server-side code:

from flask import Flask,render_template,render_template_string
from flask_socketio import SocketIO, emit

app = Flask(__name__) 
app.config['SECRET KEY'] = 'random'
socketio = SocketIO(app)

@app.route('/') 
def index():
    return render_template('index.html')

@socketio.on('test_timer')
def Timer(seconds=3600):
    def hms(seconds): # hour minute second function
        h = seconds // 3600
        m = seconds % 3600 // 60
        s = seconds % 3600 % 60
        return '{:02d}:{:02d}:{:02d}'.format(h, m, s)

    for i in range(seconds):
        emit(hms(seconds-i),broadcast=True)
        emit.sleep(1)

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

This is my client-side javascript code:

<!DOCTYPE html>

<html>
    <body>
        <script src="//cdnjs.cloudflare.com/ajax/libs/socket.io/2.2.0/socket.io.js" integrity="sha256-yr4fRk/GU1ehYJPAs8P4JlTgu0Hdsp4ZKrx8bDEDC3I=" crossorigin="anonymous"></script>
        <script type="text/javascript" charset="utf-8">
            var socket = io().connect('http://127.0.0.1:5000');
            socket.on('test_timer', function(receiving_data) {
            console.log(receiving_data);
            });
        </script>
    </body>
</html>

Even if I run the server, console.log doesn't print any timer data in client-side.

The main problem is that your client-side is never sending test_timer to the server; all you're doing is setting up a reaction to that message on both sides, but you're never emitting it anywhere. — user5734311
– user5734311, Commented Jul 24, 2020 at 9:48

Oluwafemi Sule · Accepted Answer · 2020-07-24 09:43:02Z

2

The below lines show the signature for the emit function

   def emit(self, event, data=None, room=None, include_self=True,
             namespace=None, callback=None):

In your Flask app, you have a handler registered for test_timer event. In this handler, you need to emit a response event followed by data and not just data. For example,

@socketio.on('test_timer')
def Timer(seconds=3600):
    def hms(seconds): # hour minute second function
        h = seconds // 3600
        m = seconds % 3600 // 60
        s = seconds % 3600 % 60
        return '{:02d}:{:02d}:{:02d}'.format(h, m, s)

    for i in range(seconds):
        emit('test_timer_reply', hms(seconds-i),broadcast=True)
        socketio.sleep(1)

In your client side code, you also need to emit test_timer and register a listener for test_timer_reply.

var socket = io().connect('http://127.0.0.1:5000');
socket.emit('test_timer')
socket.on('test_timer_reply', function(receiving_data) {
  console.log(receiving_data);
});

answered Jul 24, 2020 at 9:43

Oluwafemi Sule

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5 Comments

thisismynewdisplayname Over a year ago

Thanks a lot, works successfully but when one time send data, the for loop breaks. How can I prevent this?

Shivansh Potdar Over a year ago

Put it in a while True loop to keep sending if that's what you meant @skipsbiceps

thisismynewdisplayname Over a year ago

It works successfully, but the timer restarts when the page is refreshed. why?? my broadcast option is True

Sade Over a year ago

It just outputting bad request 400. I have completed the changes mentioned but still not successful. Any suggestions?

Sade Over a year ago

I have solved the bad request by uninstalling Flask-SocketIO,python-engineio,python-socketio and then re-installing and restarting machine. Make sure you re-install the following versions Flask-SocketIO==4.3.1 python-engineio==3.13.2 python-socketio==4.6.0 . I have successfully ran this example. However, when I use the same set up to update a chart, it requires a reload stackoverflow.com/questions/68376011/… . @Oluwafemi Sule any suggestions?

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