How to send information from client to server (Flask - python)

Out of the box, you can't use persistent requests or websockets with something like Flask. However, you don't necessarily need this - you can use AJAX with a simple polling mechanism.

Client side:

$('button').click(function() {
    var state = $(this).attr('data-state');

    $.post('/clients/', { state: state });
});

// Check for messages every 1 second
var checkDelay = 1000;
var lastMessageSeen = new Date().getTime();

setInterval(function() {
    $.get('/clients/', function(result) {
        if(result.ready and result.timestamp > lastMessageSeen) {
            lastMessageSeen = result.timestamp;
            console.log('Message received: ' + result.msg);
        }
    });
}, checkDelay);

Server side:

from flask import request, jsonify

@app.route('/clients/', methods=['POST'])
def client_broadcast():
    state = request.form['state']

    # here you can store the message under a key in Memcached, Redis or another in-memory cache server
    store_in_cache(state)

    return jsonify(stored=True)

@app.route('/clients/', methods=['GET'])
def client_retrieve():
    # retrieve messages from cache
    msg, timestamp = retrieve_from_cache()

    if msg:
        return jsonify(ready=True, msg=msg, timestamp=timestamp)
    else:
        return jsonify(ready=False)

I've left out the store_in_cache and retrieve_from_cache functions, because it depends on how you want to handle these messages. Are they global to all browser clients? Do you want to have a queue of messages?