39, 40行目のコメントアウトを切り替えると動かなくなる。その場合は67行目を68行目で置き換えれば動く。

flask_debug.py

#!/usr/bin/env python
from __future__ import print_function
from flask import Flask, render_template, Response
import socket
from contextlib import closing
from threading import Thread
from time import sleep

PORT = 4444

app = Flask(__name__)
shared = [None]

def udp_fire():
    """ sender """
    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    with closing(sock):
        while True:
            sock.sendto("this is udp", ("127.0.0.1", PORT))
            sleep(0.5)

def recv_mock():
    """ receiver """
    while True:
        yield "this is mock"
        sleep(0.5)

def recv_udp():
    sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
    with closing(sock):
        sock.bind(("0.0.0.0", PORT))
        while True:
            yield sock.recv(14000)

def prod():

    # switch these two data
    gen = (
        #recv_mock()
        recv_udp()
    )
    while True:
        data = gen.next()
        shared[0] = data
        print("prod(): [{d}]".format(d=shared[0]))

def cons():
    while True:
        data = shared[0]
        print("cons(): [{d}]".format(d=data))
        yield data

consumer = cons()
@app.route('/')
def index():
    return consumer.next()

if __name__ == '__main__':
    udp_sender = Thread(target=udp_fire)
    udp_sender.daemon = True
    udp_sender.start()

    producer = Thread(target=prod)
    producer.daemon = True
    producer.start()

    app.run(host='0.0.0.0', debug=True)
    # app.run(host='0.0.0.0') # <- it works well with UDP