python web.py 下的全局变量的问题

import threading
import time
import web

global bianliang
bianliang=0

class Test:
    global bianliang
    def GET(self):
        return str(bianliang)

def _listen_yushui():
    while True:
        time.sleep(2)
        global bianliang
        print bianliang

def _add():
    while True:
        time.sleep(2)
        global bianliang
        bianliang=bianliang+1
urls=(
    '/test','Test'
)
application = web.application(urls, globals())

if __name__ == "__main__":
    yushui = threading.Thread(target=_listen_yushui)
    yushui.setDaemon(True)
    yushui.start()
    add = threading.Thread(target=_add)
    add.setDaemon(True)
    add.start()
    application.run()

上面的代码在运行时，控制台输出可以看到 bianliang 的值在不断的增加；但是发送一个网络请求'your_url/test'，请求到的结果却是 bianliang 的初始值 0 ，这是为什么？