python 实现定时循环触发某个方法

直接贴上代码

import threading
def sayhello():
        print "hello world"
        global t        #Notice: use global variable!
        t = threading.Timer(5.0, sayhello)
        t.start() 

t = threading.Timer(5.0, sayhello)
t.start() 

线程t不断的改变