from Tkinter import *
import time

s = 1

def stopit():
    global s
    s = 1
    print "stopped"

def callback():
    s = 0
    if s == 0:
        print "called the callback!"
    root.after(3000, callback)

root = Tk()
root.geometry("200x200+200+200")

b = Button(root, text="START", command=callback)
b2 = Button(root, text="STOP", command=stopit)
b.pack()
b2.pack()
root.mainloop()