'add standard button box.
override if you do not want the standard buttons'

def buttonbox(self):

    box = Frame(self)
    w = Button(box, text='OK', width=10, command=self.ok, default=ACTIVE)
    w.pack(side=LEFT, padx=5, pady=5)
    w = Button(box, text='Cancel', width=10, command=self.cancel)
    w.pack(side=LEFT, padx=5, pady=5)
    self.bind('<Return>', self.ok)
    self.bind('<Escape>', self.cancel)
    box.pack()
