python Tkinter按钮上文字变化怎么弄

#!/usr/bin/python2.7
#! -*- coding: utf-8 -*-
import Tkinter
root = Tkinter.Tk()
root.title("My tools")
root.geometry('300x300+300+300')
def on_off():
   if btonoff['text'] == 'on':
       btonoff['text'] = 'off'
   else:
       btonoff['text'] = 'on'
btonoff = Tkinter.Button(root, text="on", command=on_off)
btonoff.place(x=100, y=160, width=100, height=40)
root.mainloop()

　　

#!/usr/bin/python2.7

#! -*- coding: utf-8 -*-

import Tkinter

root = Tkinter.Tk()

root.title("My tools")

root.geometry('300x300+300+300')

def on_off():

   if btonoff['text'] == 'on':

       btonoff['text'] = 'off'

   else:

       btonoff['text'] = 'on'

btonoff = Tkinter.Button(root, text="on", command=on_off)

btonoff.place(x=100, y=160, width=100, height=40)

root.mainloop()