Python tkinter label 更新方法

from tkinter import Tk, Checkbutton, Label
from tkinter import StringVar, IntVar

root = Tk()

text = StringVar()
text.set('old')
status = IntVar()

def change():
 if status.get() == 1: # if clicked
  text.set('new')
 else:
  text.set('old')

cb = Checkbutton(root, variable=status, command=change)
lb = Label(root, textvariable=text)
cb.pack()
lb.pack()

root.mainloop()

from tkinter import *
import time


class StopWatch(Frame):
 '''实现一个秒表部件'''
 msec = 50
 def __init__(self, parent=None, **kw):
  Frame.__init__(self, parent, kw)
  self._start = 0.0
  self._elapsedtime = 0.0
  self._running = False
  self.timestr = StringVar()
  self.makeWidgets()
  self.flag = True
 def makeWidgets(self):
  '''制作时间标签'''
  l = Label(self, textvariable = self.timestr)
  self._setTime(self._elapsedtime)
  l.pack(fill = X, expand = NO, pady = 2, padx = 2)
 def _update(self):
  self._elapsedtime = time.time() - self._start
  self._setTime(self._elapsedtime)
  self._timer = self.after(self.msec, self._update)
 def _setTime(self, elap):
  '''将时间格式改为 分：秒：百分秒'''
  minutes = int(elap/60)
  seconds = int(elap-minutes*60.0)
  hseconds = int((elap - minutes*60.0 - seconds) *100)
  self.timestr.set('%2d:%2d:%2d' %(minutes, seconds, hseconds))
 def Start(self):
  if not self._running:
   self._start = time.time() - self._elapsedtime
   self._update()
   self._running = True
 def Stop(self):
  '''停止秒表'''
  if self._running:
   self.after_cancel(self._timer)
   self._elapsedtime = time.time() - self._start
   self._setTime(self._elapsedtime)
   self._running = False
 def Reset(self):
  '''重设秒表'''
  self._start = time.time()
  self._elapsedtime = 0.0
  self._setTime(self._elapsedtime)


 def stopwatch(self):
  if self.flag == True:
   self.pack(side = TOP)
   Button(self, text = 'start', command = self.Start).pack(side = LEFT)
   Button(self, text = 'stop', command = self.Stop).pack(side = LEFT)
   Button(self, text = 'reset', command = self.Reset).pack(side = LEFT)
   Button(self, text = 'quit', command = self.quit).pack(side = LEFT)  
  self.flag = False


class Watch(Frame):
 msec = 1000
 def __init__(self, parent=None, **kw):
   Frame.__init__(self, parent, kw)
   self._running = False
   self.timestr1 = StringVar()
   self.timestr2 = StringVar()
   self.makeWidgets()
   self.flag = True
 def makeWidgets(self):
  l1 = Label(self, textvariable = self.timestr1)
  l2 = Label(self, textvariable = self.timestr2)
  l1.pack()
  l2.pack()
 def _update(self):
  self._settime()
  self.timer = self.after(self.msec, self._update)
 def _settime(self):
  today1 = str(time.strftime('%Y-%m-%d', time.localtime(time.time())))
  time1 = str(time.strftime('%H:%M:%S', time.localtime(time.time())))
  self.timestr1.set(today1)
  self.timestr2.set(time1)
 def start(self):
  self._update()
  self.pack(side = TOP)

if __name__ == '__main__':
 def main():
  root = Tk()
  root.geometry('250x150')
  frame1 = Frame(root)
  frame1.pack(side = BOTTOM)
  sw = StopWatch(root)
  stpwtch = Button(frame1, text = '秒表', command = sw.stopwatch)
  stpwtch.pack(side = RIGHT)
  mw = Watch(root)
  mywatch = Button(frame1, text = '时钟', command = mw.start)
  mywatch.pack(side = LEFT)
  root.mainloop()
 main()