python tkinter 组件config报错

Question

 import sys
import time
from tkinter import *

clock_lab = None

def refresh_time():
    clock_lab.config(text = time.localtime(time.time()))    

myclock = Tk()
myclock.title("myclock")
myclock.geometry("400x200")
clock_lab = Label(myclock,text= time.localtime(time.time())).pack()
clock_bt = Button(myclock, text="clock", command =lambda:refresh_time()).pack()

myclock.mainloop()

按过按钮之后报错，有没有人知道是怎么回事

Exception in Tkinter callback
Traceback (most recent call last):
File "C:\Python34\lib\tkinter__init__.py", line 1533, in call
return self.func(*args)
File "F:/code/python/clock.py", line 14, in
clock_bt = Button(myclock, text="clock", command =lambda:refresh_time()).pack()
File "F:/code/python/clock.py", line 8, in refresh_time
clock_lab.config(text = time.localtime(time.time()))
AttributeError: 'NoneType' object has no attribute 'config'

Answer 1

import sys
import time
from tkinter import *

clock_lab = None

def refresh_time():
clock_lab.config(text = time.localtime(time.time()))

myclock = Tk()
myclock.title("myclock")
myclock.geometry("400x200")
clock_lab = Label(myclock,text= time.localtime(time.time()))
clock_lab.pack()
clock_bt = Button(myclock, text="clock", command =lambda:refresh_time())
clock_bt.pack()

myclock.mainloop()

把pack()单独调用就好了，有没有大神解释一下是怎么回事啊