python2.7 Why can’t I click the button on tkinter, the red part in the picture, after clicking the "Start" button? How can I click the minimize, maximize and close buttons of tkinter after clicking the "Start" button?
The code is:

# -*- coding: UTF-8 -*-

from Tkinter import *
import os
import tkMessageBox
import time
root = Tk()
today_path = time.strftime('%Y-%m-%d')
work_path = 'C:\yes_pic\' + today_path
def start():
    while True:
        doThis(work_path)
        time.sleep(5)

def doThis(dirr):
    if not os.path.exists(dirr):
        pass
    else:
        if os.path.isdir(dirr):
            for p in os.listdir(dirr):
                d  = os.path.join(dirr,p)
                if (os.path.isdir(d) == True):
                    doThis(d)
        if  os.listdir(dirr):
            if dirr.count('\')!=2:
                tkMessageBox.showwarning("提示", "路径"+dirr+"有文件!")


button = Button(root, text="开始", command=start,width=20,height=10)
button.pack()
root.geometry('300x200+500+300')
root.mainloop()