python 窗体设计 Label Button

import tkinter as tk
window=tk.Tk()
window.title('我的窗体')
window.geometry('500x300')
var=tk.StringVar()
var.set('你好哈')

l=tk.Label(window,textvariable=var,bg="Green",font=('黑体',12),width=30,height=2)
l.pack()
on_hit=False
def hit_me():
    global on_hit
    if on_hit==False:
        on_hit=True
        var.set('你也好哈')
    else:
        on_hit=False
        var.set('你好哈')
    
b=tk.Button(window,text = 'hit me',font=('黑体',12),width=20,height=1,command=hit_me)
b.pack()
window.mainloop()