mainloop()的有关问题

from Tkinter import *
widget = Button(text='Spam', padx=10, pady=10)
widget.pack(padx=20, pady=20)
widget.config(cursor='gumby')
widget.config(bd=8, relief=RAISED)
widget.config(bg='dark green', fg='white')
widget.config(font=('helvetica', 20, 'underline italic'))
widget.mainloop()
可以执行,将widget.mainloop()写成mainloop()也可以执行.
为何下面,就不行
from Tkinter import *              # get base widget set
from dialogTable import demos      # button callback handlers
from quitter import Quitter        # attach a quit object to me
    
class Demo(Frame):
    def __init__(self, parent=None):
        Frame.__init__(self, parent)
        self.pack()
        Label(self, text="Basic demos").pack()
        for (key, value) in demos.items():
            Button(self, text=key, command=value).pack(side=TOP, fill=BOTH)
        Quitter(self).pack(side=TOP, fill=BOTH)
    
if __name__ == '__main__':
    Demo().mainloop()
不可将Demo().mainloop()写成mainloop(),请问,为什么? 

放到格式化标签里。

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