win = Frame()
win.pack()
Label(win, text='Basic demos').pack()
for(key, value) in demos.items():
    Button(win, text=key, command=value).pack(side=TOP, fill=BOTH)
win.mainloop() 

class Demo(Frame):
    def __init__(self, parent=None, **options):
        Frame.__init__(self, parent, **options).pack()
        Label(self, text='Basic demos').pack()
        for(key, value) in demos.items():
            Button(self, text=key, command=value).pack(side=TOP, fill=BOTH)
Demo.mainloop()

+Demo.mainloop() 
-Demo().mainloop()