from Tkinter import *

class HelloCallable:
    def __init__(self):                      # __init__ run on object creation
        self.msg = 'Hello __call__ world' 
    def __call__(self):
        print self.msg                       # __call__ run later when called 
        import sys; sys.exit()               # class object looks like a function

widget = Button(None, text='Hello event world', command=HelloCallable())
widget.pack()
widget.mainloop()