Chain of responsibility in python (Design Patterns - Gamma, Helm, Johnson, Vlissides)

chainofresponsibility.py

PRINT_TOPIC = 1
PAPER_ORIENTATION = 2
APPLICATION_TOPIC = 3
NO_HELP_TOPIC = -1

class HelpHandler (object):
  def __init__(self, successor=0, topic=NO_HELP_TOPIC):
	self._successor = successor
        self._topic = topic
    def HasHelp(self):
        return self._topic != NO_HELP_TOPIC;
    def HandleHelp(self):
        if self._successor != 0:
            self._successor.HandleHelp()
    def SetHandler(self,h,t):
        self._successor = h
        self._h = t

class Widget(HelpHandler):
    def __init__(self, w, t):
        super(Widget,self).__init__(w,t)

class Button(Widget):
    def __init__(self, d, t):
        super(Button,self).__init__(d,t)
    def HandleHelp(self):
        if self.HasHelp():
            print "Button Help"
        else:
            return super(Button, self).HandleHelp()

class Dialog(Widget):
    def __init__(self, h, t):
        super(Dialog,self).__init__(h,t)
        self.SetHandler(h,t)
    def HandleHelp(self):
        if self.HasHelp():
            print "Dialog Help"
        else:
            return super(Dialog, self).HandleHelp()

class Application(HelpHandler):
    def __init__(self, t):
        super(Application,self).__init__(0,t)
    def HandleHelp(self):
        print "Application Help"

def main():

    application = Application(APPLICATION_TOPIC)
    dialog = Dialog(application, PRINT_TOPIC)
    button = Button(dialog, NO_HELP_TOPIC)

    # invoking help in the chain
    button.HandleHelp()

if __name__ == "__main__":
    main()