Note, this is only accurate for PyGTK 2.6 or later.

First, subclass the type properly and make sure the constructor is chained to properly:

 class MyButton(gtk.Button):
    def __init__(self):
      gtk.Button.__init__(self)

Secondly, override the expose-event virtual method, it will be called as soon as the widget is asked to redraw it self

    def do_expose_event(self, event):

The first thing we want to do here, is to call the gtk.Button code, to draw something which looks like a button. It's simple, just chain up to the expose_event of the button itself:

     retval = gtk.Button.do_expose_event(self, event)

At this point, the button is drawn. Now you can make your own "drawings" on top of the widget.

    self.window.draw_ ....()

Finally return the same return value as we got from the parents call

    return retval

Don't forget to register the type:

 gobject.type_register(MyButton)