The following worked in the static GTK+ 2 bindings, only the 'do_clicked' method of SubButton is called:

{{{
import gtk
import gobject

def _destroy_cb(widget, data=None):
    gtk.main_quit()

class Button(gtk.Button):
    __gtype_name__ = 'SugarButton'
    def __init__(self, **kwargs):
	gtk.Button.__init__(self, **kwargs)

    def do_clicked(self):
        print 'Button clicked'

class SubButton(Button):
    __gtype_name__ = 'SugarSubButton'
    def __init__(self, **kwargs):
        Button.__init__(self, **kwargs)

    def do_clicked(self):
	print 'SubButton clicked'

window = gtk.Window()
window.set_default_size(450, 450)
window.connect("destroy", _destroy_cb)

b = SubButton(label='hallo')
window.add(b)
b.show()

window.show()

gtk.main()
}}}

In GTK+3 with the dynamic bindings, the  'do_clicked' method of Button gets called not the overriden one of SubButton.

{{{
from gi.repository import Gtk

def _destroy_cb(widget, data=None):
    Gtk.main_quit()

class Button(Gtk.Button):
    __gtype_name__ = 'SugarButton'
    def __init__(self, **kwargs):
        Gtk.Button.__init__(self, **kwargs)

    def do_clicked(self):
        print 'Button clicked'

class SubButton(Button):
    __gtype_name__ = 'SugarSubButton'
    def __init__(self, **kwargs):
        Button.__init__(self, **kwargs)

    def do_clicked(self):
	print 'SubButton clicked'

window = Gtk.Window()
window.set_default_size(450, 450)
window.connect("destroy", _destroy_cb)

b = SubButton(label='hallo')
window.add(b)
b.show()

window.show()

Gtk.main()
}}}