import gtk

class Hello:
    def __init__(self):
        self.window = gtk.Window()
        self.window.connect('destroy', self.on_quit)
        self.window.set_border_width(10)

        self.button = gtk.Button('Hello World')
        self.button.connect('clicked', self.on_hello_clicked)
        self.window.add(self.button)

        self.window.show_all()
        
    def on_quit(self, widget):
        gtk.main_quit()

    def on_hello_clicked(self, widget):
        print 'Hello world!'

if __name__ == "__main__":
    Hello()
    gtk.main()

Traceback (most recent call last):
  Line 1, in <module>
    import gtk
ImportError: No module named gtk