Mine

    def expose(self, event):
        if event.window == self.get_window(gtk.TEXT_WINDOW_LEFT):
            y1 = self.window_to_buffer_coords (gtk.TEXT_WINDOW_LEFT, 0, event.area.y)[1]
            y2 = self.window_to_buffer_coords (gtk.TEXT_WINDOW_LEFT, 0, event.area.y + event.area.height)[1]

            print y1, y2

            i = self.get_line_at_y(y1)[0]

            layout = self.create_pango_layout("pippo") # TODO: get a line specific string

            while True:
                (y, h) = self.get_line_yrange(i)
                print y

                if y > y2:
                    break

                self.style.paint_layout(event.window, self.state, False, event.area, self, None, 0, y, layout)

                if not i.forward_line():
                    break