'Button press event handler'

def _press(self, event):

    if ((self._polygon_completed or (u'move_vertex' in self.state)) and (len(self._xs) > 0)):
        (h_idx, h_dist) = self._polygon_handles.closest(event.x, event.y)
        if (h_dist < self.vertex_select_radius):
            self._active_handle_idx = h_idx
    (self._xs_at_press, self._ys_at_press) = (self._xs[:], self._ys[:])
