'Perform a Turtle-data update.'

def _update(self):

    screen = self.screen
    if (screen._tracing == 0):
        return
    elif (screen._tracing == 1):
        self._update_data()
        self._drawturtle()
        screen._update()
        screen._delay(screen._delayvalue)
    else:
        self._update_data()
        if (screen._updatecounter == 0):
            for t in screen.turtles():
                t._drawturtle()
            screen._update()
