class Screen(object):    @property    def width(self):        return self._width    @property    def height(self):        return self._height    @width.setter    def width(self, value):        self._width = value    @height.setter    def height(self, value):        self._height = value    @property    def resolution(self):        return self._width*self._heighta = Screen()a.width = 100a.height = 20print(a.resolution)