class Screen(object):
    @property
    def width (self):
        return self._width

    @width.setter
    def width(self,value):
        self._width = value

    @property
    def height (self):
        return self._height

    @height.setter
    def height(self,value):
        self._height = value

    @property
    def resolution (self):
        return self._width * self._height

s = Screen()
s.width = 1024
s.height = 768
print(s.resolution)
assert s.resolution == 786432
print('1024 * 768 = %d' % s.resolution)