@property

class Screen(object):
    @property
    def width(self):
        return self._width
    @width.setter
    def width(self,data):
        self._width=data

    @property
    def height(self):
        return self._height
    @height.setter
    def height(self,data):
        self._height=data

    @property
    def square(self):
        return self._width*self._height
r=Screen()
r.width=20
r.height=9
print('the square is %d'%r.square)