https://v2ex.com/t/676315

dependent_point.py

class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __repr__(self):
        return "({}, {})".format(self.x, self.y)

    def __add__(self, other):
        def generator():
            return Point(self.x + other.x, self.y + other.y)
        return DependentPoint(generator)

    def __sub__(self, other):
        def generator():
            return Point(self.x - other.x, self.y - other.y)
        return DependentPoint(generator)


class DependentPoint(Point):
    def __init__(self, generator):
        self.generator = generator

    @property
    def x(self):
        return self.generator().x

    @property
    def y(self):
        return self.generator().y
      
a = Point(1, 2)
b = Point(3, 4)
c = Point(5, 6)
d = a + b - c
print(d) # (-1, 0)
a.x = 3
print(d) # (1, 0)
c.y = 1
print(d) # (1, 5)