设置属性的属性

Question

我正在尝试找出一种方法，让属性的属性的设置器触发顶级类的某些操作。

作为一个虚拟示例，假设我的顶级课程是Segment。如果我将其端点的坐标直接存储为对象x0、y0、x1 和y1 的属性，并让每个对象的设置器触发所选操作，那么一切都很好。

但是如果我想将它们分成两个Point成员作为属性p0和p1，每个属性都有x和y，每当修改其中一个坐标时，没有明显的方法告诉Segment 做某事。这就是我希望能够做到的：

>>> segment = Segment(Point(0, 0), Point(3, 3))
>>> segment.p0
Point(0, 0)
>>> segment.p0.x
0
>>> segment.p1.y = 4
Length of segment changed to 5.0!  # This can only be printed by segment, not p1!

问题是segment.p1.y = 4这行首先在segment实例上调用p1的getter，然后在上一个调用的返回上调用y的setter，此时不简单让segment 实例知道已进行更改的方式。

我现在能想到的最好的事情是这样的：

class Point(object):
    def __init__(self, x, y, parent=None, name=None):
        self.parent, self.name = parent, name
        self._x, self._y = x, y

    @property
    def x(self):
        return self._x
    @x.setter
    def x(self, value):
        self._x = value
        if parent is not None:
            setattr(self.parent, self.name, self)

    # Similar code for y goes here...


class Segment(object):
    def __init__(self, p0, p1):
        self.p0, self.p1 = p0, p1

    @property
    def p0(self):
        return self._p0
    @p0.setter
    def p0(self, point):
        self._p0 = point
        self.p0.parent = self
        self.p0.name = 'p0'
        if not self._silent:
            self.do_something()  # This would print the length in the above example

    # Similar code for p1 goes here...

虽然这可以满足我的要求，但我不太喜欢手动将该链接添加回父级，也不喜欢制作大量 Point 对象的冗余副本，或者冒险如果执行以下操作会出现有趣的错误：

p0, p1, p2 = Point(0, 0), Point(1, 1), Point(2, 2)
seg0 = Segment(p0, p1)
seg1 = Segment(p0, p2)
# The following line changes the value on both seg0 and seg1, but triggers
# the do_something call on seg1 only!
seg0.p0.x = 6

有现成的食谱吗？任何人都可以想出更好的方法吗？

Answer 1

也许您正在寻找Observer design pattern：

import math

class Point(object):
    def __init__(self, x, y, name=None):
        self.name = name
        self._x, self._y = x, y
        self.observers = []

    def observe(self, observer):
        self.observers.append(observer)

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

    @property
    def x(self):
        return self._x
    @x.setter
    def x(self, value):
        self._x = value
        for o in self.observers:
            o.notify()

    @property
    def y(self):
        return self._y
    @y.setter
    def y(self, value):
        self._y = value
        for o in self.observers:
            o.notify()

class Segment(object):
    def __init__(self, p0, p1):
        self._p0, self._p1 = p0, p1
        p0.observe(self)
        p1.observe(self)

    def __repr__(self):
        return 'Segment({}, {})'.format(self.p0, self.p1)

    def notify(self):
        print('Length of {} changed to {}'.format(self, self.length()))

    def length(self):
        return math.sqrt((self.p0.x - self.p1.x)**2
                         + (self.p0.y - self.p1.y)**2)

    @property
    def p0(self):
        return self._p0
    @p0.setter
    def p0(self, point):
        self._p0 = point

    @property
    def p1(self):
        return self._p1
    @p1.setter
    def p1(self, point):
        self._p1 = point

---

segment = Segment(Point(0, 0), Point(3, 3))
print(segment.p0)
# Point(0, 0)
print(segment.p0.x)
# 0
segment.p1.y = 4

产量

Length of Segment(Point(0, 0), Point(3, 4)) changed to 5.0

和

p0, p1, p2 = Point(0, 0), Point(1, 1), Point(2, 2)
seg0 = Segment(p0, p1)
seg1 = Segment(p0, p2)
seg0.p0.x = 6

产量

Length of Segment(Point(6, 0), Point(1, 1)) changed to 5.09901951359
Length of Segment(Point(6, 0), Point(2, 2)) changed to 4.472135955