装饰一个类来监控属性变化

Question

我希望类能够在订阅者的某个属性发生变化时自动向订阅者发送通知。所以如果我要写这段代码：

@ChangeMonitor
class ChangingClass(object):

    def __init__(self, x):
        self.x = x


changer = ChangingClass(5)
print("Going to change x.")
changer.x = 6
print("Going to not change x.")
changer.x = 6
print("End of program")

输出将是：

Going to change x
Old x = 5, new x = 6
Going to not change x.
End of program.

我的问题是如何实现 ChangeMonitor 装饰器类。在上面的示例中，我假设它会打印一行指示属性的更改，但出于有用的目的，它可以向订阅的对象发送通知。

Answer 1

你必须添加一个__setattr__() method：

def ChangeMonitor(cls):
    _sentinel = object()
    old_setattr = getattr(cls, '__setattr__', None)
    def __setattr__(self, name, value):
        old = getattr(self, name, _sentinel)
        if old not is _sentinel and old != value:
            print "Old {0} = {1!r}, new {0} = {2!r}".format(name, old, value)
        if old_setattr:
            old_setattr(self, name, value)
        else:
            # Old-style class
            self.__dict__[name] = value

    cls.__setattr__ = __setattr__

    return cls

这也应该处理现有的__setattr__ 钩子。 _sentinel 也用于允许 None 作为旧值。

演示：

>>> changer = ChangingClass(5)
>>> changer.x = 6
Old x = 5, new x = 6
>>> changer.x = 6
>>> # nothing printed
...
>>> changer.x = None
Old x = 6, new x = None
>>> changer.x = 6
Old x = None, new x = 6