装饰一个类来监控属性变化

Question

我想让类在订阅者的属性之一发生变化时自动向订阅者发送通知。所以如果我写这个代码：

@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。在上面的例子中，我假设它会打印一条指示属性更改的行，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__()方法：

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。将_sentinel用于允许None作为旧值了。

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