是否可以覆盖类的__setattr__？

Question

在Python中，我知道可以说像

这样的东西

>>> class C:
...   def __setattr__(self, name, value):
...     print("Hey, you can't set {0} to {1}!".format(name, value))
...
>>> x = C()
>>> x.y = 5
Hey, you can't set y to 5!

但以下情况仍有效：

>>> C.y = 5
>>> print(C.y)
5

是否可以获得如下功能：

>>> C.y = 5
Hey, you can't set y to 5!

完全出于好奇，因为我无法想到一个真正实用的例子。

Answer 1

像所有特殊方法一样，__setattr__ is accessed on the type;对于类的实例，对于类，即元类。

您必须在自定义元类上定义它，而不是直接在类本身上定义它：

class SetAttrMeta(type):
    def __setattr__(cls, name, value):
        print("Hey, you can't set {0} to {1}!".format(name, value))

class C(metaclass=SetAttrMeta):
    pass

然后，Python会在__setattr__的返回值上查找type(C)，此处为SetAttrMeta。

演示：

>>> class SetAttrMeta(type):
...     def __setattr__(cls, name, value):
...         print("Hey, you can't set {0} to {1}!".format(name, value))
... 
>>> class C(metaclass=SetAttrMeta):
...     pass
... 
>>> C.spam = 'eggs'
Hey, you can't set spam to eggs!