python - 是否可以覆盖类的 __setattr__ ？

Question

在 Python 中，我知道可以这样说

>>> 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!

但以下仍然有效:

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

是否有可能获得如下功能:

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

纯粹出于好奇而提问，因为我真的想不出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，就是元类。

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

Python 然后在 type(C) 的返回值上查找 __setattr__，这里是 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!