Python 3.9元类属性与Classmethod属性

Question

考虑下面的代码

from abc import ABC, ABCMeta


class MyMetaClass(ABCMeta):
    @property
    def metaclass_property(cls):
        return "result"

    # def __dir__(cls):
    #     return list(super().__dir__()) + ['metaclass_property']


class MyBaseClass(ABC, metaclass=MyMetaClass):
    @classmethod
    @property
    def baseclass_property(cls):
        return "result"


class MyClass(MyBaseClass, metaclass=MyMetaClass):
    pass


assert MyClass.baseclass_property == "result"
assert hasattr(MyClass, 'baseclass_property')
assert 'baseclass_property' in dir(MyClass)

assert MyClass.metaclass_property == "result"
assert hasattr(MyClass, 'metaclass_property')
assert 'metaclass_property' in dir(MyClass)

我注意到这在最后一行抛出了一个断言错误。（python v3.9.6）。为什么呢？是虫子吗？可以通过按照两个未注释行将其手动添加到__dir__来修复它

我的问题是，解决这个问题的最好办法是什么？classmethod属性和元类属性之间有什么根本区别吗？（即，是否有我能做或不能做的事情，但不能做其他事情？）

Answer 1

我认为这与属性、元类或abc无关

一个简单的例子：

>>> int.__mro__
(<class 'int'>, <class 'object'>)
>>> isinstance(int, type)
True
>>> '__mro__' in dir(int)
False
>>> '__mro__' in dir(type)
True

在本例中，object是int的基类，而type是int的元类

dir函数的official documentation明确表示

attempts to produce the most relevant, rather than complete, information

及

If the object is a type or class object, the list contains the names of its attributes, and recursively of the attributes of its bases.

及

metaclass attributes are not in the result list when the argument is a class