是否有类似 getattr/hasattr 但绕过实例属性的函数？

Question

是否有类似标准库中的内置函数getattr 和hasattr 但在属性查找期间绕过实例属性 的函数，例如implicit lookup of special methods？

我们将这些假设函数称为getclassattr 和hasclassattr。以下是我期望的实现：

null = object()

def getclassattr(obj, name, default=null, /):
    if not isinstance(name, str):
        raise TypeError('getclassattr(): attribute name must be string')
    try:
        classmro = vars(type)['__mro__'].__get__(type(obj))
        for cls in classmro:
            classdict = vars(type)['__dict__'].__get__(cls)
            if name in classdict:
                attr = classdict[name]
                attrclassmro = vars(type)['__mro__'].__get__(type(attr))
                for attrclass in attrclassmro:
                    attrclassdict = vars(type)['__dict__'].__get__(attrclass)
                    if '__get__' in attrclassdict:
                        return attrclassdict['__get__'](attr, obj, type(obj))
                return attr
        classname = vars(type)['__name__'].__get__(type(obj))
        raise AttributeError(f'{classname!r} object has no attribute {name!r}')
    except AttributeError as exc:
        try:
            classmro = vars(type)['__mro__'].__get__(type(obj))
            for cls in classmro:
                classdict = vars(type)['__dict__'].__get__(cls)
                if '__getattr__' in classdict:
                    return classdict['__getattr__'](obj, name)
        except AttributeError as exc_2:
            exc = exc_2
        except BaseException as exc_2:
            raise exc_2 from None
        if default is not null:
            return default
        raise exc from None

def hasclassattr(obj, name, /):
    try:
        getclassattr(obj, name)
    except AttributeError:
        return False
    return True

用例是内置类classmethod:*的纯Python实现

import types

class ClassMethod:

    def __init__(self, function):
        self.__func__ = function

    def __get__(self, instance, owner=None):
        if instance is None and owner is None:
            raise TypeError('__get__(None, None) is invalid')
        if owner is None:
            owner = type(instance)
        # NOT hasattr.
        if hasclassattr(self.__func__, '__get__'):
            # NOT getattr.
            return getclassattr(self.__func__, '__get__')(owner, type(owner))
        return types.MethodType(self.__func__, owner)

    @property
    def __isabstractmethod__(self):
        return hasattr(self.__func__, '__isabstractmethod__')

---

* 请注意，此实现不适用于内置函数 getattr 和 hasattr，因为它们首先在实例属性中查找，因为与内置类 classmethod 的比较表明：

>>> import types
>>> class ClassMethod:
...     def __init__(self, function):
...         self.__func__ = function
...     def __get__(self, instance, owner=None):
...         if instance is None and owner is None:
...             raise TypeError('__get__(None, None) is invalid')
...         if owner is None:
...             owner = type(instance)
...         if hasattr(self.__func__, '__get__'):
...             return getattr(self.__func__, '__get__')(owner, type(owner))
...         return types.MethodType(self.__func__, owner)
...     @property
...     def __isabstractmethod__(self):
...         return hasattr(self.__func__, '__isabstractmethod__')
... 
>>> class M(type):
...     def __get__(self, instance, owner=None): return 'metaclass'
... 
>>> class A(metaclass=M):
...     def __get__(self, instance, owner=None): return 'class'
>>> ClassMethod(A).__get__('foo')
'class'
>>> classmethod(A).__get__('foo')
'metaclass'

Answer 1

与引入新函数 getclassattr 和 hasclassattr 以在属性查找期间绕过实例属性（如 implicit lookup of special methods）不同，另一种方法是引入代理类（我们称之为 bypass），它覆盖方法__getattribute__。我认为这可能是一种更好的方法，因为方法__getattribute__ 是一个为自定义属性查找而设计的钩子，它可以与内置函数getattr 和hasattr 一起使用，也可以与属性检索运算符. 一起使用：

class bypass:

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

    def __getattribute__(self, name):
        obj = super().__getattribute__('subject')
        classmro = vars(type)['__mro__'].__get__(type(obj))
        for cls in classmro:
            classdict = vars(type)['__dict__'].__get__(cls)
            if name in classdict:
                attr = classdict[name]
                attrclassmro = vars(type)['__mro__'].__get__(type(attr))
                for attrclass in attrclassmro:
                    attrclassdict = vars(type)['__dict__'].__get__(attrclass)
                    if '__get__' in attrclassdict:
                        return attrclassdict['__get__'](attr, obj, type(obj))
                return attr
        classname = vars(type)['__name__'].__get__(type(obj))
        raise AttributeError(f'{classname!r} object has no attribute {name!r}')

class M(type):
    x = 'metaclass'

class A(metaclass=M):
    x = 'class'

a = A()
a.x = 'object'

assert getattr(a, 'x') == 'object' and getattr(bypass(a), 'x') == 'class'
assert getattr(A, 'x') == 'class' and getattr(bypass(A), 'x') == 'metaclass'