python - 获取描述符对象的好方法

Question

在 Python 3 中

class A(object):
    attr = SomeDescriptor()
    ...
    def somewhere(self):
        # need to check is type of self.attr is SomeDescriptor()
        desc = self.__class__.__dict__[attr_name]
        return isinstance(desc, SomeDescriptor)

有更好的方法吗？我不喜欢这个 self.__class__.__dict__ 东西

Answer 1

A.attr 导致 Python 调用 SomeDescriptor().__get__(None, A) 所以如果你有 SomeDescriptor.__get__ 返回 self 当inst 为None 时，A.attr 将返回描述符:

class SomeDescriptor():
    def __get__(self, inst, instcls):
        if inst is None:
            # instance attribute accessed on class, return self
            return self

然后你访问描述符

desc  = type(self).attr

如果属性的名称只是一个字符串，attr_name，那么您将使用

desc  = getattr(type(self), attr_name)

即使 self 是 A 的 子类 的实例，这仍然有效，而

desc = self.__class__.__dict__[attr_name]

仅当 self 是 A 的实例时才有效。

---

class SomeDescriptor():
    def __get__(self, inst, instcls):
        if inst is None:
            # instance attribute accessed on class, return self
            return self
        return 4

class A():
    attr = SomeDescriptor()
    def somewhere(self):
        attr_name = 'attr'
        desc  = getattr(type(self), attr_name)
        # desc = self.__class__.__dict__[attr_name]  # b.somewhere() would raise KeyError
        return isinstance(desc, SomeDescriptor)

这表明 A.attr 返回描述符，并且 a.somewhere() 按预期工作:

a = A()
print(A.attr)
# <__main__.SomeDescriptor object at 0xb7395fcc>    
print(a.attr)
# 4    
print(a.somewhere())
# True

这表明它也适用于 A 的子类。如果你取消评论desc = self.__class__.__dict__[attr_name]，你会看到b.somewhere() 引发 KeyError:

class B(A): pass
b = B()
print(B.attr)
# <__main__.SomeDescriptor object at 0xb7395fcc>   
print(b.attr)
# 4    
print(b.somewhere())
# True

---

顺便说一句，即使你不能完全控制 SomeDescriptor 的定义，你仍然可以将它包装在一个描述符中，当 inst 为 None 时返回 self :

def wrapper(Desc):
    class Wrapper(Desc):
        def __get__(self, inst, instcls):
            if inst is None: return self
            return super().__get__(inst, instcls)
    return Wrapper

class A():
    attr = wrapper(SomeDescriptor)()
    def somewhere(self):
        desc  = type(self).attr
        # desc = self.__class__.__dict__[attr_name]  # b.somewhere() would raise KeyError
        return isinstance(desc, SomeDescriptor)

所以没必要用

desc = self.__class__.__dict__[attr_name]

或

desc = vars(type(self))['attr']

遇到同样的问题。