python 使用 __dict__ 获取对象属性

Question

使用 py3，我有一个使用 @property 装饰器的对象

class O(object):
    def __init__(self):
        self._a = None

    @property
    def a(self):
        return 1

通过 __dict__(使用 _a)访问属性 a 似乎不会返回属性修饰值，而是返回初始化值 None

o = O()
print(o.a, o.__dict__['_a'])
>>> 1, None

是否有一种通用的方法来完成这项工作？我主要需要这个

def __str__(self):
    return ' '.join('{}: {}'.format(key, val) for key, val in self.__dict__.items())

Answer 1

当然 self.__dict__["_a"] 将返回 self._a(实际上它是相反的 - self._a 将返回 self.__dict__["_a"] - 但无论如何)，而不是 self.a。 property 在这里做的唯一一件事就是自动调用它的 getter(你的 a(self) 函数)所以你不必输入括号，否则它只是一个普通的方法调用。

如果你也想要一些与属性一起工作的东西，你必须手动从 dir(self.__class__) 和 getattr(self.__class__, name)，即:

def __str__(self):
    # py2
    attribs = self.__dict__.items()
    # py3
    # attribs = list(self.__dict__.items())

    for name in dir(self.__class__):
        obj = getattr(self.__class__, name)
        if isinstance(obj, property):
           val = obj.__get__(self, self.__class__)
           attribs.append((name, val))

    return ' '.join('{}: {}'.format(key, val) for key, val in attribs)

请注意，这不会阻止 _a 出现在 attribs 中 - 如果您想避免这种情况，您还必须从 中过滤掉 protected 名称>attribs 列表(所有 protected 名称，因为您要求的是通用名称):

def __str__(self):
    attribs = [(k, v) for k, v in self.__dict__.items() if not k.startswith("_")]

    for name in dir(self.__class__):
        # a protected property is somewhat uncommon but
        # let's stay consistent with plain attribs
        if name.startswith("_"):
            continue  
        obj = getattr(self.__class__, name)
        if isinstance(obj, property):
           val = obj.__get__(self, self.__class__)
           attribs.append((name, val))

    return ' '.join('{}: {}'.format(key, val) for key, val in attribs)

另请注意，这不会处理其他计算属性(property 只是描述符协议(protocol)的一种通用实现)。在这一点上，对于仍然尽可能通用但可以根据需要进行自定义的东西，最好的选择是将上面的内容实现为带有几个用于特化的钩子(Hook)的混合类:

class PropStrMixin(object):

    # add other descriptor types you want to include in the 
    # attribs list
    _COMPUTED_ATTRIBUTES_CLASSES = [property,] 

    def _get_attr_list(self):
        attribs = [(k, v) for k, v in self.__dict__.items() if not k.startswith("_")]

        for name in dir(self.__class__):
            # a protected property is somewhat uncommon but
            # let's stay consistent with plain attribs
            if name.startswith("_"):
                continue  
            obj = getattr(self.__class__, name)
            if isinstance(obj, *self._COMPUTED_ATTRIBUTES_CLASSES):
               val = obj.__get__(self, self.__class__)
               attribs.append((name, val))
        return attribs 

    def __str__(self):
        attribs = self._get_attr_list()
        return ' '.join('{}: {}'.format(key, val) for key, val in attribs)


class YouClass(SomeParent, PropStrMixin):
    # here you can add to _COMPUTED_ATTRIBUTES_CLASSES
    _COMPUTED_ATTRIBUTES_CLASSES = PropStrMixin + [SomeCustomDescriptor])