python - 类型和 __class__ 之间的区别

Question

Django 源代码:

def new_method_proxy(func):
    def inner(self, *args):
        if self._wrapped is empty:
            self._setup()
        return func(self._wrapped, *args)
    return inner

class LazyObject(object):

    _wrapped = None

    def __init__(self):
        self._wrapped = empty

    __getattr__ = new_method_proxy(getattr)

    def _setup(self):
        # Must be implemented by subclasses to initialize the wrapped object.
        raise NotImplementedError('subclasses of LazyObject must provide a _setup() method')

    def __deepcopy__(self, memo):
        if self._wrapped is empty:
            # We have to use type(self), not self.__class__, because the
            # latter is proxied.
            result = type(self)()
            memo[id(self)] = result
            return result
        return copy.deepcopy(self._wrapped, memo)

在__deepcopy__()函数中，注释:

We have to use type(self), not self.__class__, because the latter is proxied.

这是什么意思？ type() 和 __class__ 有什么区别？

Answer 1

在 source code 中稍往下:

# Need to pretend to be the wrapped class, for the sake of objects that
# care about this (especially in equality tests)
__class__ = property(new_method_proxy(operator.attrgetter("__class__")))

在这里您可以看到 __class__ 属性(最终只是一个普通属性)被自定义定义覆盖了。这个新定义是一个返回代理类值的属性。

type(obj) 绕过新的 __class__ 属性，并返回 LazyObject 类，而不是代理类。