由于__attrs__，python pickle加载对象AttributeError

Question

我正在使用pickle将我的类转储到一个文件，如下所示，但是在打印加载对象的属性时我得到了AttributeError。

import cPickle as pickle
from requests.sessions import Session

class B(Session):
    def __init__(self, b):
        super(B, self).__init__()
        self.b = b

if __name__ == '__main__':
    b = B(2,)

    with open('/tmp/b.pl', 'wb') as f:
        pickle.dump(b, f)

    with open('/tmp/b.pl', 'rb') as f:
        t = pickle.load(f)
        print t.headers
        print t.b # AttributeError

无论如何，我找到了如何解决这个问题，只需定义B，如下所示：

class B(Session):
    __attrs__ = [
        'headers', 'cookies', 'auth', 'proxies', 'hooks', 'params', 'verify',
        'cert', 'prefetch', 'adapters', 'stream', 'trust_env',
        'max_redirects', 'b'
    ]
    def __init__(self, b):
        super(B, self).__init__()
        self.b = b

那时没关系。似乎原因是__attrs__，然后我试着测试一下：

class A(object):
    __attrs__ = []
    def __init__(self, a):
        self.a = a
class C(A):
    def __init__(self, a, c):
        super(C, self).__init__(a)
        self.c = c

if __name__ == '__main__':
    b = B(2,)
    c = C(1, 2)
    with open('/tmp/c.pl', 'wb') as f:
        pickle.dump(c, f)

    with open('/tmp/c.pl', 'rb') as f:
        t = pickle.load(f)
        print t.a
        print t.c

但是，我没有错误，为什么会这样？