Pickling子类不保存新的实例属性

Question

Pickling 添加新属性的子类似乎不包含新属性。 例如：

import requests
import pickle

class MySession(requests.Session):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.details = False

data = MySession()
file = "/tmp/pickle"

with open(file, "wb") as f:
    from pprint import pprint
    print(f"Saving session {data} to {file} with {data.details}")
    pprint(data.__dict__)
    pickle.dump(data, f)
print("Validating")
with open(file, "rb") as f:
    newdata = pickle.load(f)
    pprint(newdata.__dict__)
    print(data.details)
    print(newdata.details)

产生以下异常：

Saving session <__main__.MySession object at 0x7f0976cb9400> to /tmp/pickle with False
{'adapters': OrderedDict([('https://',
                           <requests.adapters.HTTPAdapter object at 0x7f0975dca4e0>),
                          ('http://',
                           <requests.adapters.HTTPAdapter object at 0x7f0976be9ca0>)]),
 'auth': None,
 'cert': None,
 'cookies': <RequestsCookieJar[]>,
 'details': False,
 'headers': {'User-Agent': 'python-requests/2.32.3', 'Accept-Encoding': 'gzip, deflate, br', 'Accept': '*/*', 'Connection': 'keep-alive'},
 'hooks': {'response': []},
 'max_redirects': 30,
 'params': {},
 'proxies': {},
 'stream': False,
 'trust_env': True,
 'verify': True}
Validating
{'adapters': OrderedDict([('https://',
                           <requests.adapters.HTTPAdapter object at 0x7f09759c1010>),
                          ('http://',
                           <requests.adapters.HTTPAdapter object at 0x7f0975a4e960>)]),
 'auth': None,
 'cert': None,
 'cookies': <RequestsCookieJar[]>,
 'headers': {'User-Agent': 'python-requests/2.32.3', 'Accept-Encoding': 'gzip, deflate, br', 'Accept': '*/*', 'Connection': 'keep-alive'},
 'hooks': {'response': []},
 'max_redirects': 30,
 'params': {},
 'proxies': {},
 'stream': False,
 'trust_env': True,
 'verify': True}
False
Traceback (most recent call last):
  File "/tmp/repro.py", line 22, in <module>
    print(newdata.details)
          ^^^^^^^^^^^^^^^
AttributeError: 'MySession' object has no attribute 'details'

details

的

MySession

属性未保存在 pickle 文件中。 但为什么呢？

忽略这一行下面的内容： 现在我只是要闲聊，因为 SO 认为我的帖子包含太多代码而没有足够的细节，但是当一张图片值一千个字时，为什么我们需要在一张非常不言自明的图片上添加 10,000 个字？

Answer 1

这是因为

requests.Session

类定义了自定义

__getstate__

和

__setstate__

方法：

    def __getstate__(self):
        state = {attr: getattr(self, attr, None) for attr in self.__attrs__}
        return state

    def __setstate__(self, state):
        for attr, value in state.items():
            setattr(self, attr, value)

并且

self.__attrs__

是一个类变量，定义如下：

class Session(SessionRedirectMixin):
    """A Requests session.

    Provides cookie persistence, connection-pooling, and configuration.

    Basic Usage::

      >>> import requests
      >>> s = requests.Session()
      >>> s.get('https://httpbin.org/get')
      <Response [200]>

    Or as a context manager::

      >>> with requests.Session() as s:
      ...     s.get('https://httpbin.org/get')
      <Response [200]>
    """

    __attrs__ = [
        "headers",
        "cookies",
        "auth",
        "proxies",
        "hooks",
        "params",
        "verify",
        "cert",
        "adapters",
        "stream",
        "trust_env",
        "max_redirects",
    ]

因此，一种解决方案是实现如下所示的

__getstate__

：

def __getstate__(self):
    state = super().__getstate__()
    state['details'] = self.details
    return state