Pickling a subclass that adds a new attribute doesn’t seem to include the new attribute. For example:

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)

produces the following exception:

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'

The details attribute of MySession didn’t get saved in the pickle file. But why?

Disregard below this line:
Now I am just going to ramble because SO thinks my post contains too much code and not enough details, but when a picture is worth a thousand words, why do we need to add 10,000 words to a pretty self explanatory picture?