CachedResponse.history is not fully deserialized on python 3.7 and 3.8

[JWCook]

Found during #835; related to python-attrs/cattrs#206

Example error:

requests_cache/session.py:103: in get
    return self.request('GET', url, params=params, **kwargs)
requests_cache/session.py:159: in request
    return super().request(method, url, *args, headers=headers, **kwargs)  # type: ignore
.venv/lib/python3.7/site-packages/requests/sessions.py:589: in request
    resp = self.send(prep, **send_kwargs)
requests_cache/session.py:195: in send
    actions.update_from_cached_response(cached_response, self.cache.create_key, **kwargs)
requests_cache/policy/actions.py:194: in update_from_cached_response
    elif create_key and not self._validate_vary(cached_response, create_key, **key_kwargs):
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

self = CacheActions(expire_after=-1)
cached_response = CachedResponse(_content=b'mock redirect response with Vary header (2/2)', created_at='2023-06-05 17:56:56.815238', ela.../requests-cache.com/vary-redirect-target'), status_code=200, url='http+mock://requests-cache.com/vary-redirect-target')
create_key = <bound method BaseCache.create_key of <SQLiteCache(name=/tmp/pytest-of-runner/pytest-0/popen-gw1/test_match_headers__vary_with_4/24501901-26eb-4b6b-a6d0-6b12df83c48b.db)>>
key_kwargs = {'allow_redirects': True, 'cert': None, 'match_headers': ['Accept-Language'], 'proxies': OrderedDict(), ...}
vary = 'Accept-Language'

    def _validate_vary(
        self, cached_response: 'CachedResponse', create_key: KeyCallback, **key_kwargs
    ) -> bool:
        """If the cached response contains Vary, check that the specified request headers match"""
        vary = cached_response.headers.get('Vary')
        if not vary:
            return True
        elif vary == '*':
            return False
    
        # Generate a secondary cache key based on Vary for both the cached request and new request.
        # If there are redirects, compare the new request against the last request in the chain.
        key_kwargs['match_headers'] = [k.strip() for k in vary.split(',')]
        vary_request = (
            cached_response.history[-1].request
>           if cached_response.history
            else cached_response.request
        )
E       AttributeError: 'dict' object has no attribute 'request'

requests_cache/policy/actions.py:308: AttributeError

CachedResponse.history should contain CachedResponse when there is redirect history available. Instead, it contains dicts, meaning the structure hook is not working.

[JWCook]

Posting this for future reference.

It turns out that CachedResponse.history wasn't being fully deserialized on python<=3.8 due to an unevaluated ForwardRef, causing the cattrs unstructure hook to fail.

When not yet evaluated, ForwardRef.__forward_value__ is None, so this call:

converter.structure(obj, cls.__forward_value__)

no longer works.

I'm unsure why this behaves differently on python 3.9+. Anyway, the workaround is to manually evaluate the ForwardRef:

if not cls.__forward_evaluated__:
    cls._evaluate(globals(), locals())

In python 3.9+, the call would be:

cls._evaluate(globals(), locals(), frozenset())

but that seems to be unnecessary here.