Error when running on python 3.10.2 & 3.9.10 - TypeError: ForwardRef('...') is not a class

[cheginit]

The problem

It seems that requests-cache is not compatible with Python 3.10.2 (it works with 3.10.1).

$ python -c "from requests_cache import CachedSession"             
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/tchegini/.local/apps/mambaforge/envs/ts/lib/python3.10/site-packages/requests_cache/__init__.py", line 11, in <module>
    from .backends import *
  File "/Users/tchegini/.local/apps/mambaforge/envs/ts/lib/python3.10/site-packages/requests_cache/backends/__init__.py", line 7, in <module>
    from .base import BaseCache, BaseStorage, DictStorage
  File "/Users/tchegini/.local/apps/mambaforge/envs/ts/lib/python3.10/site-packages/requests_cache/backends/base.py", line 18, in <module>
    from ..serializers import init_serializer
  File "/Users/tchegini/.local/apps/mambaforge/envs/ts/lib/python3.10/site-packages/requests_cache/serializers/__init__.py", line 6, in <module>
    from .preconf import (
  File "/Users/tchegini/.local/apps/mambaforge/envs/ts/lib/python3.10/site-packages/requests_cache/serializers/preconf.py", line 27, in <module>
    CattrStage()
  File "/Users/tchegini/.local/apps/mambaforge/envs/ts/lib/python3.10/site-packages/requests_cache/serializers/cattrs.py", line 32, in __init__
    self.converter = init_converter(factory)
  File "/Users/tchegini/.local/apps/mambaforge/envs/ts/lib/python3.10/site-packages/requests_cache/serializers/cattrs.py", line 69, in init_converter
    converter.register_structure_hook(
  File "/Users/tchegini/.local/apps/mambaforge/envs/ts/lib/python3.10/site-packages/cattr/converters.py", line 269, in register_structure_hook
    self._structure_func.register_cls_list([(cl, func)])
  File "/Users/tchegini/.local/apps/mambaforge/envs/ts/lib/python3.10/site-packages/cattr/dispatch.py", line 57, in register_cls_list
    self._single_dispatch.register(cls, handler)
  File "/Users/tchegini/.local/apps/mambaforge/envs/ts/lib/python3.10/functools.py", line 856, in register
    raise TypeError(
TypeError: Invalid first argument to `register()`. ForwardRef('CachedResponse') is not a class.

Expected behavior

Since Python 3.10.2 is a patch release requests-cache should work.

Steps to reproduce the behavior

Create an environment with python 3.10.2 and then just try importing CachedResponse:

python -c "from requests_cache import CachedSession"

Workarounds

Not that I know of.

Environment

• requests-cache version: Tried with both the stable release, 0.9.0 and master from Github
• Python version: 3.10.2
• Platform: MacOSX 12.1

[JWCook]

Thanks for reporting this. This might be a difficult one to figure out.

I don't see anything in the python 3.10.2 changelog that indicates different behavior with ForwardRef. This may be something that would require changes in the cattrs library, but I'm not 100% sure yet.

I should have some time to look into this next week. If I can't figure out the root cause, I can at least make an ugly temporary workaround.

[cheginit]

I don't see anything in the python 3.10.2 changelog that indicates different behavior with ForwardRef.

Right, I was kinda surprised too.

I should have some time to look into this next week. If I can't figure out the root cause, I can at least make an ugly temporary workaround.

Thanks, that would be great.

[JWCook]

While writing up python-attrs/cattrs#206, I found a fix for this in python-attrs/cattrs#201 (comment).

Brief explanation for anyone else who encounters a similar problem: In python 3.10.2, some behavior unexpectedly changed regarding ForwardRef and register() with functools single-dispatch generic functions.

This caused the following method of structuring ForwardRef (using cattrs) to fail:

from attr import define, field
from cattr import GenConverter
from typing import ForwardRef, List

@define()
class MyClass:
    history: List['MyClass'] = field(factory=list)

converter = GenConverter()

# Resolve MyClass forward reference during deserialization
converter.register_structure_hook(
    ForwardRef('MyClass'),
    lambda obj, _: converter.structure(obj, MyClass)
)

A solution for this is to register a function that tests whether a type is a ForwardRef, rather than registering a ForwardRef instance:

# Resolve any forward references during deserialization
converter.register_structure_hook_func(
    lambda cls: cls.__class__ is typing.ForwardRef,
    lambda obj, cls: converter .structure(obj, cls.__forward_value__),
)

This is also an all-around better solution since it generically resolves any forward references instead of a specific one, and avoids potential naming conflicts in forward references across different modules.

[dargor]

I encountered the same problem with python 3.9.10, too. I patched with #503, now it works fine. Thanks !

[JWCook]

This fix is now in the latest stable version (requests-cache 0.9.1).

[cheginit]

@JWCook Will you be releasing 0.9.1 in conda-forge as well? The conda-forge build of my package (conda-forge/pygeoogc-feedstock#25) is failing due to requests-cache.

[JWCook]

That is automated, it just takes some time to pick up a new release from PyPI sometimes. Looks like it just finished a few minutes ago:
conda-forge/requests-cache-feedstock#31
https://anaconda.org/conda-forge/requests-cache

[cheginit]

Great! Thanks.

[Tinche]

FWIW I think they broke it with changes to singledispatch. (https://bugs.python.org/issue46032)