Dataclass fields not being fetched for resolve_types

[praateekmahajan]

Disclaimer : This might be a cattrs issue (please let me know if I should close this and repost there).

Raising here since after my limited investigation it seems like it might be solvable within _funcs.py (might break other things). Per my understanding, the issue primarily stems from PEP 563. So the behavior changes based on if you use from __future__ import annotations in your code.

from __future__ import annotations # comment this line and rerun for success
from dataclasses import dataclass
from cattr import structure


@dataclass
class Foo:
    bar: int


print(structure({"bar": 123}, Foo))

This seems to be the code flow

1. gen_structure_attrs_fromdict is called https://github.com/python-attrs/cattrs/blob/20ca64405944ea81ac207e070e180d2d805c8fd1/src/cattr/converters.py#L730-L734
2. That calls fields in _compat.py in cattr. This fields works as expected. https://github.com/python-attrs/cattrs/blob/20ca64405944ea81ac207e070e180d2d805c8fd1/src/cattr/_compat.py#L49-L56
3. For each of the field above if isisntance(field, str) then resolve_types is called in _funcs.py in attr.

   attrs/src/attr/_funcs.py

   Line 413 in 3df7bbe

   for field in fields(cls) if attribs is None else attribs:

4. The resolve_types calls the fields in _make.py which raises NotAnAttrsClassError

   attrs/src/attr/_make.py

   Lines 2029 to 2054 in 3df7bbe

   	def fields(cls):
   	"""
   	Return the tuple of ``attrs`` attributes for a class.
   	The tuple also allows accessing the fields by their names (see below for
   	examples).
   	:param type cls: Class to introspect.
   	:raise TypeError: If *cls* is not a class.
   	:raise attr.exceptions.NotAnAttrsClassError: If *cls* is not an ``attrs``
   	class.
   	:rtype: tuple (with name accessors) of `attrs.Attribute`
   	.. versionchanged:: 16.2.0 Returned tuple allows accessing the fields
   	by name.
   	"""
   	if not isclass(cls):
   	raise TypeError("Passed object must be a class.")
   	attrs = getattr(cls, "__attrs_attrs__", None)
   	if attrs is None:
   	raise NotAnAttrsClassError(
   	"{cls!r} is not an attrs-decorated class.".format(cls=cls)
   	)
   	return attrs

To me it seems like the fields in step 2 and 4 could/should be the same? I might be missing context, happy to work on this given some general guidance.

Thank you for the two powerful libraries!

[Tinche]

Does it work with cattrs 1.10?

[euresti]

Did you mean to do from dataclasses import dataclass? Because then it's not an attrs class.

[praateekmahajan]

@Tinche Just saw python-attrs/cattrs#195 and tried 1.10 out, seems like that works.

@euresti dataclasses was intentional, opened the issue in attr since seemed like a generic solution might belong in attr but seems like its being taken care of in cattrs.

Thanks, will close the ticket!