kw_only tuple support?

• cattrs version: 1.10.0
• Python version: 3.10.2
• Operating System: macOS

Description

I have a nested data structure using attrs and kw_only. Converting to a tuple works beautifully, letting me load this structure into a SQLite3 database.

import attrs
import cattrs

@attrs.define(kw_only=True)
class A:
    a: str = ""

@attrs.define(kw_only=True)
class B(A):
    b: int

# PS: The reason for kw_only is primarily because it makes subclassing work properly
# for building data structures with arbitrary defaults - the above requires
# kw_only in either attrs or Python 3.10's dataclasses.

converter = cattrs.GenConverter()
converter.unstructure_attrs_astuple(B(b=2)) # -> ('', 2)

However, the other direction fails:

>>> converter.structure_attrs_fromtuple(('', 2), B)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/henryschreiner/git/CMS/hypernewsviewer/.venv/lib/python3.10/site-packages/cattr/converters.py", line 424, in structure_attrs_fromtuple
    return cl(*conv_obj)  # type: ignore
TypeError: B.__init__() takes 1 positional argument but 3 were given

My current workaround is a little ugly and possibly not very efficient:

>>> converter.structure_attrs_fromdict({n.name:i for n,i in zip(attrs.fields(B), ('', 2))}, B)
B(a='', b=2) 

Would it be possible to make fromtuple work on a kw_only class, like astuple does? IMO, kw_only is about subclassing & controlling manual creation, not raw conversion.

Might be hard/impossible, but thought I'd ask.

Start in cattr/converters.py at structure_attrs_fromtuple, then compare it with unstructure_attrs_astuple and the structure_attrs_fromdict workaround shown in the issue. Reproduce the B example with Python 3.10 and attrs kw_only classes. Done means tuple conversion constructs B(a='', b=2) without passing positional arguments, with tests covering the nested inheritance case.