IndexError: tuple index out of range when unstructure dataclass with typing.List annotation

[DriverX]

• cattrs version: 1.10.0
• Python version: 3.8.12
• Operating System: Centos

Description

Got IndexError: tuple index out of range error when unstructure dataclass with only List attribute annotation.

What I Did

import attr
from typing import List
import cattr


@attr.s(slots=True, kw_only=True)
class A:
    texts: List = attr.ib()


print(cattr.unstructure(A(texts=[])))

Got exceeption

Traceback (most recent call last):
  File "test.py", line 11, in <module>
    print(cattr.unstructure(A(texts=[])))
  File "python3.8/site-packages/cattr/converters.py", line 197, in unstructure
    return self._unstructure_func.dispatch(
  File "python3.8/site-packages/cattr/dispatch.py", line 49, in _dispatch
    return self._function_dispatch.dispatch(cl)
  File "python3.8/site-packages/cattr/dispatch.py", line 126, in dispatch
    return handler(typ)
  File "python3.8/site-packages/cattr/converters.py", line 722, in gen_unstructure_attrs_fromdict
    h = make_dict_unstructure_fn(
  File "python3.8/site-packages/cattr/gen.py", line 125, in make_dict_unstructure_fn
    handler = converter._unstructure_func.dispatch(t)
  File "python3.8/site-packages/cattr/dispatch.py", line 49, in _dispatch
    return self._function_dispatch.dispatch(cl)
  File "python3.8/site-packages/cattr/dispatch.py", line 126, in dispatch
    return handler(typ)
  File "python3.8/site-packages/cattr/converters.py", line 754, in gen_unstructure_iterable
    h = make_iterable_unstructure_fn(
  File "python3.8/site-packages/cattr/gen.py", line 339, in make_iterable_unstructure_fn
    type_arg = get_args(cl)[0]
IndexError: tuple index out of range

Replace List -> List[Any] or List -> Optional[List] fix this problem.

import attr
from typing import List, Optional
import cattr


@attr.s(slots=True, kw_only=True)
class A:
    texts: Optional[List] = attr.ib()


print(cattr.unstructure(A(texts=[])))

Produce correct result

{'texts': []}

[Tinche]

Ah I forgot to support the bare List. If you were using Python 3.9+ I would suggest using:

from attr import define

import cattr


@define(kw_only=True)
class A:
    texts: list


print(cattr.unstructure(A(texts=[])))

I can add a fix for this, it's not hard, but with all the possible ways of writing type annotations it's going to be tricky to get em all.

Thanks for the report.

[Tinche]

Fixed on main, alongside a test.