can't structure dataclass with PEP563 postponed annotations or forward references

[anthrotype]

• cattrs version: 1.8.0
• Python version: 3.10.0
• Operating System: macOS 12.0.1

Description

I would like to use cattrs with dataclasses (for this project I can't use attrs classes). I get the following errors when I try to structure a dataclass with circular, nested definitions that contain string literals to be post-evaluated (e.g. via typing.get_type_hints).
It seems that this currently only works with attrs classes, but not dataclasses.

What I Did

from __future__ import annotations
from typing import List
from dataclasses import dataclass, field
from cattr import Converter, GenConverter


@dataclass
class Node:
    children: List[Node] = field(default_factory=list)


tree = Node([Node([Node(), Node()]), Node()])

c = GenConverter()
data = c.unstructure(tree)
# Raises: NotAnAttrsClassError: <class '__main__.Node'> is not an attrs-decorated class.
c.structure(data, Node)


c = Converter()
data = c.unstructure(tree)
# Raises: StructureHandlerNotFoundError: Unsupported type: List[Node]. Register a structure hook for it.
c.structure(data, Node)

If I remove from __future__ import annotations and type the nested children: List["Node"] I get this other error:

StructureHandlerNotFoundError: Unsupported type: ForwardRef('Node'). Register a structure hook for it.

[anthrotype]

Note the same happens with the newly released cattrs 1.9.0

[Tinche]

Hm, for attrs classes I use attr.resolve_types, and that doesn't exist for dataclasses. I can try using typing.get_type_hints directly, but that won't cover some of the use cases (like when you need to resolve them yourself, in a special way).

[anthrotype]

using typing.get_type_hints directly

yeah, I suppose that's the way to go. You can have a resolve_types in _compat that delegates to attrs.resolve_types for attrs classes or does the same thing for dataclasses.

won't cover some of the use cases (like when you need to resolve them yourself, in a special way)

not sure what you mean by that

[Tinche]

So for example cattrs doing resolve_types on a class can fail. That's why resolve_types takes some arguments so you can help it before you actually use the class.

Also resolve_types is a stateful change - it updates the fields. get_type_hints just returns the annotations. It'll probably be enough to start though.

[Tinche]

I just pushed support for this to the main branch, feel free to test it out.

[anthrotype]

Thanks, it works now!