PEP 649 - ForwardRef

pep649.py

import dataclasses
import typing
from typing import *

TypeAlias = Any

# ******* Implementation *******


@dataclasses.dataclass
class ForwardRef:
  resolve: Callable[[], Any]


def type_hints(cls):
  return {
      k: resolve_forward_ref(v) for k, v in cls.__annotations__.items()
  }


def resolve_forward_ref(hint: TypeAlias):
    origin = typing.get_origin(hint)
    return _ORIGIN_TO_RESOLVER[origin](hint)


def _list_resolver(hint: TypeAlias):
    (item_hint,) = typing.get_args(hint)
    return list[resolve_forward_ref(item_hint)]


def _union_resolver(hint: TypeAlias):
    item_hints = typing.get_args(hint)
    return Union[tuple(resolve_forward_ref(i) for i in item_hints)]


def _default_resolver(hint: TypeAlias):
    if isinstance(hint, ForwardRef):
      return hint.resolve()
    return hint


_ORIGIN_TO_RESOLVER = {
    list: _list_resolver,
    typing.Union: _union_resolver,
    None: _default_resolver,
}


# ******* Example *******

@dataclasses.dataclass
class User:
    name: str
    group: ForwardRef(lambda: Group)

@dataclasses.dataclass
class Group:
    name: str
    admins: list[ForwardRef(lambda: User)]


def fn():

  @dataclasses.dataclass
  class User0:
      name: str
      group: ForwardRef(lambda: Group0)

  @dataclasses.dataclass
  class Group0:
      name: str
      admins: list[ForwardRef(lambda: User0)]
  
  return User0, Group0


cls0, cls1 = fn()



print(type_hints(User))
print(type_hints(Group))
print(type_hints(cls0))
print(type_hints(cls1))

This display:

{'name': <class 'str'>, 'group': <class '__main__.Group'>}
{'name': <class 'str'>, 'admins': list[__main__.User]}
{'name': <class 'str'>, 'group': <class '__main__.fn.<locals>.Group0'>}
{'name': <class 'str'>, 'admins': list[__main__.fn.<locals>.User0]}