Type checking fails when registering a generic structure hook #106
Description
- cattrs version: 1.1.1
- Python version: 3.8.6, mypy 0.790
- Operating System: macOS 10.15.7
Description
I might be misspecifying it, but I'm not sure how to register a generic structure hook function that can return more than one type.
mypy didn't complain about cattrs 1.0 and it continues to execute fine.
What I Did
from typing import Any, Protocol, Type, TypeVar
import attr
import cattr
class Nameable(Protocol):
def __init__(self, name: str) -> None: ...
@attr.s(auto_attribs=True)
class A:
name: str
@attr.s(auto_attribs=True)
class B:
name: str
@attr.s(auto_attribs=True)
class Widget:
spam: A
eggs: B
T = TypeVar("T", bound=Nameable)
def instantiate_from_name(value: Any, klass: Type[T]) -> T:
return klass(name=value)
cattr.register_structure_hook(A, instantiate_from_name)
cattr.register_structure_hook(B, instantiate_from_name)
print(cattr.structure({"spam": "Hormel", "eggs": "Farm Fresh"}, Widget))$ python demo3.py
Widget(spam=A(name='Hormel'), eggs=B(name='Farm Fresh'))
$ mypy demo3.py
demo3.py:33: error: Argument 2 has incompatible type "Callable[[Any, Type[T]], T]"; expected "Callable[[Any, Type[T]], A]"
demo3.py:34: error: Argument 2 has incompatible type "Callable[[Any, Type[T]], T]"; expected "Callable[[Any, Type[T]], B]"
Found 2 errors in 1 file (checked 1 source file)
Changing the signature on register_structure_hook from self, cl: Type[T], func: Callable[[Any, Type[V]], T] to self, cl: Type[T], func: Callable[[Any, Type[T]], T] resolves this and I'm not sure why the latter is wrong but I'm probably missing some subtlety.