Type checking fails when registering a structure hook on a Union

[tdsmith]

• cattrs version: 1.1.1
• Python version: 3.8.6, mypy 0.790
• Operating System: macOS 10.15.7

Description

After upgrading from cattrs 1.0, mypy has started rejecting Union types as the type argument to Converter.register_structure_hook.

I think the docs at https://cattrs.readthedocs.io/en/latest/structuring.html#manual-disambiguation say that this is the supported way to structure a union type. It executes fine, but mypy rejects it.

What I Did

from typing import Any, ClassVar, List, Literal, Union, Type

import attr
import cattr

@attr.s(auto_attribs=True)
class Foo:
    kind: ClassVar[Literal["foo"]] = "foo"
    value: int

@attr.s(auto_attribs=True)
class Bar:
    kind: ClassVar[Literal["bar"]] = "bar"
    value: int

FooBar = Union[Foo, Bar]

def discriminate_foobar(value: Any, _klass: Type) -> FooBar:
    kind, int_value = value["kind"], value["value"]
    return Foo(int_value) if kind == "foo" else Bar(int_value)

cattr.register_structure_hook(FooBar, discriminate_foobar)

print(cattr.structure([{"kind": "foo", "value": 3}, {"kind": "bar", "value": 42}], List[FooBar]))

$ python demo2.py
[Foo(value=3), Bar(value=42)]

$ mypy demo2.py
demo2.py:22: error: Argument 1 has incompatible type "object"; expected "Type[Any]"

[Tinche]

Looks like a mypy bug to me, feel free to go over to their issue tracker and log this.

For example,

>>> reveal_type(Union[int, float])
note: Revealed type is 'builtins.object'

[tdsmith]

I went over to the python/typing gitter and got some feedback from oremanj that this is correct. Unions describe types but don't meet the type contract; they e.g. don't work with isinstance() or issubclass() and they're not notionally instantiable.

We also can't express "a Type or a Union of Types" directly; you can't construct a type hint that accepts any old Union because the type of a Union[...] isn't a Union, it's just object.

Do you buy that? I could go ahead and open an issue against mypy to collect some more feedback, especially on the second point -- my guess is that mypy will be reluctant to make a change but we can explain the use case.

[Tinche]

Yeah, I buy that a subset of the type system doesn't fulfil the Type[T] contract. Looks like a Sequence[int] isn't a valid Type[T] either.

This means we can't annotate this method properly, though. It would be nice to have a type to express a valid type hint though. In any case, looks like this is a missing feature instead of a bug in Mypy. I'll take a PR changing this type annotation to Any :)

If you feel like it, feel free to make an issue over at Mypy explaining the situation.

[Tinche]

Fixed on master.