"field" and "exclude" arguments of model_to_dict() do not accept sets

[mthuurne]

Bug report

What's wrong

Our test suite contains code that simplifies to this fragment:

from typing import Mapping

from django.db.models.base import Model
from django.forms import model_to_dict


def check(instance: Model, data: Mapping[str, object]) -> None:
    assert data == model_to_dict(instance, fields=data.keys())

When checking that with mypy, it reports:

testcase.py:8: error: Argument "fields" to "model_to_dict" has incompatible type "AbstractSet[str]";
expected "Union[List[Union[Callable[..., Any], str]], Sequence[str], Literal['__all__'], None]"
[arg-type]
        assert data == model_to_dict(instance, fields=data.keys())

How is that should be

The implementation of model_to_dict() only needs __bool__() and __contains__() to be provided by the fields and exclude arguments, so passing a keys set should not be flagged as an error.

I think a solution could be to replace Sequence in the stubs annotation with Collection.

System information

• OS: Ubuntu Linux 18.04
• python version: 3.6.9
• django version: 2.2.1
• mypy version: 0.812
• django-stubs version: 1.8.0

[sobolevn]

@mthuurne please, feel free to send a PR! 👍