Over-specific return type required for ListView.get_queryset overrides

[vgel]

Bug report

What's wrong

Right now django-stubs requires that the return type of a views.generic.ListView subclass' get_queryset method be a queryset, which seems reasonable. However, django doesn't actually require that, and this bit me while adding typing to a project that uses a non-queryset iterable in a view.

See here:
https://github.com/django/django/blob/f87b0ecd37e64e7a019d472de37d0789a8790f1f/django/views/generic/list.py#L21

The return value must be an iterable and may be an instance of QuerySet in which case QuerySet specific behavior will be enabled. (emphasis mine)

V.s. here:

django-stubs/django-stubs/views/generic/list.pyi

Line 20 in a8fdd4c

def get_queryset(self) -> QuerySet: ...

Repro case:

from typing import Iterable, Any
from django.views import generic

class TestcaseView(generic.ListView):
    def get_queryset(self) -> Iterable[Any]:
        return [1, 2, 3]

Message:

Return type "Iterable[Any]" of "get_queryset" incompatible with return type "QuerySet[Any]" in supertype "MultipleObjectMixin"

This issue might also affect other view methods -- for example it seems DetailView also is required to require a QuerySet right now:

django-stubs/django-stubs/views/generic/detail.pyi

Line 17 in a8fdd4c

def get_queryset(self) -> models.query.QuerySet: ...

, but I haven't tested whether django agrees with that restriction.

System information

• OS: Linux marina 5.4.0-7634-generic CharField with blank=True should not be considered nullable #38159249712920.04~9a1ea2e-Ubuntu SMP Fri Jun 19 22:43:37 UTC x86_64 x86_64 x86_64 GNU/Linux
• python version: 3.8.2
• django version: 3.1.2
• mypy version: 0.782
• django-stubs version: 1.6.0

I can submit a pull for this if there's interest.

[LucidDan]

PR #897 seems to be aimed at changing this but isn't linked to the issue.

One concern I have - if this is changed to an Iterable[T] or even Union[Iterable[T], QuerySet[T]], suddenly anyone relying on the Django default CBV behaviour, which is that it WILL always be a QuerySet will need to start doing an isinstance() check before using queryset methods, or they'll get type errors; e.g

qs = get_queryset()
ids = qs.values_list("id") if isinstance(QuerySet, qs) else [obj.id for obj in qs]

But if you are just using the standard CBV classes, the isinstance check should be unnecessary, it will always be a queryset.

Is there a better solution?

[flaeppe]

With the changes in #2191 and the argument in #2174 in mind, I will close this as a won't fix. Since it's totally fair that get_queryset returns a QuerySet