id	summary	reporter	owner	description	type	status	component	version	severity	resolution	keywords	cc	stage	has_patch	needs_docs	needs_tests	needs_better_patch	easy	ui_ux
36342	Slicing a QuerySet with a result cache results in a list?	Willem Van Onsem		"As [https://github.com/django/django/blob/adf2991d32c24f8b2e549a25a7eda52f317a91a6/django/db/models/query.py#L401-L436 per code] if you slice a QuerySet with a result cache, we return the sliced result cache, this thus means that for a `QuerySet`:

{{{
from django.contrib.auth.model import User

qs = User.objects.all()
bool(qs)  # enable/disable

qs[:3].values('id')
}}}

will raise an error because `qs[:3]` returns a list, whereas if we comment out the `bool`, it will still work.

This is done probably because of performance reasons: if we have a `QuerySet`, and we already know the results, we can just work with these results.

But I'm wondering if we ""can have the cake and eat it too"". We could for example create a sliced copy of the queryset, and populate the result cache of the queryset. Something along the lines of:

{{{
    def __getitem__(self, k):
        """"""Retrieve an item or slice from the set of results.""""""
        if not isinstance(k, (int, slice)):
            raise TypeError(
                ""QuerySet indices must be integers or slices, not %s.""
                % type(k).__name__
            )
        if (isinstance(k, int) and k < 0) or (
            isinstance(k, slice)
            and (
                (k.start is not None and k.start < 0)
                or (k.stop is not None and k.stop < 0)
            )
        ):
            raise ValueError(""Negative indexing is not supported."")

        # remove below
        # if self._result_cache is not None:
        #    return self._result_cache[k]

        if isinstance(k, slice):
            qs = self._chain()
            if k.start is not None:
                start = int(k.start)
            else:
                start = None
            if k.stop is not None:
                stop = int(k.stop)
            else:
                stop = None
            qs.query.set_limits(start, stop)
            if self._result_cache is not None:
                # populate the QuerySet
                qs._result_cache = self._result_cache[k]
            return list(qs)[:: k.step] if k.step else qs
}}}

this thus means that, (a) unless we use a step, we always get a `QuerySet` for slicing (since it is not always known in advance *if* the `QuerySet` has a result cache, that can be the result of complicated code flows); and (b) if the result cache was present, the queryset we generate will not have to fetch the data, if we don't make more `QuerySet` calls."	Cleanup/optimization	closed	Database layer (models, ORM)	5.2	Normal	invalid			Unreviewed	0	0	0	0	0	0