[Django]-Django: When to use QuerySet none()

In cases where you want to append to querysets but want an empty one to begin with
Similar to conditions where we instantiate an empty list to begin with but gradually keep appending meaningful values to it
example..

def get_me_queryset(conditionA, conditionB, conditionC):
    queryset = Model.objects.none()

    if conditionA:
        queryset |= some_complex_computation(conditionA)
    elif conditionB:
        queryset |= some_complex_computation(conditionB)

    if conditionC:
        queryset |= some_simple_computation(conditionC)

    return queryset

get_me_queryset should almost always return instance of django.db.models.query.QuerySet (because good programming) and not None or [], or else it will introduce headaches later..

This way even if none of the conditions come True, your code will still remain intact. No more type checking

For those who do not undestand | operator’s usage here:

queryset |= queryset2

It translates to:

queryset = queryset + queryset

another use of queryset.none is when you don’t know if there will be objects but do not want to raise an error.

example:

class DummyMixin(object):

    def get_context_data(self,**kwargs):
        """ Return all the pks of objects into the context """

        context = super(DummyMixin, self).get_context_data(**kwargs)

        objects_pks = context.get(
            "object_list",
            Mymodel.objects.none()
        ).values_list("pk", flat=True)

        context["objects_pks"] = objects_pks

Another good use case for this is if some calling method wants to call .values_list() or similar on results. If the method returned None, you’d get an error like

AttributeError: 'list' object has no attribute 'values_list'

But if your clause returns MyModel.objects.none() instead of None, the calling code will be happy, since the returned data is an empty queryset rather than a None object.

Another way of putting it is that it allows you to not mix up return types (like “this function returns a QuerySet or None,” which is messy).

It’s useful to see where qs.none() is used in other examples in the Django docs. For example, when initializing a model formset using a queryset if you want the resulting formset to be empty the example given is:

formset = AuthorFormSet(queryset=Author.objects.none())

none() is used in get_queryset() to return an empty queryset depending on the state of has_view_or_change_permission() as shown below:

class BaseModelAdmin(metaclass=forms.MediaDefiningClass):

    # ...

    def has_view_or_change_permission(self, request, obj=None):
        return self.has_view_permission(request, obj) or self.has_change_permission(
            request, obj
        )

# ...

class InlineModelAdmin(BaseModelAdmin):

    # ...

    def get_queryset(self, request):
        queryset = super().get_queryset(request)
        if not self.has_view_or_change_permission(request):
            queryset = queryset.none() # Here
        return queryset