如何在 Django 中测试空查询集？

Question

I'm testing a view in Django that should remove all the tags from an object. For that i use this assertion:

self.assertEqual(list(Tag.objects.get_for_object(Animal.objects.get(pk=1))),[])

That works well, as i get an empty list in return. I wrapped the Django queryset in a list to avoid this:

AssertionError: [] != []

where an empty Django queryset is compared with an empty list.

But as this is not something i like a lot, i wondered if there is a nicer way to do that test.

Answer 1

只是使用exists

self.assertFalse(Tag.objects.get_for_object(Animal.objects.get(pk=1)).exists())

Answer 2

self.assertEqual(Tag.objects.get_for_object(Animal.objects.get(pk=1).count(), 0)

You could also use len() if you want to enforce the queryset being evaluated as a list!

Alternately also assertQuerysetEqual is useful, you could do a comparison with an instance 0f django.db.models.query.EmptyQuerySet ! But using count() should be the fastest way in most cases!

Answer 3

Chris's answer works in this case. But, you can also use:

# the_list = list(some_queryset)   # converting a queryset into a list
self.assertEqual(len(the_list), 0)

# to go directly from queryset:
self.assertEqual(some_queryset.count(), 0)

Answer 4

As @Bernhard pointed, you could use self.assertQuerysetEquals , and compare your query set with an empty queryset, although it is an elegant solution, it may not be efficient.

The other solutions are also valid, but here is mine:

self.assertEquals(Tag.objects.get_for_object(Animal.objects.get(pk=1), None)

I strongly suggest to don't convert a django queryset in a list. This is because converting a queryset to a list needs to load all the queryset in memory and convert it in a Python object.

For that purpose exist the queryset methods count() , exists() and etc..