[1.5.x] Fixed #19261 -- Delayed Queryset evaluation in paginators

Thanks trbs for the report and the patch.
Backport of 1b307d6c8f from master.

django/core/paginator.py

@@ -1,5 +1,8 @@
 from math import ceil
 
+from django.utils import six
+
+
 class InvalidPage(Exception):
     pass
 
@@ -88,6 +91,8 @@ class Page(object):
         return len(self.object_list)
 
     def __getitem__(self, index):
+        if not isinstance(index, (slice,) + six.integer_types):
+            raise TypeError
         # The object_list is converted to a list so that if it was a QuerySet
         # it won't be a database hit per __getitem__.
         return list(self.object_list)[index]

tests/regressiontests/pagination/tests.py

@@ -266,3 +266,25 @@ class ModelPaginationTests(TestCase):
         self.assertEqual(1, p.previous_page_number())
         self.assertEqual(6, p.start_index())
         self.assertEqual(9, p.end_index())
+
+    def test_page_getitem(self):
+        """
+        Tests proper behaviour of a paginator page __getitem__ (queryset
+        evaluation, slicing, exception raised).
+        """
+        paginator = Paginator(Article.objects.all(), 5)
+        p = paginator.page(1)
+
+        # Make sure object_list queryset is not evaluated by an invalid __getitem__ call.
+        # (this happens from the template engine when using eg: {% page_obj.has_previous %})
+        self.assertIsNone(p.object_list._result_cache)
+        self.assertRaises(TypeError, lambda: p['has_previous'])
+        self.assertIsNone(p.object_list._result_cache)
+
+        # Make sure slicing the Page object with numbers and slice objects work.
+        self.assertEqual(p[0], Article.objects.get(headline='Article 1'))
+        self.assertQuerysetEqual(p[slice(2)], [
+                "<Article: Article 1>",
+                "<Article: Article 2>",
+            ]
+        )