Refs #32508 -- Raised TypeError instead of using "assert" on unsupported operations for sliced querysets.

django/db/models/query.py

@@ -654,9 +654,6 @@ class QuerySet:
                 "earliest() and latest() require either fields as positional "
                 "arguments or 'get_latest_by' in the model's Meta."
             )
-
-        assert not self.query.is_sliced, \
-            "Cannot change a query once a slice has been taken."
         obj = self._chain()
         obj.query.set_limits(high=1)
         obj.query.clear_ordering(force_empty=True)
@@ -664,9 +661,13 @@ class QuerySet:
         return obj.get()
 
     def earliest(self, *fields):
+        if self.query.is_sliced:
+            raise TypeError('Cannot change a query once a slice has been taken.')
         return self._earliest(*fields)
 
     def latest(self, *fields):
+        if self.query.is_sliced:
+            raise TypeError('Cannot change a query once a slice has been taken.')
         return self.reverse()._earliest(*fields)
 
     def first(self):
@@ -684,8 +685,8 @@ class QuerySet:
         Return a dictionary mapping each of the given IDs to the object with
         that ID. If `id_list` isn't provided, evaluate the entire QuerySet.
         """
-        assert not self.query.is_sliced, \
+        if self.query.is_sliced:
-            "Cannot use 'limit' or 'offset' with in_bulk"
+            raise TypeError("Cannot use 'limit' or 'offset' with in_bulk().")
         opts = self.model._meta
         unique_fields = [
             constraint.fields[0]
@@ -721,9 +722,8 @@ class QuerySet:
     def delete(self):
         """Delete the records in the current QuerySet."""
         self._not_support_combined_queries('delete')
-        assert not self.query.is_sliced, \
+        if self.query.is_sliced:
-            "Cannot use 'limit' or 'offset' with delete."
+            raise TypeError("Cannot use 'limit' or 'offset' with delete().")
-
         if self.query.distinct or self.query.distinct_fields:
             raise TypeError('Cannot call delete() after .distinct().')
         if self._fields is not None:
@@ -772,8 +772,8 @@ class QuerySet:
         fields to the appropriate values.
         """
         self._not_support_combined_queries('update')
-        assert not self.query.is_sliced, \
+        if self.query.is_sliced:
-            "Cannot update a query once a slice has been taken."
+            raise TypeError('Cannot update a query once a slice has been taken.')
         self._for_write = True
         query = self.query.chain(sql.UpdateQuery)
         query.add_update_values(kwargs)
@@ -792,8 +792,8 @@ class QuerySet:
         code (it requires too much poking around at model internals to be
         useful at that level).
         """
-        assert not self.query.is_sliced, \
+        if self.query.is_sliced:
-            "Cannot update a query once a slice has been taken."
+            raise TypeError('Cannot update a query once a slice has been taken.')
         query = self.query.chain(sql.UpdateQuery)
         query.add_update_fields(values)
         # Clear any annotations so that they won't be present in subqueries.
@@ -970,10 +970,8 @@ class QuerySet:
         return self._filter_or_exclude(True, args, kwargs)
 
     def _filter_or_exclude(self, negate, args, kwargs):
-        if args or kwargs:
+        if (args or kwargs) and self.query.is_sliced:
-            assert not self.query.is_sliced, \
+            raise TypeError('Cannot filter a query once a slice has been taken.')
-                "Cannot filter a query once a slice has been taken."
-
         clone = self._chain()
         if self._defer_next_filter:
             self._defer_next_filter = False
@@ -1163,8 +1161,8 @@ class QuerySet:
 
     def order_by(self, *field_names):
         """Return a new QuerySet instance with the ordering changed."""
-        assert not self.query.is_sliced, \
+        if self.query.is_sliced:
-            "Cannot reorder a query once a slice has been taken."
+            raise TypeError('Cannot reorder a query once a slice has been taken.')
         obj = self._chain()
         obj.query.clear_ordering(force_empty=False)
         obj.query.add_ordering(*field_names)
@@ -1175,8 +1173,8 @@ class QuerySet:
         Return a new QuerySet instance that will select only distinct results.
         """
         self._not_support_combined_queries('distinct')
-        assert not self.query.is_sliced, \
+        if self.query.is_sliced:
-            "Cannot create distinct fields once a slice has been taken."
+            raise TypeError('Cannot create distinct fields once a slice has been taken.')
         obj = self._chain()
         obj.query.add_distinct_fields(*field_names)
         return obj
@@ -1185,8 +1183,8 @@ class QuerySet:
               order_by=None, select_params=None):
         """Add extra SQL fragments to the query."""
         self._not_support_combined_queries('extra')
-        assert not self.query.is_sliced, \
+        if self.query.is_sliced:
-            "Cannot change a query once a slice has been taken"
+            raise TypeError('Cannot change a query once a slice has been taken.')
         clone = self._chain()
         clone.query.add_extra(select, select_params, where, params, tables, order_by)
         return clone

django/db/models/sql/query.py

@@ -574,8 +574,8 @@ class Query(BaseExpression):
         """
         assert self.model == rhs.model, \
             "Cannot combine queries on two different base models."
-        assert not self.is_sliced, \
+        if self.is_sliced:
-            "Cannot combine queries once a slice has been taken."
+            raise TypeError('Cannot combine queries once a slice has been taken.')
         assert self.distinct == rhs.distinct, \
             "Cannot combine a unique query with a non-unique query."
         assert self.distinct_fields == rhs.distinct_fields, \

docs/releases/4.0.txt

@@ -318,6 +318,9 @@ Miscellaneous
   rather than via the recommended ``AdminSite.urls`` property, or
   ``AdminSite.get_urls()`` method.
 
+* Unsupported operations on a sliced queryset now raise ``TypeError`` instead
+  of ``AssertionError``.
+
 .. _deprecated-features-4.0:
 
 Features deprecated in 4.0

tests/delete/tests.py

@@ -275,6 +275,10 @@ class OnDeleteTests(TestCase):
 
 
 class DeletionTests(TestCase):
+    def test_sliced_queryset(self):
+        msg = "Cannot use 'limit' or 'offset' with delete()."
+        with self.assertRaisesMessage(TypeError, msg):
+            M.objects.all()[0:5].delete()
 
     def test_m2m(self):
         m = M.objects.create()

tests/distinct_on_fields/tests.py

@@ -98,6 +98,11 @@ class DistinctOnTests(TestCase):
         c2 = c1.distinct('pk')
         self.assertNotIn('OUTER JOIN', str(c2.query))
 
+    def test_sliced_queryset(self):
+        msg = 'Cannot create distinct fields once a slice has been taken.'
+        with self.assertRaisesMessage(TypeError, msg):
+            Staff.objects.all()[0:5].distinct('name')
+
     def test_transform(self):
         new_name = self.t1.name.upper()
         self.assertNotEqual(self.t1.name, new_name)

tests/get_earliest_or_latest/tests.py

@@ -81,6 +81,11 @@ class EarliestOrLatestTests(TestCase):
         Article.objects.model._meta.get_latest_by = ('pub_date', 'expire_date')
         self.assertEqual(Article.objects.filter(pub_date=datetime(2005, 7, 28)).earliest(), a4)
 
+    def test_earliest_sliced_queryset(self):
+        msg = 'Cannot change a query once a slice has been taken.'
+        with self.assertRaisesMessage(TypeError, msg):
+            Article.objects.all()[0:5].earliest()
+
     def test_latest(self):
         # Because no Articles exist yet, latest() raises ArticleDoesNotExist.
         with self.assertRaises(Article.DoesNotExist):
@@ -143,6 +148,11 @@ class EarliestOrLatestTests(TestCase):
         Article.objects.model._meta.get_latest_by = ('pub_date', 'expire_date')
         self.assertEqual(Article.objects.filter(pub_date=datetime(2005, 7, 27)).latest(), a3)
 
+    def test_latest_sliced_queryset(self):
+        msg = 'Cannot change a query once a slice has been taken.'
+        with self.assertRaisesMessage(TypeError, msg):
+            Article.objects.all()[0:5].latest()
+
     def test_latest_manual(self):
         # You can still use latest() with a model that doesn't have
         # "get_latest_by" set -- just pass in the field name manually.

tests/lookup/tests.py

@@ -244,6 +244,11 @@ class LookupTests(TestCase):
                 with self.assertRaisesMessage(ValueError, msg % field_name):
                     Model.objects.in_bulk(field_name=field_name)
 
+    def test_in_bulk_sliced_queryset(self):
+        msg = "Cannot use 'limit' or 'offset' with in_bulk()."
+        with self.assertRaisesMessage(TypeError, msg):
+            Article.objects.all()[0:5].in_bulk([self.a1.id, self.a2.id])
+
     def test_values(self):
         # values() returns a list of dictionaries instead of object instances --
         # and you can specify which fields you want to retrieve.

tests/queries/tests.py

@@ -700,7 +700,8 @@ class Queries1Tests(TestCase):
             )
             self.assertQuerysetEqual(q.reverse(), [])
             q.query.low_mark = 1
-            with self.assertRaisesMessage(AssertionError, 'Cannot change a query once a slice has been taken'):
+            msg = 'Cannot change a query once a slice has been taken.'
+            with self.assertRaisesMessage(TypeError, msg):
                 q.extra(select={'foo': "1"})
             self.assertQuerysetEqual(q.defer('meal'), [])
             self.assertQuerysetEqual(q.only('meal'), [])
@@ -2359,15 +2360,18 @@ class QuerySetSupportsPythonIdioms(TestCase):
         )
 
     def test_slicing_cannot_filter_queryset_once_sliced(self):
-        with self.assertRaisesMessage(AssertionError, "Cannot filter a query once a slice has been taken."):
+        msg = 'Cannot filter a query once a slice has been taken.'
+        with self.assertRaisesMessage(TypeError, msg):
             Article.objects.all()[0:5].filter(id=1)
 
     def test_slicing_cannot_reorder_queryset_once_sliced(self):
-        with self.assertRaisesMessage(AssertionError, "Cannot reorder a query once a slice has been taken."):
+        msg = 'Cannot reorder a query once a slice has been taken.'
+        with self.assertRaisesMessage(TypeError, msg):
             Article.objects.all()[0:5].order_by('id')
 
     def test_slicing_cannot_combine_queries_once_sliced(self):
-        with self.assertRaisesMessage(AssertionError, "Cannot combine queries once a slice has been taken."):
+        msg = 'Cannot combine queries once a slice has been taken.'
+        with self.assertRaisesMessage(TypeError, msg):
             Article.objects.all()[0:1] & Article.objects.all()[4:5]
 
     def test_slicing_negative_indexing_not_supported_for_single_element(self):
@@ -2417,7 +2421,8 @@ class WeirdQuerysetSlicingTests(TestCase):
         self.assertQuerysetEqual(Article.objects.all()[0:0], [])
         self.assertQuerysetEqual(Article.objects.all()[0:0][:10], [])
         self.assertEqual(Article.objects.all()[:0].count(), 0)
-        with self.assertRaisesMessage(TypeError, 'Cannot reverse a query once a slice has been taken.'):
+        msg = 'Cannot change a query once a slice has been taken.'
+        with self.assertRaisesMessage(TypeError, msg):
             Article.objects.all()[:0].latest('created')
 
     def test_empty_resultset_sql(self):

tests/update/tests.py

@@ -130,7 +130,7 @@ class AdvancedTests(TestCase):
         """
         method = DataPoint.objects.all()[:2].update
         msg = 'Cannot update a query once a slice has been taken.'
-        with self.assertRaisesMessage(AssertionError, msg):
+        with self.assertRaisesMessage(TypeError, msg):
             method(another_value='another thing')
 
     def test_update_respects_to_field(self):