[1.10.x] Fixed #24931 -- Made namespaced RegexURLResolvers populate callback strings.

Fixed a regression in 2f16ff5a6c.

Thanks Tim Graham for the review.

Backport of 625b8e9295 from master

tests/urlpatterns_reverse/nested_urls.py

@@ -0,0 +1,25 @@
+from django.conf.urls import include, url
+from django.views import View
+
+
+def view1(request):
+    pass
+
+
+def view2(request):
+    pass
+
+
+class View3(View):
+    pass
+
+
+nested = ([
+    url(r'^view1/$', view1, name='view1'),
+    url(r'^view3/$', View3.as_view(), name='view3'),
+], 'backend')
+
+urlpatterns = [
+    url(r'^some/path/', include(nested, namespace='nested')),
+    url(r'^view2/$', view2, name='view2'),
+]

tests/urlpatterns_reverse/tests.py

@@ -428,6 +428,13 @@ class ResolverTests(unittest.TestCase):
                                 'Wrong URL name.  Expected "%s", got "%s".' % (e['name'], t.name)
                             )
 
+    def test_namespaced_view_detail(self):
+        resolver = get_resolver('urlpatterns_reverse.nested_urls')
+        self.assertTrue(resolver._is_callback('urlpatterns_reverse.nested_urls.view1'))
+        self.assertTrue(resolver._is_callback('urlpatterns_reverse.nested_urls.view2'))
+        self.assertTrue(resolver._is_callback('urlpatterns_reverse.nested_urls.View3'))
+        self.assertFalse(resolver._is_callback('urlpatterns_reverse.nested_urls.blub'))
+
 
 @override_settings(ROOT_URLCONF='urlpatterns_reverse.reverse_lazy_urls')
 class ReverseLazyTest(TestCase):