Fixed #29758 -- Documented how to test custom error views.

docs/topics/http/views.txt

@@ -166,3 +166,39 @@ The :func:`~django.views.defaults.bad_request` view is overridden by
 
     Use the :setting:`CSRF_FAILURE_VIEW` setting to override the CSRF error
     view.
+
+Testing custom error views
+--------------------------
+
+To test the response of a custom error handler, raise the appropriate exception
+in a test view. For example::
+
+    from django.core.exceptions import PermissionDenied
+    from django.http import HttpResponse
+    from django.test import SimpleTestCase, override_settings
+    from django.urls import path
+
+
+    def response_error_handler(request, exception=None):
+        return HttpResponse('Error handler content', status=403)
+
+
+    def permission_denied_view(request):
+        raise PermissionDenied
+
+
+    urlpatterns = [
+        path('403/', permission_denied_view),
+    ]
+
+    handler403 = response_error_handler
+
+
+    # ROOT_URLCONF must specify the module that contains handler403 = ...
+    @override_settings(ROOT_URLCONF=__name__)
+    class CustomErrorHandlerTests(SimpleTestCase):
+
+        def test_handler_renders_template_response(self):
+            response = self.client.get('/403/')
+            # Make assertions on the response here. For example:
+            self.assertContains(response, 'Error handler content', status_code=403)