FIX: Raise error if service revision doesn't exist when no registry

octue/cloud/pub_sub/service.py

@@ -338,7 +338,18 @@ def ask(
                     service_registries=self.service_registries,
                 )
 
-        elif not service_revision_tag:
+        elif service_revision_tag:
+            pub_sub_id = convert_service_id_to_pub_sub_form(service_id)
+
+            service_revision_subscription = Subscription(
+                name=".".join((OCTUE_SERVICES_PREFIX, pub_sub_id)),
+                topic=self.services_topic,
+            )
+
+            if not service_revision_subscription.exists():
+                raise octue.exceptions.ServiceNotFound(f"Service revision {service_id!r} not found.")
+
+        else:
             raise octue.exceptions.InvalidServiceID(
                 f"A service revision tag for {service_id!r} must be provided if service registries aren't being used."
             )

tests/cloud/pub_sub/test_service.py

@@ -121,6 +121,13 @@ def test_missing_services_topic_results_in_error(self):
             with self.assertRaises(exceptions.ServiceNotFound):
                 service.services_topic
 
+    def test_error_raised_if_service_revision_not_found_when_not_using_service_registry(self):
+        """Test that an error is raised if a service revision isn't found when not using a service registry."""
+        service = MockService(backend=BACKEND)
+
+        with self.assertRaises(exceptions.ServiceNotFound):
+            service.ask("non/existent:service")
+
     def test_ask_unregistered_service_revision_when_service_registries_specified_results_in_error(self):
         """Test that an error is raised if attempting to ask an unregistered service a question when service registries
         are being used.