Improve the error message on out-of-sync flow ids (#1171)

* Improve the error message on out-of-sync flow ids

* Add more meaningful messages on test fail

Author: PGijsbers

openml/setups/functions.py

@@ -42,7 +42,10 @@ def setup_exists(flow) -> int:
     # checks whether the flow exists on the server and flow ids align
     exists = flow_exists(flow.name, flow.external_version)
     if exists != flow.flow_id:
-        raise ValueError("This should not happen!")
+        raise ValueError(
+            f"Local flow id ({flow.id}) differs from server id ({exists}). "
+            "If this issue persists, please contact the developers."
+        )
 
     openml_param_settings = flow.extension.obtain_parameter_values(flow)
     description = xmltodict.unparse(_to_dict(flow.flow_id, openml_param_settings), pretty=True)

tests/test_runs/test_run_functions.py

@@ -1112,13 +1112,13 @@ def test__run_exists(self):
 
             flow = self.extension.model_to_flow(clf)
             flow_exists = openml.flows.flow_exists(flow.name, flow.external_version)
-            self.assertGreater(flow_exists, 0)
+            self.assertGreater(flow_exists, 0, "Server says flow from run does not exist.")
             # Do NOT use get_flow reinitialization, this potentially sets
             # hyperparameter values wrong. Rather use the local model.
             downloaded_flow = openml.flows.get_flow(flow_exists)
             downloaded_flow.model = clf
             setup_exists = openml.setups.setup_exists(downloaded_flow)
-            self.assertGreater(setup_exists, 0)
+            self.assertGreater(setup_exists, 0, "Server says setup of run does not exist.")
             run_ids = run_exists(task.task_id, setup_exists)
             self.assertTrue(run_ids, msg=(run_ids, clf))