[Python][ML] Raise if ImplicitMT is enabled for the RDataLoader

bindings/pyroot/pythonizations/python/ROOT/_pythonization/_ml_dataloader.py

@@ -44,6 +44,16 @@ def _exclude_target_and_weights(columns: list[str], target_columns: list[str], w
     return [str(f) for f in columns if str(f) not in excluded]
 
 
+def _raise_if_implicit_mt(rdfs: list) -> None:
+    """Raise if ImplicitMT is enabled, or was when one of the RDataFrames was created."""
+    import ROOT
+
+    if ROOT.IsImplicitMTEnabled() or any(rdf.GetNSlots() > 1 for rdf in rdfs):
+        raise RuntimeError(
+            "Cannot use RDataLoader with implicit multi-threading enabled, call ROOT.DisableImplicitMT() before creating the data loader and its inputs."
+        )
+
+
 class _RDataLoader:
     def _resolve_columns(
         self,
@@ -205,6 +215,7 @@ def __init__(
         if not hasattr(rdataframes, "__iter__"):
             rdataframes = [rdataframes]
         self.noded_rdfs = [RDF.AsRNode(rdf) for rdf in rdataframes]
+        _raise_if_implicit_mt(self.noded_rdfs)
 
         self.target_columns = _normalize_target(target)
         self.weights_column = weights
@@ -272,10 +283,12 @@ def DeActivate(self):
 
     def ActivateTrainingEpoch(self):
         """Activate the training epoch of the generator"""
+        _raise_if_implicit_mt(self.noded_rdfs)
         self.engine.ActivateTrainingEpoch()
 
     def ActivateValidationEpoch(self):
         """Activate the validation epoch of the generator"""
+        _raise_if_implicit_mt(self.noded_rdfs)
         self.engine.ActivateValidationEpoch()
 
     def DeActivateTrainingEpoch(self):
@@ -493,6 +506,7 @@ def GetValidationBatch(self) -> Any:
         return batch if (batch and batch.GetSize() > 0) else None
 
     def Save(self, dataset_name: str, filename: str, is_training: bool, output_format: str) -> None:
+        _raise_if_implicit_mt(self.noded_rdfs)
         self.engine.Save(dataset_name, filename, is_training, output_format)
 
 

bindings/pyroot/pythonizations/test/ml_dataloader.py

@@ -8123,5 +8123,22 @@ def test08_round_trip(self):
         np.testing.assert_array_equal(w.flatten(), np.full(20, 0.5))
 
 
+class DataLoaderImplicitMT(unittest.TestCase):
+    file_name = "dataloader_imt.root"
+    tree_name = "mytree"
+
+    def tearDown(self):
+        ROOT.DisableImplicitMT()
+        os.remove(self.file_name)
+
+    def test01_implicit_mt_not_supported(self):
+        ROOT.RDataFrame(10).Define("b1", "(int) rdfentry_").Snapshot(self.tree_name, self.file_name)
+        ROOT.EnableImplicitMT(2)
+        df = ROOT.RDataFrame(self.tree_name, self.file_name)
+
+        with self.assertRaisesRegex(RuntimeError, "Cannot use RDataLoader with implicit multi-threading enabled"):
+            ROOT.Experimental.ML.RDataLoader(df, batch_size=2).as_numpy()
+
+
 if __name__ == "__main__":
     unittest.main()