multi groups in the constraints (#7711)

python-package/xgboost/core.py

@@ -1393,9 +1393,9 @@ class Booster:
                     + ",".join(
                         [feature_idx_mapping[feature_name] for feature_name in constraint]
                     )
-                    + "]"
+                    + "],"
                 )
-            return s + "]"
+            return s[:-1] + "]"
         except KeyError as e:
             raise ValueError(
                 "Constrained features are not a subset of training data feature names"

tests/python/test_interaction_constraints.py

@@ -78,6 +78,12 @@ class TestInteractionConstraints:
                                          feature_names=feature_names,
                                          interaction_constraints=constraints)
 
+        constraints = [['feature_0', 'feature_1'], ['feature_2']]
+        feature_names = ['feature_0', 'feature_1', 'feature_2']
+        self.run_interaction_constraints(tree_method='exact',
+                                         feature_names=feature_names,
+                                         interaction_constraints=constraints)
+
     @pytest.mark.skipif(**tm.no_sklearn())
     def training_accuracy(self, tree_method):
         """Test accuracy, reused by GPU tests."""