Fix for issue #911 (#926)

Author: woodsp-ibm

qiskit_machine_learning/kernels/trainable_fidelity_quantum_kernel.py

@@ -1,6 +1,6 @@
 # This code is part of a Qiskit project.
 #
-# (C) Copyright IBM 2022, 2024.
+# (C) Copyright IBM 2022, 2025.
 #
 # This code is licensed under the Apache License, Version 2.0. You may
 # obtain a copy of this license in the LICENSE.txt file in the root directory
@@ -92,13 +92,13 @@ def __init__(
         )
 
         # override the num of features defined in the base class
-        self._num_features = feature_map.num_parameters - self._num_training_parameters
+        self._num_features = self.feature_map.num_parameters - self._num_training_parameters
         self._feature_parameters = [
             parameter
-            for parameter in feature_map.parameters
+            for parameter in self.feature_map.parameters
             if parameter not in self._training_parameters
         ]
-        self._parameter_dict = {parameter: None for parameter in feature_map.parameters}
+        self._parameter_dict = {parameter: None for parameter in self.feature_map.parameters}
 
     def _get_parameterization(
         self, x_vec: np.ndarray, y_vec: np.ndarray

qiskit_machine_learning/kernels/trainable_fidelity_statevector_kernel.py

@@ -1,6 +1,6 @@
 # This code is part of a Qiskit project.
 #
-# (C) Copyright IBM 2023.
+# (C) Copyright IBM 2023, 2025.
 #
 # This code is licensed under the Apache License, Version 2.0. You may
 # obtain a copy of this license in the LICENSE.txt file in the root directory
@@ -88,10 +88,10 @@ def __init__(
         )
 
         # Override the number of features defined in the base class.
-        self._num_features = feature_map.num_parameters - self._num_training_parameters
+        self._num_features = self.feature_map.num_parameters - self._num_training_parameters
         self._feature_parameters = [
             parameter
-            for parameter in feature_map.parameters
+            for parameter in self.feature_map.parameters
             if parameter not in self._training_parameters
         ]
         self._parameter_dict = {parameter: None for parameter in self.feature_map.parameters}

releasenotes/notes/fix-trainable-kernel-16e11f1ac80a0b37.yaml

@@ -0,0 +1,8 @@
+---
+fixes:
+  - |
+    Fixed a bug in :class:`.TrainableFidelityQuantumKernel` and
+    :class:`.TrainableFidelityStatevectorKernel` where not passing a feature map
+    (or explicitly passing None to do the equivalent) would fail internally with
+    a NoneType AttributeError as described by
+    `issue 911 <https://github.com/qiskit-community/qiskit-machine-learning/issues/911>`__.

test/kernels/test_trainable_fidelity_qkernel.py

@@ -1,6 +1,6 @@
 # This code is part of a Qiskit project.
 #
-# (C) Copyright IBM 2021, 2023.
+# (C) Copyright IBM 2021, 2025.
 #
 # This code is licensed under the Apache License, Version 2.0. You may
 # obtain a copy of this license in the LICENSE.txt file in the root directory
@@ -181,6 +181,24 @@ def test_properties(self, trainable_kernel_type):
         self.assertEqual(len(self.training_parameters), kernel.num_training_parameters)
         self.assertEqual(self.num_features, kernel.num_features)
 
+    # Testing changes related to the bug fix for
+    # https://github.com/qiskit-community/qiskit-machine-learning/issues/911
+    @data(TrainableFidelityQuantumKernel, TrainableFidelityStatevectorKernel)
+    def test_default_feature_map(self, trainable_kernel_type):
+        """Test properties of the trainable quantum kernel."""
+        with self.subTest("Do not pass feature map at all"):
+            kernel = trainable_kernel_type()
+            # The above would crash as per the reference issue. This following checks
+            # just make sure feature map is present and built as expected
+            self.assertIsNotNone(kernel.feature_map)
+            self.assertEqual(len(kernel.feature_map.parameters), 2)
+
+        # As above but explicitly pass None
+        with self.subTest("Pass feature map with value None"):
+            kernel = trainable_kernel_type(feature_map=None)
+            self.assertIsNotNone(kernel.feature_map)
+            self.assertEqual(len(kernel.feature_map.parameters), 2)
+
 
 if __name__ == "__main__":
     unittest.main()