Bug/607 fidelity statevector kernel cannot be pickled

.gitignore

@@ -127,3 +127,4 @@ venv.bak/
 
 # stestr repostiory
 .stestr/
+bug607_replicated.py

qiskit_machine_learning/kernels/fidelity_statevector_kernel.py

@@ -1,6 +1,6 @@
 # This code is part of a Qiskit project.
 #
-# (C) Copyright IBM 2023.
+# (C) Copyright IBM 2023, 2024.
 #
 # 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
@@ -97,7 +97,7 @@ def __init__(
         self._auto_clear_cache = auto_clear_cache
         self._shots = shots
         self._enforce_psd = enforce_psd
-
+        self._cache_size = cache_size
         # Create the statevector cache at the instance level.
         self._get_statevector = lru_cache(maxsize=cache_size)(self._get_statevector_)
 
@@ -160,3 +160,12 @@ def clear_cache(self):
         """Clear the statevector cache."""
         # pylint: disable=no-member
         self._get_statevector.cache_clear()
+
+    def __getstate__(self) -> dict:
+        kernel = dict(self.__dict__)
+        kernel["_get_statevector"] = None
+        return kernel
+
+    def __setstate__(self, kernel):
+        self.__dict__ = kernel
+        self._get_statevector = lru_cache(maxsize=self._cache_size)(self._get_statevector_)

releasenotes/notes/fix-fid_statevector_kernel-pickling-b7fa2b13a15ec9c6.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    Fixed a bug where :class:`.FidelityQuantumKernel` threw an error when pickled.

test/kernels/test_fidelity_statevector_kernel.py

@@ -1,6 +1,6 @@
 # This code is part of a Qiskit project.
 #
-# (C) Copyright IBM 2023.
+# (C) Copyright IBM 2023, 2024.
 #
 # 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
@@ -17,6 +17,7 @@
 import itertools
 import sys
 import unittest
+import pickle
 
 from test import QiskitMachineLearningTestCase
 
@@ -343,6 +344,55 @@ def test_properties(self):
         self.assertEqual(qc, kernel.feature_map)
         self.assertEqual(1, kernel.num_features)
 
+    def test_pickling(self):
+        """Test that the kernel can be pickled correctly and without error."""
+        # Compares original kernel with copies made using pickle module and get & set state directly
+        qc = QuantumCircuit(1)
+        qc.ry(Parameter("w"), 0)
+        kernel1 = FidelityStatevectorKernel(feature_map=qc)
+
+        pickled_obj = pickle.dumps(kernel1)
+        kernel2 = pickle.loads(pickled_obj)
+
+        kernel3 = FidelityStatevectorKernel()
+        kernel3.__setstate__(kernel1.__getstate__())
+
+        with self.subTest("Pickle fail, kernels are not the same type"):
+            self.assertEqual(type(kernel1), type(kernel2))
+
+        with self.subTest("Pickle fail, kernels are not the same type"):
+            self.assertEqual(type(kernel1), type(kernel3))
+
+        with self.subTest("Pickle fail, kernels are not unique objects"):
+            self.assertNotEqual(kernel1, kernel2)
+
+        with self.subTest("Pickle fail, kernels are not unique objects"):
+            self.assertNotEqual(kernel1, kernel3)
+
+        with self.subTest("Pickle fail, caches are not the same type"):
+            self.assertEqual(type(kernel1._get_statevector), type(kernel2._get_statevector))
+
+        with self.subTest("Pickel fail, caches are not the same type"):
+            self.assertEqual(type(kernel1._get_statevector), type(kernel3._get_statevector))
+
+        # Remove cache to check dict properties are otherwise identical.
+        # - caches are never identical as they have different RAM locations.
+        kernel1.__dict__["_get_statevector"] = None
+        kernel2.__dict__["_get_statevector"] = None
+        kernel3.__dict__["_get_statevector"] = None
+
+        # Confirm changes were made.
+        with self.subTest("Pickle fail, caches have not been removed from kernels"):
+            self.assertEqual(kernel1._get_statevector, None)
+            self.assertEqual(kernel2._get_statevector, None)
+            self.assertEqual(kernel3._get_statevector, None)
+
+        with self.subTest("Pickle fail, properties of kernels (bar cache) are not identical"):
+            self.assertEqual(kernel1.__dict__, kernel2.__dict__)
+
+        with self.subTest("Pickle fail, properties of kernels (bar cache) are not identical"):
+            self.assertEqual(kernel1.__dict__, kernel3.__dict__)
+
 
 @ddt
 class TestStatevectorKernelDuplicates(QiskitMachineLearningTestCase):