Serialize numpy arrays with object types

qiskit/providers/ibmq/runtime/utils.py

@@ -185,6 +185,8 @@ def default(self, obj: Any) -> Any:  # pylint: disable=arguments-differ
         if isinstance(obj, complex):
             return {'__type__': 'complex', '__value__': [obj.real, obj.imag]}
         if isinstance(obj, np.ndarray):
+            if obj.dtype == object:
+                return {"__type__": "ndarray", "__value__": obj.tolist()}
             value = _serialize_and_encode(obj, np.save, allow_pickle=False)
             return {'__type__': 'ndarray', '__value__': value}
         if isinstance(obj, set):
@@ -255,6 +257,8 @@ def object_hook(self, obj: Any) -> Any:
             if obj_type == 'complex':
                 return obj_val[0] + 1j * obj_val[1]
             if obj_type == 'ndarray':
+                if isinstance(obj_val, list):
+                    return np.array(obj_val)
                 return _decode_and_deserialize(obj_val, np.load)
             if obj_type == 'set':
                 return set(obj_val)

releasenotes/notes/numpy-ndarray-encoder-12745f022ec8ef0b.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Fixed an issue where numpy ndarrays with object types could not be 
+    serialized. :class:`qiskit.providers.ibmq.runtime.RuntimeEncoder` and 
+    :class:`qiskit.providers.ibmq.runtime.RuntimeDecoder` have been updated 
+    to handle these ndarrays.

test/ibmq/runtime/test_runtime.py

@@ -257,6 +257,19 @@ def test_encoder_datetime(self):
             decoded = json.loads(encoded, cls=RuntimeDecoder)
             self.assertEqual(decoded, obj)
 
+    def test_encoder_ndarray(self):
+        """Test encoding and decoding a numpy ndarray."""
+        subtests = (
+            {"ndarray": np.array([[1, 2, 3], [{"obj": 123}, 5, 6]], dtype=object)},
+            {"ndarray": np.array([[1, 2, 3], [{"obj": 123}, 5, 6]])},
+            {"ndarray": np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]], dtype=int)},
+        )
+        for obj in subtests:
+            encoded = json.dumps(obj, cls=RuntimeEncoder)
+            self.assertIsInstance(encoded, str)
+            decoded = json.loads(encoded, cls=RuntimeDecoder)
+            self.assertTrue(np.array_equal(decoded["ndarray"], obj["ndarray"]))
+
     def test_encoder_instruction(self):
         """Test encoding and decoding instructions"""
         subtests = (