Make sure lists are pickable, too.

primitives/test_primitives/sum.py

@@ -94,14 +94,20 @@ class SumPrimitive(base.SingletonOutputMixin[Inputs, Outputs, None, Hyperparams]
         if DOCKER_KEY not in self.docker_containers:
             raise ValueError("Docker key '{docker_key}' missing among provided Docker containers.".format(docker_key=DOCKER_KEY))
 
+    def _convert_value(self, value: typing.Any) -> typing.Union[numpy.ndarray, typing.List, typing.Any]:
+        # Server does not know about container types, just standard numpy arrays and lists.
+        if isinstance(value, container.ndarray):
+            return value.view(numpy.ndarray)
+        elif isinstance(value, container.List):
+            return [self._convert_value(v) for v in value]
+        else:
+            return value
+
     def produce(self, *, inputs: Inputs, timeout: float = None, iterations: int = None) -> base.CallResult[Outputs]:
         # In the future, we should store here data in Arrow format into
         # Plasma store and just pass an ObjectId of data over HTTP.
-        if isinstance(inputs, container.ndarray):
-            value = inputs.view(numpy.ndarray)
-            data = pickle.dumps(value)
-        else:
-            data = pickle.dumps(inputs)
+        value = self._convert_value(inputs)
+        data = pickle.dumps(value)
 
         # TODO: Retry if connection fails.
         #       This connection can sometimes fail because the service inside a Docker container