how to solv: gPRC with python,"the JSON object must be str, bytes or bytearray, not RepeatedScalarContainer"

Question

This is my first question on this site, and my English is not so good.So,if there is any misunderstanding, please tell me. Thank you.

I am trying use gPRC to send a message ,which is defined as :

message PrepareMsg{
    message Data{
        string node_id = 1;
        string vote = 2;
        }
    Data data = 1;
    repeated string signature = 2;
}

My code for Cilent just like this:

1     def pre_prepare(self, block):
2         request = grpc_pb2.PrePrepareMsg()
3         request.data.node_id = self.node_id
4         request.data.block.CopyFrom(block)
5         a = request.data.SerializeToString()
6         temp_sign = signing(self.signer, a)
7         for i in temp_sign:
8             request.signature.append(hex(i))
9         self_node = set()
10        self_node.add(p2p.SELF_IP_PORT)
11        print(self_node)
12        nodes = set(p2p.Node.get_nodes_list()) - self_node
13        print("print nodes in broadcast:")
14        print(nodes)
15        for i in nodes:
16            channel = grpc.insecure_channel(i)
17            stub = grpc_pb2_grpc.ConsensusStub(channel)
18            try:
19                response = stub.PrePrepare(request)
20                print(response.Result)
21            except Exception as e:
22                print("get except: %s" % str(e))

I find there is a error :"Exception calling application: the JSON object must be str, bytes or bytearray, not RepeatedScalarContainer" I don't know why it happened.