Why does python protobuf json_format.Parse throw a TypeError?

Question

I am experimenting with protobuf serialization to JSON. I made a simple proto file with the following messages:

syntax = "proto3";
message Bool {
    bool data = 1;
}

message BoolArray {
    repeated Bool bools = 1;
}

I then run some basic code to build the message, push to Json, then read it back in:

pb_bool_array = pb_bool.BoolArray()
b = pb_bool_array.bools.add()
b.data = True
bools_as_json = MessageToJson( pb_bool_array )

Parse(bools_as_json, proto.bool_pb2.BoolArray )

but the Parse function throws a TypeError with the following message:

google.protobuf.json_format.ParseError: Failed to parse bools field: unbound method ClearField() must be called with BoolArray instance as first argument (got str instance instead).

I traced the Parse function and this error fires off on line 519 in Google's json_format code. Why would this TypeError occur? Am I missing something in my proto spec and/or abusing the python API?

Thanks!

Patrick Martin · Accepted Answer · 2018-07-27 16:14:53Z

9

After further analysis of the json_format.Parse() function, I realized that I was abusing the API.

Parse(bools_as_json, proto.bool_pb2.BoolArray )

should really be:

Parse(bools_as_json, proto.bool_pb2.BoolArray() )

The API expects a message instance to fill, not the type of message. Everything works as expected.

answered Jul 27, 2018 at 16:14

Patrick Martin

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