protocol buffers - Why is my Protobuf message (in Python) ignoring zero values?

Question

I've been working on implementing protobufs for IPC for a project. For some reason, values that are set to 0 are not being set/serialized. For context, the .proto file contains the following message:

syntax = "proto3";

enum SetGet {
    SET = 0;
    GET = 1;
}

message State {
    SetGet setget = 1;
    double x = 2;
    double y = 3;
    double depth = 4;
    double yaw = 5;
    double pitch = 6;
    double roll = 7; 
}

I compile the file to a Python _pb2 file with protoc, and then I try running the following test script:

import filename_pb2 as pb

state = pb.State()
state.x = 0
state.y = 0
state.depth = 0
state.yaw = 0
state.pitch = 0
state.roll = 0
state.setget = pb.SET

print("State: {}".format(state))

state2 = pb.State()
state2.ParseFromString(state.SerializeToString())

print("State2: {}".format(state2))

When I run it, the following output is printed:

State: 
State2: 

It seems that nothing is being set, or that the zero values are somehow being ignored. However, when I change the values (x, y, depth, etc.) to something nonzero, say 0.1, I get the following, expected result:

State: x: 0.1
y: 0.1
depth: 0.1
yaw: 0.1
pitch: 0.1
roll: 0.1

State2: x: 0.1
y: 0.1
depth: 0.1
yaw: 0.1
pitch: 0.1
roll: 0.1

Even though the numbers are printed out, for some reason the enum still isn't. Why does this happen with protobufs? Is the double type 0 by default, so the protobuf serializer saves on space by ignoring them? Why, then, are they not being restored when State2 is parsed in? Is there some line in the documentation that I missed? Thanks in advance!

-- Tim

Answer 1

Yes, 0 is the default. This case is mentioned explicitly in the documentation:

Note that for scalar message fields, once a message is parsed there's no way of telling whether a field was explicitly set to the default value (for example whether a boolean was set to false) or just not set at all: you should bear this in mind when defining your message types. For example, don't have a boolean that switches on some behaviour when set to false if you don't want that behaviour to also happen by default. Also note that if a scalar message field is set to its default, the value will not be serialized on the wire.