How to define an optional field in protobuf 3

Question

I need to specify a message with an optional field in protobuf (proto3 syntax). In terms of proto 2 syntax, the message I want to express is something like:

Answer 1

Another way to encode the message you intend is to add another field to track "set" fields:

syntax="proto3";

package qtprotobuf.examples;

message SparseMessage {
    repeated uint32 fieldsUsed = 1;
    bool   attendedParty = 2;
    uint32 numberOfKids  = 3;
    string nickName      = 4;
}

message ExplicitMessage {
    enum PARTY_STATUS {ATTENDED=0; DIDNT_ATTEND=1; DIDNT_ASK=2;};
    PARTY_STATUS attendedParty = 1;
    bool   indicatedKids = 2;
    uint32 numberOfKids  = 3;
    enum NO_NICK_STATUS {HAS_NO_NICKNAME=0; WOULD_NOT_ADMIT_TO_HAVING_HAD_NICKNAME=1;};
    NO_NICK_STATUS noNickStatus = 4;
    string nickName      = 5;
}

This is especially appropriate if there is a large number of fields and only a small number of them have been assigned.

In python, usage would look like this:

import field_enum_example_pb2
m = field_enum_example_pb2.SparseMessage()
m.attendedParty = True
m.fieldsUsed.append(field_enum_example_pb2.SparseMessages.ATTENDEDPARTY_FIELD_NUMBER)