Why default value of a field does not change in the compiled proto file?

Question

I have a proto file as below:

message MyMessage
{
    optional uint32 field1 = 1 [default = 5];
}

And when I compile it with below command,

protoc -I=. --python_betterproto_out=. my_message.proto

I get below init.py file:

@dataclass(eq=False, repr=False)
class MyMessage(betterproto.Message):
    field1: int = betterproto.uint32_field(1)

Finally, I want to get the binary encoded representation of a message object as below, :

meessage = MyMessage()
message.field1 = 0
encoded = bytes(message)

It does not serialize field1. The reason is specified in the __bytes__ function (the link):

            # Default (zero) values are not serialized. Two exceptions are
            # if this is the selected oneof item or if we know we have to
            # serialize an empty message (i.e. zero value was explicitly
            # set by the user).
            

I want to know, why the zero is still the default value, despite of setting it to 5 in `.proto' file? and how can I fix it?

I tried the oneof solution and it worked. But I don't want to use that.

Answer 1

Here's my repro that demonstrates that betterproto does not appear to reflect the [default = 5] while protobuf does.

python3 -m venv venv
source venv/bin/activate

python3 -m pip "betterproto[compiler]"
python3 -m pip protobuf

Ensure protoc is in the PATH:

PATH=${PATH}:/path/to/protoc-22.4-linux-x86_64/bin

Then:

NAME="foo.proto"
protoc \
--proto_path=${PWD} \
--python_betterproto_out=${PWD} \
--python_out=${PWD} \
--pyi_out=${PWD} \
${PWD}/${NAME}

Then: main.py:

import foo
import foo_pb2

# betterproto prints default Default (!) of 0
m = foo.MyMessage()
print(m.field1)

# protobuf prints annotated Default of 5
m = foo_pb2.MyMessage()
print(m.field1)

Prints:

0
5

And, protobuf generates ${NAME}_pb2.py which includes a serialized copy of the FileDescriptor which uses FileDescriptorProto.

We can use protoc to decode this:

FILE="..." # AddSerializedFile byte string from ${NAME}_pb2.py

printf ${FILE} \
| protoc \
  --decode=google.protobuf.FileDescriptorProto \
  --proto_path=path/to/descriptor.proto

Which yields:

name: "{NAME}"
package: "{PACKAGE}"
message_type {
  name: "MyMessage"
  field {
    name: "field1"
    number: 1
    label: LABEL_OPTIONAL
    type: TYPE_UINT32
    default_value: "5"
  }
}