Oneof behaviour not according to protobuf Language Guide

[benedikt-bartscher]

https://protobuf.dev/programming-guides/proto3/#oneof

Oneof fields are like optional fields except all the fields in a oneof share memory, and at most one field can be set at the same time. Setting any member of the oneof automatically clears all the other members. You can check which value in a oneof is set (if any) using a special case() or WhichOneof() method, depending on your chosen language.

Note that if multiple values are set, the last set value as determined by the order in the proto will overwrite all previous ones.

this currently passes:

def test_multiple_oneof() -> None:
    msg = OneofTest(
        field_one=1,
        field_two=2,
    )
    assert msg.field_one == 1
    assert msg.field_two == 2

in the previous version (python-betterproto) it raised an AttributeError

[benedikt-bartscher]

Funny thing: betterproto2.which_one_of actually raises for this case. Which seems kind of inconsistent. I suggest storing which field number is active for every oneof, f.e. in a dict[str, int]. This way we can clear out all other oneofs during setattr if the group key is already present in that dict. Also which_one_of does not need to raise any more and does not need to getattr-iterate over all fields