Document how to use module_pb2.Message.WhichOneof

[tseaver]

The docs for oneof fields do not specify how to query which of a given set of oneof fields is present in a message.

The workaround I have found is to use "fieldname" in message, with the proviso that some field names may be aliased (e.g., "set" -> "set_"). This workaround isn't adequate for using the selected oneof label as a key into a dict, for instance.

[busunkim96]

@software-dov. Is there a good way to do this?

The workaround I used in googleapis/python-monitoring#91 is to convert the message into a pb. However, this has the same issue that it won't work if any field names are aliased.

from google.cloud import monitoring_v3

typed_value = "double_value"
value_type = monitoring_v3.TypedValue.pb(typed_value).WhichOneof("value")

class TypedValue(proto.Message):
    r"""A single strongly-typed value.

    Attributes:
        bool_value (bool):
            A Boolean value: ``true`` or ``false``.
        int64_value (int):
            A 64-bit integer. Its range is approximately
            &plusmn;9.2x10<sup>18</sup>.
        double_value (float):
            A 64-bit double-precision floating-point
            number. Its magnitude is approximately
            &plusmn;10<sup>&plusmn;300</sup> and it has 16
            significant digits of precision.
        string_value (str):
            A variable-length string value.
        distribution_value (~.distribution.Distribution):
            A distribution value.
    """

    bool_value = proto.Field(proto.BOOL, number=1, oneof="value")

    int64_value = proto.Field(proto.INT64, number=2, oneof="value")

    double_value = proto.Field(proto.DOUBLE, number=3, oneof="value")

    string_value = proto.Field(proto.STRING, number=4, oneof="value")

    distribution_value = proto.Field(
        proto.MESSAGE, number=5, oneof="value", message=distribution.Distribution,
    )

[achew22]

The best way that I've found to do this is, unfortunately, to poke through to the raw _pb.

message._pb. WhichOneof("field_name")

Is there a preferred way to do this?

[dpowley]

The best way that I've found to do this is, unfortunately, to poke through to the raw _pb.

message._pb.WhichOneOf("field_name")

Is there a preferred way to do this?

For future onlookers: message._pb.WhichOneof("field_name"). Note for some reason WhichOneof is not actually PascalCase