如果子消息没有字段，如何在 protobuf 消息上分配一个字段？

Question

我想创建一个 BigTable DeleteFromRow 变更。 Mutation 和 DeleteFromRow 的原型如下所示：

oneof mutation {
    // Set a cell's value.
    SetCell set_cell = 1;

    // Deletes cells from a column.
    DeleteFromColumn delete_from_column = 2;

    // Deletes cells from a column family.
    DeleteFromFamily delete_from_family = 3;

    // Deletes cells from the entire row.
    DeleteFromRow delete_from_row = 4;
  }
}

message DeleteFromRow {

}

在 Python 中，您不能直接实例化 DeleteFromRow 对象并将 Mutation 的 delete_from_row 字段设置为该对象。

所以这个行不通:

request = bigtable_pb2.MutateRowRequest(table_name='tablename', row_key=row_key)
mutation = request.mutations.add()
mutation.delete_from_row = data_pb2.Mutation.DeleteFromRow()

正如其他 SO 用户所提出的（参见 this question），这导致

AttributeError: Assignment not allowed to composite field "delete_from_row" in protocol message object.

根据 protobuf docs，您应该通过设置子字段之一来设置一个字段。因此，应该以这种方式创建 DeleteFromFamily 突变：

mutation.delete_from_family.family_name = 'some_family'

但是，如何为没有字段的 DeleteFromRow 消息执行此操作？

Answer 1

您可以启动 DeleteFromRow 对象并使用关键字参数 delete_from_row:

创建一个变异

dfr = data_pb2.Mutation.DeleteFromRow()
mutation = data_pb2.Mutation(delete_from_row=dfr)

虽然您不能 add 或 append 对请求的重复突变字段进行此突变（尽管在我看来这就是文档所说的 here），但您可以extend它:

request = bigtable_pb2.MutateRowRequest(table_name='tablename', row_key=row_key)
request.mutations.extend([mutation])

Answer 2

您可以使用 Message.SetInParent:

Mark this as present in the parent.

This normally happens automatically when you assign a field of a sub-message, but sometimes you want to make the sub-message present while keeping it empty. If you find yourself using this, you may want to reconsider your design.

示例：

message Msg {
  oneof kind {
    int64 int_field = 1;
    EmptyMsg msg_field = 1;
  }
}

message EmptyMsg {}

msg = Msg()
print(msg.WhichOneof('kind'))  # None

msg.msg_field  # No-op (return EmptyMsg but don't set oneof field)
print(msg.WhichOneof('kind'))  # None

msg.msg_field.SetInParent()
print(v.WhichOneof('kind'))  # msg_field

如果子消息没有字段，如何在 protobuf 消息上分配一个字段？

How do I assign a oneof field on a protobuf message if the child message has no fields?

python

bigtable

grpc