Python: Handle ros2 uint8[] field as str without ValueError

[pezy]

Public-Facing Changes

None

Description

When calling the write_message method, the message argument often originates from some other serialized form, like Protobuf or JSON. However, bytes fields can become mismatched during these serialization processes. For instance, even though a Protobuf message may contain bytes fields, using MessageToDict to convert it to a Python dictionary will base64 encode those bytes, as it adheres to the Proto3 JSON mapping guidelines.

This leads to a confusing situation for the user. They might assume that the field is still in bytes and defined as uint8[] in the ROS2 message. But when invoking write_message, they encounter the following cryptic error:

ValueError: Field "data" is not an array (<class 'str'>) but has array type "uint8[]"

While the error is technically correct, it's confusing for users who are not aware of the limitations of JSON or the transformations performed by MessageToDict.

This pull request aims to alleviate this issue by adding an additional check to see if the problematic field is base64-encoded. If it is, we decode it and proceed without errors, as demonstrated in test_write_bytes_object_from_json_deserialization. If decoding fails, the function will still raise the original error.

[james-rms]

Thanks for the PR. I believe this change might hide encoding bugs that a user would rather fix in their client code, so I'm against merging this upstream. As an aside, since this is the mcap-ros2-support package, we don’t want any protobuf-specific logic in there and we don’t expect/assume users are using protobuf.

Since this change helps you, you're free to keep it in your own fork of mcap-ros2-support or move it to a helper function in your code that processes messages before calling writer.write_message.