Technically it is 2-way, so we could ship data Python -> MsgPack -> Java. I do think that serializing things like numpy arrays and so on (a common use case I would say) would probably not work out the box. I think pickling is better for that.

Still, perhaps this support can be expanded somewhat in future to allow more Python / Java RDD interop (with the proviso that tricky data structures may not work in terms of SerDe).