There seems to be a bug in unifhy when the files contained in the unifhy.DataSet are small enough so that they can fit in memory. This seems to be linked to the documented behaviour of cf.Field.get_filenames:

The file names in normalised, absolute form. If all of the data are in memory then an empty set is returned.

This results in the filenames attribute of a given unifhy.Variable to be an empty set. Ultimately leading to saving an empty sequence of filenames in the YAML file, so that a to_yaml > from_yaml workflow fails.

It would be good to check with cf-python whether there is another functionality that keeps track of filenames, or if it makes sense for their package to offer such functionality. If not, it will be up to unifhy to keep track of them.