Here is review303161002 which implements it.
I've added the attribute as optional (thanks to python3 extendend unpacking) so all the current view_attributes should continue working without adding the depends parameter.
Sometimes we want to be sure that a field used by a PYSON expression is present in the view to which the attribute is added.
I think we could add a fourth item which will be a depends list. It should only be added to the view if the xpath match at least one element.