AFAICT, source constraints are only useful for human consumption. But also, I don't think it's accurate to say that changing what interpreter constraints mean would make the py-constraints goal noticeably less useful: it just means that edits in the middle of the graph rather than the bottom can't be encoded in the interpreter constraints. But since that code can't ever be checked to actually be compatible by running it, source constraints are only ever a human marker anyway... so could just as well be a tag.