Uneeded error when delete on empty restriction in merge table

[samuelbray32]

• In the merge table delete function here it iterates of each part table with matching entries.
• Merge.merge_get_part() return None if there are no matching parts. This is non-iterable and causes an error

Solution:
Either:

• check return value of Merge.merge_get_part() before iterating here
• change the return value of Merge.merge_get_part() for an empty case

@CBroz1, do you have any known reason to prefer either option?

[CBroz1]

Thanks for posting, Sam. It makes more sense to me for the delete to check the value. I don't think someone who used merge_get_part alone would expect an empty iterable in the null case, and I wouldn't want to change the behavior of one func to suit another. It's then a question of whether or not to flip return_empties - and I think it's more computation if we're running null deletes for each part. I might adjust delete to the following

class Merge(dj.Manual):
    ...
    def delete(self, force_permission=False, *args, **kwargs):
        """Alias for cautious_delete, overwrites datajoint.table.Table.delete"""
        if not (parts := self.merge_get_part(
            restriction=self.restriction,
            multi_source=True,
            return_empties=False,
        )):
            return
        for part in parts:        
            part.delete(force_permission=force_permission, *args, **kwargs)