def check_item_type_uniqueness(tag, previous_tags):
    """
    Check the uniqueness of the 'item type' for an object.
    """
    fail = False
    #If the tag is being created...
    if not tag.id:
        #... and the new item type is different from previous item types (for
        #example, different from the first of them), fail
        fail = previous_tags and tag.item_type != previous_tags[0].item_type
    #If the tag is being modifying...
    else:
        #... but there is only one previous tag (the one that is being
        #modifying), do not fail
        fail = previous_tags.count() > 1 and \
               tag.item_type != previous_tags[0].item_type
    return fail
Check the uniqueness of the 'item type' for an object.