gistfile1.txt

cat_to_remove = <category_publishable_is_being_removed_from>
categories = [all_current_leaf_categories_publishable_is_in];
categories.remove(cat_to_remove);
all_cats = set()
for cat in categories:
    while cat:
        all_cats.add(cat)
        cat = cat.parent
# all_cats should now be a set of all categories the publishable should still be in after the removal

while cat_to_remove:
    if cat_to_remove in all_cats:
        continue
    remove(pub.id, cat_to_remove)
    cat_to_remove = cat_to_remove.parent