Ensure an item is appended to a list or create a new empty list

        :param new_items: the item(s) to append
        :type new_items: list(obj)
        :param append_to: the list on which to append the items
        :type append_to: list()
        :param index: index of the list on which to append the items
        :type index: int

def _ensure_append(self, new_items, append_to, index=0):
        """Ensure an item is appended to a list or create a new empty list

        :param new_items: the item(s) to append
        :type new_items: list(obj)
        :param append_to: the list on which to append the items
        :type append_to: list()
        :param index: index of the list on which to append the items
        :type index: int
        """
        append_to = append_to or []
        append_to.insert(index, new_items)
        return append_to