def backfill_append(listobject, index, item):
    """Returns a newlist with `item` inserted at `index`.
    Inserts None between last used index of `listobject` and
    `index` at which `item` is inserted::

       >>> backfill_append(['1', '2'], 5, '6')
       ['1', '2', None, None, None, '6']

    """
    temp_list = listobject[:]
    if index >= len(temp_list):
        difference = index - len(temp_list)
        backfill = [None for x in range(difference)]
        backfill.append(item)
        temp_list.extend(backfill)
    else:
        temp_list[index] = item
    return temp_list