def fill_list(short_list, fill_value, length):
    """Extend a list to specified length by filling it with the fill_value.

    Examples:
    >>> fill_list(["a"], "b", 3)
    ['a', 'b', 'b']

    """
    res = list(short_list)
    diff = length - len(short_list)
    assert diff >= 0, "short_list has to be shorter than length."
    if diff >= 1:
        res += [fill_value] * diff
    return res