def fill_width(bytes_, width):
    """
    Ensure a byte string representing a positive integer is a specific width
    (in bytes)

    :param bytes_:
        The integer byte string

    :param width:
        The desired width as an integer

    :return:
        A byte string of the width specified
    """

    while len(bytes_) < width:
        bytes_ = b'\x00' + bytes_
    return bytes_