def pretty(hex_string, seperator=' '):
    """
    Prettify a hex string.

    >>> pretty("\x01\x02\x03\xff")
    '01 02 03 FF'
    """
    hex_string = hex_string.encode('hex')
    out = ''

    for i in range(len(hex_string)):
        if not i % 2:
            out = out + seperator
        out = out + hex_string[i].capitalize()

    return out