def _gen_ticket(prefix=None, lg=settings.CAS_TICKET_LEN):
    """
        Generate a ticket with prefix ``prefix`` and length ``lg``

        :param unicode prefix: An optional prefix (probably ST, PT, PGT or PGTIOU)
        :param int lg: The length of the generated ticket (with the prefix)
        :return: A randomlly generated ticket of length ``lg``
        :rtype: unicode
    """
    random_part = u''.join(
        random.choice(
            string.ascii_letters + string.digits
        ) for _ in range(lg - len(prefix or "") - 1)
    )
    if prefix is not None:
        return u'%s-%s' % (prefix, random_part)
    else:
        return random_part
Generate a ticket with prefix ``prefix`` and length ``lg``

        :param unicode prefix: An optional prefix (probably ST, PT, PGT or PGTIOU)
        :param int lg: The length of the generated ticket (with the prefix)
        :return: A randomlly generated ticket of length ``lg``
        :rtype: unicode