Maintain a relationship between a minion and a dns name

def _id_map(minion_id, dns_name):
    '''
    Maintain a relationship between a minion and a dns name
    '''
    bank = 'digicert/minions'
    cache = salt.cache.Cache(__opts__, syspaths.CACHE_DIR)
    dns_names = cache.fetch(bank, minion_id)
    if not isinstance(dns_names, list):
        dns_names = []
    if dns_name not in dns_names:
        dns_names.append(dns_name)
    cache.store(bank, minion_id, dns_names)