Get a default dones object for ns.  If no dones object exists for ns yet,
    a DbDones object will be created, cached, and returned.

def get(ns, dburl=None):
    '''
    Get a default dones object for ns.  If no dones object exists for ns yet,
    a DbDones object will be created, cached, and returned.
    '''
    if dburl is None:
        dburl = DONES_DB_URL

    cache_key = (ns, dburl)
    if ns not in DONES_CACHE:
        dones_ns = 'dones_{}'.format(ns)
        DONES_CACHE[cache_key] = DbDones(ns=dones_ns, dburl=dburl)

    return DONES_CACHE[cache_key]