'Returns a (possibly empty) list of valid access tokens for a given user Account.'

@classmethod
def _by_user(cls, account):

    try:
        tba = cls._by_user_view()._byID(account._id36)
    except tdb_cassandra.NotFound:
        return []
    tokens = cls._byID(tba._values().keys())
    return [token for token in tokens.itervalues() if token.check_valid()]
