def check_op(op):
    """
    :param op: op to be checked
    :return: op if acceptable, raise error if not
    """
    acceptable_op = ["=", ">=", "<=", "CONTAINS", "!!=", ">", "<", "<>", "IS NULL", "IS NOT NULL"]

    if op not in acceptable_op:
        raise KeyError("'op' sent ('%s') not acceptable. Please check your request." % op)

    return op