Need to transform this mysql function into redshift function or stored procedure

Question

so i have an mysql function that i need to convert to a procedure or function that is compatible with aws redshift.

This is the mysql function:

    CREATE DEFINER=`software`@`%` FUNCTION `redeemed_qty`(ticketID bigint) RETURNS bigint
BEGIN
   DECLARE redeemed_qty bigint;
   SELECT
      SUM(quantity) INTO redeemed_qty
   FROM redemption
   WHERE ticket_id = ticketID
   AND (ISNULL(remark)
   OR remark <> 'Cancelled');
   IF redeemed_qty IS NULL THEN
     SELECT IFNULL(CASE WHEN SUM(quantity_total) > quantity_total THEN quantity_total END,0) INTO redeemed_qty FROM ticket WHERE parent_id = ticketID AND status = 2;
   END IF;
  RETURN redeemed_qty;
END 

When I tried transforming it into a redshift function:

CREATE FUNCTION fn_redeemedqty (ticketID bigint,remark character varying,quantity bigint)
RETURNS bigint
STABLE
AS $$
    if (ticketID is not None) & (remark <> 'Cancelled') | (remark is None):
      return sum(quantity)
    return 0

$$ LANGUAGE plpythonu
;

But how do i insert the IF statement from mysql.

I also sometimes get an error:

global name 'plpy' is not defined.

I would appreciate your answers.