def update_ticket(self, tid, tickets=None):
        """If the customer should be granted an electronic ticket as a result
        of a successful payment, the merchant may (at any time) PUT ticket
        information to this endpoint. There is an ordered list of tickets; the
        merchant may PUT several times to update the list. The PUT overwrites
        any existing content, so if adding additional tickets one must remember
        to also include the tickets previously issued.

        So far the only code type supported is "string", meaning a text code
        that is displayed to the customer, however we will add QR code,
        barcodes etc. soon.  Please contact mCASH about supporting your
        barcode.

        Arguments:
            tickets:
                List of tickets to grant customer
        """
        arguments = {'tickets': tickets}
        return self.do_req('PUT',
                           self.merchant_api_base_url + '/payment_request/' +
                           tid + '/ticket/', arguments)