def check_payment(self, payment):
        """
        Check the payment for required fields and validity.
        @param payment: The payment dict
        @return: True if valid, error string if invalid paramaters where
        encountered.
        """
        validation = ""

        if not isinstance(payment['amount'], int):
            validation += "AMOUNT_NOT_INTEGER "

        if not isinstance(payment['mandate_date'], datetime.date):
            validation += "MANDATE_DATE_INVALID_OR_NOT_DATETIME_INSTANCE"
        payment['mandate_date'] = str(payment['mandate_date'])

        if not isinstance(payment['collection_date'], datetime.date):
            validation += "COLLECTION_DATE_INVALID_OR_NOT_DATETIME_INSTANCE"
        payment['collection_date'] = str(payment['collection_date'])

        if validation == "":
            return True
        else:
            raise Exception('Payment did not validate: ' + validation)
Check the payment for required fields and validity.
        @param payment: The payment dict
        @return: True if valid, error string if invalid paramaters where
        encountered.