Write a server side script in python or frappe

from __future__ import unicode_literals
import frappe
from frappe.utils import flt
from frappe.model.document import Document

class SalesOrder(Document):
def validate(self):
total_weight = 0.0
for row in self.menu :
row.total=flt(row.rate_per_unit)*flt(row.quantity)
discount=row.total*row.discount/100
total_weight += row.total-discount
self.weight=total_weight
self.total=total_weight