Add round_price

We add a common method to round prices that are stored using the price digits.
It is useless to use the digits from the field because it can not be changed in
any other way than by configuration.

issue9146
review299521002

production.py

@@ -12,7 +12,7 @@ from trytond.pool import Pool
 from trytond.transaction import Transaction
 
 from trytond.modules.company.model import employee_field, set_employee
-from trytond.modules.product import price_digits
+from trytond.modules.product import price_digits, round_price
 
 BOM_CHANGES = ['bom', 'product', 'quantity', 'uom', 'warehouse', 'location',
     'company', 'inputs', 'outputs']
@@ -426,9 +426,7 @@ class Production(Workflow, ModelSQL, ModelView):
             else:
                 cost_price = input_.product.cost_price
             cost += (Decimal(str(input_.internal_quantity)) * cost_price)
-
-        digits = self.__class__.cost.digits
-        return cost.quantize(Decimal(str(10 ** -digits[1])))
+        return round_price(cost)
 
     @fields.depends('inputs')
     def on_change_with_cost(self):
@@ -522,7 +520,6 @@ class Production(Workflow, ModelSQL, ModelView):
         Uom = pool.get('product.uom')
         Move = pool.get('stock.move')
 
-        digits = Decimal(str(10 ** -Move.unit_price.digits[1]))
         moves = []
         for production in productions:
             sum_ = Decimal(0)
@@ -554,8 +551,7 @@ class Production(Workflow, ModelSQL, ModelView):
                 else:
                     ratio = Decimal(1) / len(production.outputs)
                 quantity = Decimal(str(output.quantity))
-                unit_price = (
-                    production.cost * ratio / quantity).quantize(digits)
+                unit_price = round_price(production.cost * ratio / quantity)
                 if output.unit_price != unit_price:
                     output.unit_price = unit_price
                     moves.append(output)