quantity_descriptor.py

class Quantity:
  __counter = 0
  
  def __init__(self):
    cls = self.__class__
    self.storage_name = f'_{cls.__name__}#{cls.__counter}'
    cls.__counter += 1
    
  def __get__(self, instance, owner):  # `owner` is the ref of the managed class 
    if instance is None:
      # when access from managed class not managed instance, e.g. `CartItem.price`
      # return descriptor instance is better 
      return self
    return getattr(instance, self.storage_name)
    
  def __set__(self, instance, val):
    if val > 0:
      setattr(instance, self.storage_name, val)
    else:
      raise ValueError()
      
class CartItem:
  weight = Quantity()
  price = Quantity()