code: def __set_quantity(self, value):
          '''
          Sets the quantity
          @param value:str
          '''
          try:
              if value < 0:
                  raise ValueError()
  
              self.__quantity = Decimal(str(value))
          except ValueError:
              raise ValueError("Quantity must be a positive number"f __set_quantity(self, value):
          '''
          Sets the quantity
          @param value:str
          '''
          try:
              if value < 0:
                  raise ValueError()
  
              self.__quantity = Decimal(str(value))
          except ValueError:
              raise ValueError("Quantity must be a positive number")
docs: Sets the quantity
          @param value:str