class Order(list):
    def __init__(self, discount, total_price, date):
        self.discount = discount 
        self._total_price = total_price
        self.date = date
        
    @property
    def discount(self):
        return self._discount
    
    @discount.setter
    def discount(self, value):
        low, high = 0, 99
        if not low <= value <= high:
            raise ValueError("should be in [{}:{}], "
                             "but passed {}".format(low, high, value))
        self._discount = value

    @property
    def total_price(self):
        return self._total_price - self._total_price * self._discount


class Item():
    def __init__(self, id, name, description, price):
        self.id = id 
        self.name = name
        self.description = description
        self.price = price


class DownloadableItem(Item):
    def __init__(self, filename, downloads_count):
        self.filename = filename
        self.downloads_count = downloads_count

>>> import itertools
>>> 
>>> class Item:
...     def __init__(self, name, price):
...         self.name = name
...         self.price = price
... 
>>> class Order(list):
...     def __init__(self, name):
...         self.name = name
...     
...     def __str__(self):
...         out = self.name + ':\n'
...         srt = sorted(self, key=lambda i: i.name)
...         grp = itertools.groupby(srt, key=lambda i: i.name)
...         for name, it in grp:
...             tup = tuple(i.price for i in it)
...             n = len(tup)
...             fmt = '  {}: number {} | price {}\n'
...             out += fmt.format(name, n, tup[0] * n)
...         return out
... 
>>> order = Order('Order 1')
>>> order.append(Item('Item 1', 1))
>>> order.append(Item('Item 2', 3))
>>> order.append(Item('Item 1', 1))
>>> order.append(Item('Item 2', 3))
>>> order.append(Item('Item 1', 1))
>>> order.append(Item('Item 2', 3))
>>> order.append(Item('Item 3', 5))
>>> order.append(Item('Item 3', 5))
>>> 
>>> print(order, end='')
Order 1:
  Item 1: number 3 | price 3
  Item 2: number 3 | price 9
  Item 3: number 2 | price 10
>>>