Паттерн Итератор (iter)

class Bag:
    def __init__(self, items=None):
        self.__bag = {}
        if items is not None:
            for item in items:
                self.add(item)
    def add(self, item):
        self.__bag[item] = self.__bag.get(item, 0) + 1
    def __delitem__(self, item):
        if self.__bag.get(item) is not None:
            self.__bag[item] -= 1
            if self.__bag[item] <= 0:
                del self.__bag[item]
        else:
            raise KeyError(str(item))
    def count(self, item):
        return self.__bag.get(item, 0)
    def __len__(self):
        return sum(count for count in self.__bag.values())
    def __contains__(self, item):
        return item in self.__bag
    def __iter__(self):
        items = []
        for item, count in self.__bag.items():
            for _ in range(count):
                items.append(item)
        return iter(items)


Example:
bag = Bag('first', 'first', 'second')