class Bag(object):    def __init__(self,maxsize=10):        self.maxsize=maxsize        self._item=list()    def add(self,item):        if len(self)>self.maxsize:            raise Exception('bag is full')        self._item.append(item)    def remove(self,item):        self._item.remove(item)    def __len__(self):        return len(self._item)    def __iter__(self):        for item in self._item:            yield itemdef test_bag():    bag=Bag()    bag.add(1)    bag.add(2)    bag.add(3)    assert len(bag)==3    bag.remove(3)    assert len(bag)==2    for i in bag:        print(i)test_bag()