python 将对象设置为可迭代的两种实现方法

class Library(object):  def __init__(self):    self.value=['a','b','c','d','e']  def __getitem__(self, i):    if i>=len(self.value):      raise IndexError("out of index")    value=self.value[i]    return value

class Library2(object):  def __init__(self):    self.value=['a','b','c','d','e']    self.i=-1  def __iter__(self):    return self  def next(self):    self.i += 1    if self.i>=len(self.value):      raise StopIteration    return self.value[self.i]     test=Library2() print test.next() print test.next()