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()