Python的迭代

这个文章不错：
Python函数式编程指南（三）：迭代器

说起来，之前的编程主要是C，没用过迭代。这跟循环有什么区别？

看《Python基础教程》，看到这里，有点晕：

一个实现了__iter__方法的对象是可迭代的，一个实现了next方法的对象是迭代器。

写个代码来看看：

class FF:
   def __init__(self):
      self.a = 0
      print ">>> in __init__: ", self.a
   def next(self):
      print ">>> in __next__: ", self.a
      if self.a > 3:
          raise StopIteration
      self.a = self.a + 1
      return self.a
   def __iter__(self):
      print ">>> in __iter__: ", self.a
      return self

print "===================="
ff=FF()
print "==== After the generation of the instance ===="

def loop(i):
  print "==== start loop ", i, "===="
  for i in ff:                             ## 调用了__iter__，然后再每次的循环中diaoyon
    print "==== start each loop ===="
    print i
    print "==== end ================"

loop(1)
print ""
loop(2)

输出：

====================
>>> in __init__:  0
==== After the generation of the instance ====
==== start loop  1 ====
>>> in __iter__:  0
>>> in __next__:  0
==== start each loop ====
1
==== end ================
>>> in __next__:  1
==== start each loop ====
2
==== end ================
>>> in __next__:  2
==== start each loop ====
3
==== end ================
>>> in __next__:  3
==== start each loop ====
4
==== end ================
>>> in __next__:  4


==== start loop  2 ====
>>> in __iter__: 4
>>> in __next__: 4

同样的实例，如果调用loop函数，直接这么操作得到一个列表：

print "===================="
ff=FF()
print "==== After the generation of the instance ===="
myiter=list(ff)
print myiter

输出结果：

====================
>>> in __init__:  0
==== After the generation of the instance ====
>>> in __iter__:  0
>>> in __next__:  0
>>> in __next__:  1
>>> in __next__:  2
>>> in __next__:  3
>>> in __next__:  4
[1, 2, 3, 4]