python之列表推导、迭代器、生成器

http://blog.chinaunix.net/uid-26722078-id-3484197.html

1.列表推导

看几个例子，一切就明白了。

 #!/usr/bin/python
 numbers = range(10)
 size = len(numbers)
 evens = []
 i = 0
 while i < size:
     if i%2 == 0:
         evens.append(i)
     i += 1
 print evens

 #!/usr/bin/python
 evens = [i for i in range(10) if i % 2 == 0]
 print evens

 #!/usr/bin/python
 def _process(pos, element):
     return '%d:%s' % (pos, element)

 seq = ['a', 'b', 'c']
 print [_process(i, el) for i, el in enumerate(seq)]

备注：注意 enumerate用法

2.迭代器

  迭代器只不过是实现迭代器协议的容器对象，基于next和__iter__两个方法

 #!/usr/bin/python
 class TestIterator(object):
     def __init__(self, step):
         self.step = step
     def next(self):
         """Return the next element"""
         if self.step == 0:
             raise StopIteration
         self.step -= 1
         return self.step
     def __iter__(self):
         """Return the iterator itself."""
         return self
 print [el for el in TestIterator(4)]

3.生成器

  next()和send()的工作机制一样，不过使用send()，yield将变成能够返回传入的值，因而可以据此来改变行为。

看两个例子，一切就明白了。

 #!/usr/bin/python
 def power(values):
     for value in values:
         print "powing %s" % value
         yield value
 def add(values):
     for value in values:
         if value % 2 == 0:
             yield value + 3
         else:
             yield value + 2
 elements = [1, 4, 7, 9, 12, 19]
 res = add(power(elements))
 res.next()

 #!/usr/bin/python
 def psychologist():
     print "Please tell me your problems"
     while True:
         answer = (yield)
         if answer is not None:
             if answer.endswith('?'):
                 print("Don't ask yourself too much questions!")
             elif 'good' in answer:
                 print("That's good, Let's go on")
             elif 'bad' in answer:
                 print("It's a pity")
 free = psychologist()