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。

 1 #!/usr/bin/python
 2 numbers = range(10)
 3 size = len(numbers)
 4 evens = []
 5 i = 0
 6 while i < size:
 7     if i%2 == 0:
 8         evens.append(i)
 9     i += 1
10 print evens
11  
12 #!/usr/bin/python
13 evens = [i for i in range(10) if i % 2 == 0]
14 print evens
15  
16 #!/usr/bin/python
17 def _process(pos, element):
18     return '%d:%s' % (pos, element)
19  
20 seq = ['a', 'b', 'c']
21 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

 1 #!/usr/bin/python
 2 class TestIterator(object):
 3     def __init__(self, step):
 4         self.step = step
 5     def next(self):
 6         """Return the next element"""
 7         if self.step == 0:
 8             raise StopIteration
 9         self.step -= 1
10         return self.step
11     def __iter__(self):
12         """Return the iterator itself."""
13         return self
14 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。

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

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