yield的用法案例

#yield.py

#! /usr/bin/env python
#coding=utf-8
class Permutation:
    def __init__(self, justalist):
        self._data = justalist[:]
        self._sofar = []
    def __iter__(self):
        return self.next()
    def next(self):
        for elem in self._data:
            if elem not in self._sofar:
                self._sofar.append(elem)
                if len(self._sofar) == len(self._data):
                    yield self._sofar[:]
                else:
                    for v in self.next():
                        yield v
                self._sofar.pop()

a = [1,2,3]
for i in Permutation(a):
    print i

$ python yield.py

[1, 2, 3]

[1, 3, 2]

[2, 1, 3]

[2, 3, 1]

[3, 1, 2]

[3, 2, 1]

 

#yield2.py

#! /usr/bin/env python
#coding=utf-8
import os
def walk(path):
    if os.path.isdir(path):
        for file in os.listdir(path):
            file_path=os.path.join(path, file);
            for sub_file in walk(file_path):
                yield sub_file
    else:
        pass
    yield path

for file in walk("."):
    print file

以上是yield的用法。这个东西不太好理解。网上有很多例子。讲的也不是很透彻。