greenlet已经实现了协程，但是这个还的人工切换，是不是觉得太麻烦了，不要捉急，python还有一个比greenlet更强大的并且能够自动切换任务的模块gevent

其原理是当一个greenlet遇到IO(指的是input output 输入输出，比如网络、文件操作等)操作时，比如访问网络，就自动切换到其他的greenlet，等到IO操作完成，再在适当的时候切换回来继续执行。

由于IO操作非常耗时，经常使程序处于等待状态，有了gevent为我们自动切换协程，就保证总有greenlet在运行，而不是等待IO

1. gevent的使用

#coding=utf-8

#请使用python 2 来执行此程序

import gevent

def f(n):
    for i in range(n):
        print gevent.getcurrent(), i

g1 = gevent.spawn(f, 5)
g2 = gevent.spawn(f, 5)
g3 = gevent.spawn(f, 5)
g1.join()
g2.join()
g3.join()

运行结果

<Greenlet at 0x10e49f550: f(5)> 0
<Greenlet at 0x10e49f550: f(5)> 1
<Greenlet at 0x10e49f550: f(5)> 2
<Greenlet at 0x10e49f550: f(5)> 3
<Greenlet at 0x10e49f550: f(5)> 4
<Greenlet at 0x10e49f910: f(5)> 0
<Greenlet at 0x10e49f910: f(5)> 1
<Greenlet at 0x10e49f910: f(5)> 2
<Greenlet at 0x10e49f910: f(5)> 3
<Greenlet at 0x10e49f910: f(5)> 4
<Greenlet at 0x10e49f4b0: f(5)> 0
<Greenlet at 0x10e49f4b0: f(5)> 1
<Greenlet at 0x10e49f4b0: f(5)> 2
<Greenlet at 0x10e49f4b0: f(5)> 3
<Greenlet at 0x10e49f4b0: f(5)> 4

可以看到，3个greenlet是依次运行而不是交替运行

2. gevent切换执行

import gevent

def f(n):
    for i in range(n):
        print gevent.getcurrent(), i
        #用来模拟一个耗时操作，注意不是time模块中的sleep
        gevent.sleep(1)

g1 = gevent.spawn(f, 5)
g2 = gevent.spawn(f, 5)
g3 = gevent.spawn(f, 5)
g1.join()
g2.join()
g3.join()

运行结果

<Greenlet at 0x7fa70ffa1c30: f(5)> 0
<Greenlet at 0x7fa70ffa1870: f(5)> 0
<Greenlet at 0x7fa70ffa1eb0: f(5)> 0
<Greenlet at 0x7fa70ffa1c30: f(5)> 1
<Greenlet at 0x7fa70ffa1870: f(5)> 1
<Greenlet at 0x7fa70ffa1eb0: f(5)> 1
<Greenlet at 0x7fa70ffa1c30: f(5)> 2
<Greenlet at 0x7fa70ffa1870: f(5)> 2
<Greenlet at 0x7fa70ffa1eb0: f(5)> 2
<Greenlet at 0x7fa70ffa1c30: f(5)> 3
<Greenlet at 0x7fa70ffa1870: f(5)> 3
<Greenlet at 0x7fa70ffa1eb0: f(5)> 3
<Greenlet at 0x7fa70ffa1c30: f(5)> 4
<Greenlet at 0x7fa70ffa1870: f(5)> 4
<Greenlet at 0x7fa70ffa1eb0: f(5)> 4

3个greenlet交替运行

3. gevent并发下载器

当然，实际代码里，我们不会用gevent.sleep()去切换协程，而是在执行到IO操作时，gevent自动切换，代码如下：

#coding=utf-8

from gevent import monkey; 
import gevent
import urllib2

#有IO才做时需要这一句
monkey.patch_all()

def myDownLoad(url):
    print('GET: %s' % url)
    resp = urllib2.urlopen(url)
    data = resp.read()
    print('%d bytes received from %s.' % (len(data), url))

gevent.joinall([
        gevent.spawn(myDownLoad, 'http://www.baidu.com/'),
        gevent.spawn(myDownLoad, 'http://www.itcast.cn/'),
        gevent.spawn(myDownLoad, 'http://www.itheima.com/'),
])

运行结果

GET: http://www.baidu.com/
GET: http://www.itcast.cn/
GET: http://www.itheima.com/
102247 bytes received from http://www.baidu.com/.
166903 bytes received from http://www.itheima.com/.
162294 bytes received from http://www.itcast.cn/.

从上能够看到是先发送的获取baidu的相关信息，然后依次是itcast、itheima，但是收到数据的先后顺序不一定与发送顺序相同，这也就体现出了异步，即不确定什么时候会收到数据，顺序不一定