Gevent

Gevent 是一个第三方库，可以轻松通过gevent实现并发同步或异步编程，在gevent中用到的主要模式是Greenlet, 它是以C扩展模块形式接入Python的轻量级协程。 Greenlet全部运行在主程序操作系统进程的内部，但它们被协作式地调度。

示例

import gevent
#gevent，自动挡切换

def func1():
    print(‘Ashley starts running.‘)    #1
    gevent.sleep(2)    #切换到2
    print(‘Ashley tumbled over Sarah.‘)    #6

def func2():
    print(‘Sarah starts running.‘)    #2
    gevent.sleep(1)    #切换到3
    print(‘Sarah tumbled over Nancy.‘)    #5

def func3():    
    print(‘Nancy starts running.‘)    #3
    gevent.sleep(0)    #切换，但无需等待
    print(‘Nancy tumbled on a hurdle.‘)    #4

gevent.joinall([
    gevent.spawn(func1),    #spawn引发引起导致造成
    gevent.spawn(func2),
    gevent.spawn(func3)
])

结果

Ashley starts running.
Sarah starts running.
Nancy starts running.
Nancy tumbled on a hurdle.
Sarah tumbled over Nancy.
Ashley tumbled over Sarah.

同步与异步的性能区别

看下面示例

import gevent

def task(pid):
    gevent.sleep(0.5)
    print(‘Task %s done‘ % pid)

def synchronous():
    for i in range(1, 10):
        task(i)    #串行，循环10次task函数

def asynchronous():
    threads = [gevent.spawn(task, i) for i in range(10)]
    #发起协程，协程执行task函数，循环10次，总计10条线程，存储在threads里
    gevent.joinall(threads)

print(‘Synchronous:‘)
synchronous()    #同步串行
print(‘Asynchronous:‘)
asynchronous()    #异步并行

结果

Synchronous:    #这里每条睡0.5秒
Task 1 done
Task 2 done
Task 3 done
Task 4 done
Task 5 done
Task 6 done
Task 7 done
Task 8 done
Task 9 done
Asynchronous:    #这里是同步执行
Task 0 done
Task 1 done
Task 2 done
Task 3 done
Task 4 done
Task 5 done
Task 6 done
Task 7 done
Task 8 done

上面程序的重要部分是将task函数封装到Greenlet内部线程的gevent.spawn。 初始化的greenlet列表存放在数组threads中，此数组被传给gevent.joinall 函数，后者阻塞当前流程，并执行所有给定的greenlet。执行流程只会在 所有greenlet执行完后才会继续向下走。　　

遇到IO阻塞时会自动切换任务，下面是一个简单的爬取网页的示例，我们看一下同步与异步的区别：

import gevent, time
from gevent import monkey    #gevent无法识别socket中的I/O操作
from urllib import request
monkey.patch_all()    #有了这个方法，相当于给socket的I/O操作加上了标识

def f(url):
    print(‘GET: %s‘ % url)
    resp = request.urlopen(url)
    data = resp.read()    #I/O任务，每当遇到I/O任务会自动切换
    print(‘%d bytes received from %s.‘ % (len(data), url))
    # print(‘Data from %s is: %s.‘ % (url, data))

urls = [‘https://www.python.org/‘,
        ‘https://www.yahoo.com/‘,
        ‘https://github.com/‘]

sync_start_time = time.time()
for url in urls:
    f(url)
print(‘同步Cost: ‘, time.time() - sync_start_time)

async_start_time = time.time()
gevent.joinall([
    gevent.spawn(f, ‘https://www.python.org/‘),    #启动协程，执行f函数，并将url传递给f
    gevent.spawn(f, ‘https://www.yahoo.com/‘),
    gevent.spawn(f, ‘https://github.com/‘),
])
print(‘异步Cost: ‘, time.time() - async_start_time)

结果

GET: https://www.python.org/
48981 bytes received from https://www.python.org/.
GET: https://www.yahoo.com/
334724 bytes received from https://www.yahoo.com/.
GET: https://github.com/
132874 bytes received from https://github.com/.
同步Cost:  25.191901445388794
GET: https://www.python.org/
GET: https://www.yahoo.com/
GET: https://github.com/
48981 bytes received from https://www.python.org/.
132874 bytes received from https://github.com/.
334179 bytes received from https://www.yahoo.com/.
异步Cost:  5.816762208938599

由于每次遇到I/O操作都会自动切换，所以，先输出三条print(‘GET: %s’ % url)，当遇到data = resp.read()这步时会自动切换。read()读取属于I/O操作。

3.4.2 Gevent

原文：https://www.cnblogs.com/infinitecodes/p/12134529.html