一、安装模块

安装：pip3 install greenlet

二、greenlet实现状态切换

from greenlet import greenlet

def eat(name):
    print('%s eat 1' %name)
    g2.switch('randy')
    print('%s eat 2' %name)
    g2.switch()
def play(name):
    print('%s play 1' %name)
    g1.switch()
    print('%s play 2' %name)

g1=greenlet(eat)
g2=greenlet(play)

g1.switch('randy')#可以在第一次switch时传入参数，以后都不需要

randy eat 1
randy play 1
randy eat 2
randy play 2

三、效率对比

单纯的切换（在没有io的情况下或者没有重复开辟内存空间的操作），反而会降低程序的执行速度。

#顺序执行
import time


def f1():
    res = 1
    for i in range(100000000):
        res += i


def f2():
    res = 1
    for i in range(100000000):
        res *= i


start = time.time()
f1()
f2()
stop = time.time()
print('run time is %s' % (stop - start))  # 10.985628366470337

run time is 13.264543294906616

# 切换
from greenlet import greenlet
import time


def f1():
    res = 1
    for i in range(100000000):
        res += i
        g2.switch()


def f2():
    res = 1
    for i in range(100000000):
        res *= i
        g1.switch()


start = time.time()
g1 = greenlet(f1)
g2 = greenlet(f2)
g1.switch()
stop = time.time()
print('run time is %s' % (stop - start))  # 52.763017892837524

greenlet只是提供了一种比generator更加便捷的切换方式，当切到一个任务执行时如果遇到io，那就原地阻塞，仍然是没有解决遇到IO自动切换来提升效率的问题。

单线程里的这20个任务的代码通常会既有计算操作又有阻塞操作，我们完全可以在执行任务1时遇到阻塞，就利用阻塞的时间去执行任务2...如此，才能提高效率，这就用到了Gevent模块。