Python 协程

协程，又称微线程，纤程。英文名Coroutine。

子程序，或者称为函数，在所有语言中都是层级调用，比如A调用B，B在执行过程中又调用了C，C执行完毕返回，B执行完毕返回，最后是A执行完毕。 所以子程序调用是通过栈实现的，一个线程就是执行一个子程序。

子程序调用总是一个入口，一次返回，调用顺序是明确的。而协程的调用和子程序不同。

协程看上去也是子程序，但执行过程中，在子程序内部可中断，然后转而执行别的子程序，在适当的时候再返回来接着执行。

注意，在一个子程序中中断，去执行其他子程序，不是函数调用，有点类似CPU的中断

看起来A、B的执行有点像多线程，但协程的特点在于是一个线程执行，那和多线程比，协程有何优势？

第一大优势就是协程极高的执行效率

因为子程序切换不是线程切换，而是由程序自身控制，因此，没有线程切换的开销，和多线程比，线程数量越多，协程的性能优势就越明显。

第二大优势就是不需要多线程的锁机制

因为只有一个线程，也不存在同时写变量冲突，在协程中控制共享资源不加锁，只需要判断状态就好了，所以执行效率比多线程高很多。

因为协程是一个线程执行，那怎么利用多核CPU呢？最简单的方法是多进程+协程，既充分利用多核，又充分发挥协程的高效率，可获得极高的性能。

Python对协程的支持还非常有限，用在generator中的yield可以一定程度上实现协程。虽然支持不完全，但已经可以发挥相当大的威力了。

import time

def generate():
    n = 0
    while True:
        n = n + 1
        print('generate is going to yield %d' % n)
        r = yield n              # 关键一句
        print('generate has got %d' % r)

def process(g):
    next(g)
    n = 0
    for i in range(10):
        n = n + 1
        print('process is going to send %d' % n)
        r = g.send(n)          # 关键一句
        print('process has got %d' % r)
        print('====================')
        time.sleep(1)
    g.close()

if __name__=='__main__':
    g = generate()
    process(g)

运行结果

generate is going to yield 1
process is going to send 1
generate has got 1
generate is going to yield 2
process has got 2
====================
process is going to send 2
generate has got 2
generate is going to yield 3
process has got 3
====================
process is going to send 3
generate has got 3
generate is going to yield 4
process has got 4
====================
process is going to send 4
generate has got 4
generate is going to yield 5
process has got 5
====================
process is going to send 5
generate has got 5
generate is going to yield 6
process has got 6
====================
process is going to send 6
generate has got 6
generate is going to yield 7
process has got 7
====================
process is going to send 7
generate has got 7
generate is going to yield 8
process has got 8
====================
process is going to send 8
generate has got 8
generate is going to yield 9
process has got 9
====================
process is going to send 9
generate has got 9
generate is going to yield 10
process has got 10
====================
process is going to send 10
generate has got 10
generate is going to yield 11
process has got 11
====================

协程看起来就好像是两个程序互为对方的子程序，程序在两个子程序之间不停的切换