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