coroutines总结整理

这次看了一些关于协程(coroutine)的资料，这是python特有的，我会在这篇文字里面慢慢总结整理下来，估计会写个1周多。

coroutine和generator的关系

generator用到了yield语法，而coroutine也是用到了yield语法，什么区别呢？大致是这样的：

1. generator是为iteration产生数据
2. coroutine是消费数据
3. coroutine也可以产生值，但目的不是为了迭代
4. 不要把2个概念混在一起，写出让大家摸不着头脑的代码

下面的代码是generator，用来倒数迭代用

# countdown.py
#
# A simple generator function

def countdown(n):
    print "Counting down from", n
    while n > 0:
        yield n
        n -= 1
    print "Done counting down"

# Example use
if __name__ == '__main__':
    for i in countdown(5):
        print i

输出是

Counting down from 5
5
4
3
2
1
Done counting down

下面代码是coroutine，用来消费字符串，过滤出有用的行

# grep.py
#
# A very simple coroutine

def grep(pattern):
    print "Looking for %s" % pattern
    while True:
        line = (yield)
        if pattern in line:
            print line,

# Example use
if __name__ == '__main__':
    g = grep("python")
    # g.send(None)
    g.next()
    g.send("Yeah, but no, but yeah, but no")
    g.send("A series of tubes")
    g.send("python generators rock!")
    g.close()

输出是

Looking for python
python generators rock!

千万不要写下面的代码，又想消费，又想迭代，结果不知所云：

# bogus.py
#
# Bogus example of a generator that produces and receives values


def countdown(n):
    print "Counting down from", n
    while n >= 0:
        newvalue = (yield n)
        # If a new value got sent in, reset n with it
        if newvalue is not None:
            n = newvalue
        else:
            n -= 1

# The holy grail countdown
c = countdown(5)
for x in c:
    print x
    if x == 5:
        c.send(3)

输出是：

Counting down from 5
5
2
1
0