Python协程：从yield/send到async/await

这个文章理好了脉落。

http://python.jobbole.com/86069/

我练 习了一番，感受好了很多。。。

Python由于众所周知的GIL的原因，导致其线程无法发挥多核的并行计算能力（当然，后来有了multiprocessing，可以实现多进程并行），显得比较鸡肋。既然在GIL之下，同一时刻只能有一个线程在运行，那么对于CPU密集的程序来说，线程之间的切换开销就成了拖累，而以I/O为瓶颈的程序正是协程所擅长的：

多任务并发（非并行），每个任务在合适的时候挂起（发起I/O）和恢复(I/O结束)

弄清楚了asyncio.coroutine和yield from之后，在Python3.5中引入的async和await就不难理解了：可以将他们理解成asyncio.coroutine/yield from的完美替身。当然，从Python设计的角度来说，async/await让协程表面上独立于生成器而存在，将细节都隐藏于asyncio模块之下，语法更清晰明了。

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import asyncio
import time
import random

'''
def old_fib(n):
    res = [0] * n
    index = 0
    a = 0
    b = 1
    while index < n:
        res[index] = b
        a, b = b, a + b
        index += 1
    return res

print("-"*10 + "test old fib " + "-"*10)
for fib_res in old_fib(20):
    print(fib_res)

def fib(n):
    index = 0
    a = 0
    b = 1
    while index < n:
        yield b
        a, b = b, a + b
        index += 1

print("-"*10 + "test yield fib " + "-"*10)
for fib_res in fib(20):
    print(fib_res)

def stupid_fib(n):
    index = 0
    a = 0
    b = 1
    while index < n:
        sleep_cnt = yield b
        print("let me think {0} secs".format(sleep_cnt))
        time.sleep(sleep_cnt)
        a, b = b, a + b
        index += 1
print("-"*10 + "test yield send " + "-"*10)
N = 20
sfib = stupid_fib(N)
fib_res = next(sfib)
while True:
    print(fib_res)
    try:
        fib_res = sfib.send(random.uniform(0, 0.5))
    except StopIteration:
        break

def copy_fib(n):
    print("I am copy from fib")
    yield from fib(n)
    print("copy end")
print("-"*10 + "test yield from " + "-"*10)
for fib_res in copy_fib(20):
    print(fib_res)

def copy_stupid_fib(n):
    print("I am copy from stupid fib")
    yield from stupid_fib(n)
    print("Copy end")

print("-"*10 + "test yield from and send" + "-"*10)
N = 20
csfib = copy_stupid_fib(N)
fib_res = next(csfib)
while True:
    print(fib_res)
    try:
        fib_res = csfib.send(random.uniform(0, 0.5))
    except StopIteration:
        break

@asyncio.coroutine
def smart_fib(n):
    index = 0
    a = 0
    b = 1
    while index < n:
        sleep_secs = random.uniform(0, 0.2)
        yield from asyncio.sleep(sleep_secs)
        print("Smart one think {} secs to get {}".format(sleep_secs, b))
        a, b = b, a + b
        index += 1

@asyncio.coroutine
def stupid_fib(n):
    index = 0
    a = 0
    b = 1
    while index < n:
        sleep_secs = random.uniform(0, 0.4)
        yield from asyncio.sleep(sleep_secs)
        print("Stupid one think {} secs to get {}".format(sleep_secs, b))
        a, b = b, a + b
        index += 1

loop = asyncio.get_event_loop()
tasks = [
    asyncio.async(smart_fib(10)),
    asyncio.async(stupid_fib(10)),
    ]
loop.run_until_complete(async.wait(tasks))
print("All fib finished.")
loop.close()

'''

async def smart_fib(n):
    index = 0
    a = 0
    b = 1
    while index < n:
        sleep_secs = random.uniform(0, 0.2)
        await asyncio.sleep(sleep_secs)
        print("Smart one think {} secs to get {}".format(sleep_secs, b))
        a, b = b, a + b
        index += 1

async def stupid_fib(n):
    index = 0
    a = 0
    b = 1
    while index < n:
        sleep_secs = random.uniform(0, 0.4)
        await asyncio.sleep(sleep_secs)
        print("Stupid one think {} secs to get {}".format(sleep_secs, b))
        a, b = b, a + b
        index += 1

loop = asyncio.get_event_loop()
tasks = [
    asyncio.async(smart_fib(10)),
    asyncio.async(stupid_fib(10)),
    ]
loop.run_until_complete(asyncio.wait(tasks))
print("All fib finished.")
loop.close()

Python中的协程经历了很长的一段发展历程。其大概经历了如下三个阶段：

1. 最初的生成器变形yield/send
2. 引入@asyncio.coroutine和yield from
3. 在最近的Python3.5版本中引入async/await关键字