python异步IO的发展历程

python中异步IO发展分为三个发展阶段

1.使用yield和send

2.使用@asyncio.coroutine和yield from

3.使用async/await关键字

一、yield和send

def 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

for res in fib(20):
print(res)

这是一段输出斐波那契数列的代码，需要经过数次迭代。这种方式的缺点是执行这种迭代运算需要占用大量内存，而我们最终的目的如果只是想得到某一个顺序位上的数字，该方法就不太合适了。

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

for res in fib(20):
print(res)

当我们使用yield 时，无须在函数内加入return，直接用for in，便可直接得到yield的值，即没运行一次，就会再执行一次next(fib(20)) （相当于执行下一次吧)。

import random
import time
def fib(n):
index = 0
a = 0
b = 1
while index < n:
sleep_cnt = yield b
print('delay {0} seconds'.format(sleep_cnt))
time.sleep(sleep_cnt)
a, b = b, a + b
index += 1

N = 20
sfib = fib(N)
res = next(sfib)
while True:
print(res)
try:
res = sfib.send(random.uniform(0, 0.5))
except StopIteration:
break

当我们想往线程中发送数据时，可以用到send函数，此处使用time库，虚拟一下io的延迟。

二、@asyncio.coroutine和yield from

def fibAgain(n):
print('I am copy from fib')
yield from fib(n)
print('Copy end')

for res in fibAgain(20):
print(res)

以上代码承接上文，由此可见，yield from相当于重构之前的yield的代码，重新来一次刷新。

import asyncio

@asyncio.coroutine
def wget(host):
print('wget %s...' % host)
connect = asyncio.open_connection(host, 80)
reader, writer = yield from connect
header = 'GET / HTTP/1.0\r\nHost: %s\r\n\r\n' % host
writer.write(header.encode('utf-8'))
yield from writer.drain()
while True:
line = yield from reader.readline()
if line == b'\r\n':
break
print('%s header > %s' % (host, line.decode('utf-8').rstrip()))
# Ignore the body, close the socket
writer.close()

loop = asyncio.get_event_loop()
tasks = [wget(host) for host in ['www.sina.com.cn', 'www.sohu.com', 'www.163.com']]
loop.run_until_complete(asyncio.wait(tasks))
loop.close()

以上是一个连接网站的一个例子，我们使用yield挂起需要异步IO的代码，在函数前加入@asyncio.coroutine关键字，再使用实现线程的并发。

三、async/await关键字

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

if __name__ == '__main__':
loop = asyncio.get_event_loop()
tasks = [
smart_fib(10)),
stupid_fib(10))
]
loop.run_until_complete(asyncio.wait(tasks))
print('All fib finished.')
loop.close()

理解了yield from之后，async/await关键字就很好理解了，其实就是对yield from的简化。