asyncio

asyncio是python3.4版本引入的标准库，直接内置了对异步IO的支持。

 

asyncio的编程模型就是一个消息循环。我们从asyncio模块中直接获取一个EventLoop的引用，把需要执行的协程扔到EventLoop中执行，就实现了异步IO.

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()

 yield from：在生成器gen中使用yield from subgen（）时，subgen会获得控制权，把产出的值传给gen的调用方（这里为事件循环），即调用方可以控制subgen，与此同时gen会阻塞，等待subgen终止。

 

yield from可用于简化for循环中的yield表达式，例如：

def gen():
    for c in 'AB':
        yield c
    for i in range(1, 3):
        yield i

list(gen())
['A', 'B', '1', '2']

可以改写为：

def gen():
    yield from 'AB'
    yield from range(1, 3)
    

list(gen())
['A', 'B', '1', '2']