python学习笔记 异步asyncio

asyncio是Python 3.4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。咱们从asyncio模块中直接获取一个EventLoop的引用，而后把须要执行的协程扔到EventLoop中执行，就实现了异步IO。

用asyncio实现Hello world代码以下：

import asyncio

@asyncio.coroutine
def hello():
    print("Hello world!")
    # 异步调用asyncio.sleep(1):
    r = yield from asyncio.sleep(1)
    print("Hello again!")

# 获取EventLoop:
loop = asyncio.get_event_loop()
# 执行coroutine
loop.run_until_complete(hello())
loop.close()

@asyncio.coroutine把一个generator标记为coroutine类型，而后，就把这个coroutine扔到eventloop中去执行

hello（）会先打印出helloworld，而后yield from能够让咱们方便的调用另外一个generator，因为asyncio.sleep(1)也是一个coroutine

因此线程不会等待asyncio.sleep而是直接中断并执行下一个消息循环，当asyncio.sleep返回的时候，线程就在yield from拿到返回值，此处是None

而后执行下一个语句

把asyncio.sleep（1）当作是一个耗时1秒的IO操做。在此期间主线程没有等待，而是去执行eventloop其余能够执行的coroutine所以能够实现并发执行

接下来封装2个coroutine试试

import threading
import asyncio

@asyncio.coroutine
def hello():
    print('Hello world! (%s)' % threading.currentThread())
    yield from asyncio.sleep(1)
    print('Hello again! (%s)' % threading.currentThread())

loop = asyncio.get_event_loop()
tasks = [hello(), hello()]
loop.run_until_complete(asyncio.wait(tasks))
loop.close()

Hello world! (<_MainThread(MainThread, started 140735195337472)>)
Hello world! (<_MainThread(MainThread, started 140735195337472)>)
(暂停约1秒)
Hello again! (<_MainThread(MainThread, started 140735195337472)>)
Hello again! (<_MainThread(MainThread, started 140735195337472)>)

由打印的当前线程名称能够看出，两个coroutine是由同一个线程并发执行的。

若是把asyncio.sleep()换成真正的IO操做，则多个coroutine就能够由一个线程并发执行。

咱们用asyncio的异步网络链接来获取sina、sohu和163的网站首页：

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()

wget www.sohu.com...
wget www.sina.com.cn...
wget www.163.com...
(等待一段时间)
(打印出sohu的header)
www.sohu.com header > HTTP/1.1 200 OK
www.sohu.com header > Content-Type: text/html
...
(打印出sina的header)
www.sina.com.cn header > HTTP/1.1 200 OK
www.sina.com.cn header > Date: Wed, 20 May 2015 04:56:33 GMT
...
(打印出163的header)
www.163.com header > HTTP/1.0 302 Moved Temporarily
www.163.com header > Server: Cdn Cache Server V2.0

asyncio提供了完善的异步IO支持；

异步操做须要在coroutine中经过yield from完成；

多个coroutine能够封装成一组Task而后并发执行。