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  • 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。我们从asyncio模块中直接获取一个EventLoop的引用,然后把需要执行的协程扔到EventLoop中执行,就实现了异步IO。

    asyncio实现Hello world代码如下:

    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,然后,就把这个coroutine扔到eventloop中去执行

    hello()会先打印出helloworld,然后yield from可以让我们方便的调用另一个generator,由于asyncio.sleep(1)也是一个coroutine

    所以线程不会等待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

    然后执行下一个语句

    把asyncio.sleep(1)看成是一个耗时1秒的IO操作。在此期间主线程没有等待,而是去执行eventloop其他可以执行的coroutine因此可以实现并发执行

    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可以看出,两个coroutine是由同一个线程并发执行的。
    
    如果把asyncio.sleep()换成真正的IO操作,则多个coroutine就可以由一个线程并发执行。
    
    我们用asyncio的异步网络连接来获取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
    Host: %s
    
    ' % host
        writer.write(header.encode('utf-8'))
        yield from writer.drain()
        while True:
            line = yield from reader.readline()
            if line == b'
    ':
                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;

    异步操作需要在coroutine中通过yield from完成;

    多个coroutine可以封装成一组Task然后并发执行。

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  • 原文地址:https://www.cnblogs.com/Commence/p/5583627.html
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