asyncio

最后更新于:2022-04-01 01:16:03

`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。 用`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,我们就把这个`coroutine`扔到`EventLoop`中执行。 `hello()`会首先打印出`Hello world!`,然后,`yield from`语法可以让我们方便地调用另一个`generator`。由于`asyncio.sleep()`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()`返回时,线程就可以从`yield from`拿到返回值(此处是`None`),然后接着执行下一行语句。 把`asyncio.sleep(1)`看成是一个耗时1秒的IO操作,在此期间,主线程并未等待,而是去执行`EventLoop`中其他可以执行的`coroutine`了,因此可以实现并发执行。 我们用Task封装两个`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() ~~~ 观察执行过程: ~~~ 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\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() ~~~ 执行结果如下: ~~~ 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 ... ~~~ 可见3个连接由一个线程通过`coroutine`并发完成。 ### 小结 `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。 ### 参考源码 [async_hello.py](https://github.com/michaelliao/learn-python3/blob/master/samples/async/async_hello.py) [async_wget.py](https://github.com/michaelliao/learn-python3/blob/master/samples/async/async_wget.py)
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