关于 yield 返回值的问题请教

def gen(): for i in range(1): print('i=',i) X = yield i print('X=',X) G = gen() print('next(G)=',next(G)) print('next(G)=',next(G)) 上面代码输出如下，可见如果等号的右边如果是 yield 语句，则等号左边的变量值默认是 None，除非用 send 语句对它赋值： i= 0 next(G)= 0 X= None Traceback (most recent call last): File "c:\Users\rubin\AppData\Roaming\Code\User\rb\t.py", line 9, in print('next(G)=',next(G)) StopIteration 但是对于下面的例子来说，即使没有用 send 语句，yield 返回的值也能赋值给等号左边的变量 reader 和 writer。 import asyncio @asyncio.coroutine def wget(host): print('wget %s...' % host) cOnnect= asyncio.open_connection(host, 80) reader, writer = yield from connect #yield 返回的值也能赋值给等号左边的变量 header = 'GET / HTTP/1.0\r\nHost: %s\r\n\r\n' % host print('reader=%s write=%s' % (reader,writer)) writer.write(header.encode('utf-8')) yield from writer.drain() writer.close() loop = asyncio.get_event_loop() tasks = [wget(host) for host in ['www.sohu.com']] loop.run_until_complete(asyncio.wait(tasks)) loop.close() 上面代码输出如下： wget www.sohu.com. .. reader=<StreamReader t=<_SelectorSocketTransport fd=592 read=polling write=<idle, bufsize=0>>> write=<StreamWriter transport=<_SelectorSocketTransport fd=592 read=polling write=<idle, bufsize=0>> reader=<StreamReader t=<_SelectorSocketTransport fd=592 read=polling write=<idle, bufsize=0>>>> 

为何等号左边的变量 reader 和 writer 不是 None ？请问这是什么原理呢？谢谢！