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！