python原生实现协程

# python 为了将语义更加明确, 就引入了async和await关键词用于定义原生的协程
# await 只能出现在 async里面   async里面不能有yield


async def downloader(url):
    return '测试'


async def download_url(url):
    print(url)
    html = await downloader(url)   # await 后面接收await对象  await对象实现了__await__方法
    return html

if __name__ == '__main__':
    res = download_url('https://www.zhijinyu.com')
    # next(None)  这样调用会报错  RuntimeWarning: coroutine 'download_url' was never awaited
    res.send(None)