httpclient import HTTPClient def synchronous_fetch(url): http_client = HTTPClient() response = http_client.fetch(url) return response.body And here is the same function rewritten asynchronously as a native coroutine: AsyncHTTPClient async def asynchronous_fetch(url): http_client = AsyncHTTPClient() response = await http_client.fetch(url) return response.body Or for compatibility with older versions of Python, using the tornado coroutine def async_fetch_gen(url): http_client = AsyncHTTPClient() response = yield http_client.fetch(url) raise gen.Return(response.body) Coroutines are a little magical, but what they do internally

HTTPClient def synchronous_fetch(url): http_client = HTTPClient() response = http_client.fetch(url) return response.body And here is the same function rewritten asynchronously as a native def asynchronous_fetch(url): http_client = AsyncHTTPClient() response = await http_client.fetch(url) return response.body Or for compatibility with older versions of Python, using the tornado def async_fetch_gen(url): http_client = AsyncHTTPClient() response = yield http_client.fetch(url) raise gen.Return(response.body) Coroutines are a little magical, but what they do internally

page_title(url: &str) -> Option { let response = trpl::get(url).await; let response_text = response.text().await; Html::parse(&response_text) .select_first("title") .map(|title_element| thread::spawn 的行为，当时传递给另一个线程的闭包会 立即开始运行。它也不同于许多其他语言实现 async 的方式。但这样做对于 Rust 提 供与迭代器相同级别的性能保证至关重要。 当我们有了 response_text 字符串后，就可以使用 Html::parse 将其解析为一个 Html 类型的 实例。不同于原始字符串，现在我们有了一个可以将 HTML 作为更丰富数据结构来操作的数 据类型。特别是我们可以使用 修改 page_title 的函数体来链式调用 trpl::get 和 text 并在其之间使用 await，如示例 17-2 所 示： 文件名：src/main.rs let response_text = trpl::get(url).await.text().await; 这样我们就成功编写了第一个异步函数！在我们向 main 加入一些代码调用它之前，让我们再 多了解下我们写了什么以及它的意义。