你可以流式传输响应中的数据，但是不能按照描述的方式动态更新模板。模板在服务器端只渲染一次，然后发送到客户端。你需要使用JavaScript读取流式响应并在客户端输出数据。

用于XMLHttpRequest向要发送数据的端点发出请求。然后定期从流中读取直到完成。

此示例假定一种非常简单的消息格式：一行数据，然后是换行符。当然，只要有一种识别每条消息的方法，解析就可能变得很复杂。例如，你可以返回一个JSON对象并在客户端上对其进行解码。

from time import sleep
from flask import Flask, render_template
from math import sqrt

app = Flask(__name__)

@app.route('/')
def index():
    # render the template (below) that will use JavaScript to read the stream
    return render_template('index.html')

@app.route('/stream_sqrt')
def stream():
    def generate():
        for i in range(500):
            yield '{}\n'.format(sqrt(i))
            sleep(1)

    return app.response_class(generate(), mimetype='text/plain')

app.run()

<p>This is the latest output: <span id="latest"></span></p>
<p>This is all the output:</p>
<ul id="output"></ul>
<script>
    var latest = document.getElementById('latest');
    var output = document.getElementById('output');

    var xhr = new XMLHttpRequest();
    xhr.open('GET', '{{ url_for('stream') }}');
    xhr.send();
    var position = 0;

    function handleNewData() {
        // the response text include the entire response so far
        // split the messages, then take the messages that haven't been handled yet
        // position tracks how many messages have been handled
        // messages end with a newline, so split will always show one extra empty message at the end
        var messages = xhr.responseText.split('\n');
        messages.slice(position, -1).forEach(function(value) {
            latest.textContent = value;  // update the latest value in place
            // build and append a new item to a list to log all output
            var item = document.createElement('li');
            item.textContent = value;
            output.appendChild(item);
        });
        position = messages.length - 1;
    }

    var timer;
    timer = setInterval(function() {
        // check the response for new data
        handleNewData();
        // stop checking once the response has ended
        if (xhr.readyState == XMLHttpRequest.DONE) {
            clearInterval(timer);
            latest.textContent = 'Done';
        }
    }, 1000);
</script>