javascript客户端到Python服务器：获取请求后XMLHttpRequest响应文本为空

我正在尝试编写一个 chrome 扩展，它能够向 python 服务器脚本发送/接收数据。目前，我正处于 js 脚本发出 GET 请求的阶段，唯一的问题是 .responseText 始终为空（即使 python 脚本以文本响应）。

popup.js

document.addEventListener('DOMContentLoaded', function() {  
    var xhr = new XMLHttpRequest();

    xhr.onreadystatechange = function() {
        if (xhr.readyState == xhr.DONE) {           
            var resptxt = xhr.responseText;

            if (resptxt) {
                document.getElementById('texthtm').innerHTML = resptxt;
            } else {
                document.getElementById('texthtm').innerHTML = "no response";
            }
        }
    }

    xhr.open("GET", "http://127.0.0.1:80", true);
    xhr.send();
});

服务器.py

import time
import BaseHTTPServer
from SimpleHTTPServer import SimpleHTTPRequestHandler

HOST_NAME = "127.0.0.1" 
PORT_NUMBER = 80 

class MyHandler(BaseHTTPServer.BaseHTTPRequestHandler):
    def do_HEAD(self):
        self.send_response(200)
        self.send_header("Content-type", "text/plain")
        self.end_headers()
    def do_GET(self):
        self.send_response(200)
        self.send_header("Content-type", "text/plain")
        self.end_headers()
        self.wfile.write("heyyy")


if __name__ == "__main__":

    server_class = BaseHTTPServer.HTTPServer
    httpd = server_class((HOST_NAME, PORT_NUMBER), MyHandler)
    print time.asctime(), "Server Starts - %s:%s" % (HOST_NAME, PORT_NUMBER)

    try:
            httpd.serve_forever()
    except KeyboardInterrupt:
            pass

    httpd.server_close()
    print time.asctime(), "Server Stops - %s:%s" % (HOST_NAME, PORT_NUMBER)

所以每次我按下扩展按钮时，它只是显示“无响应”，这意味着请求至少正在完成。我在这里做错了什么？