<?xml version="1.0" encoding="utf-8"?>
<main version="1">
    <sub number="1">first</sub>
    <sub number="2">second</sub>
</main>

from io import BytesIO
from xml.etree import ElementTree as ET
from tornado.ioloop import IOLoop
from tornado.httpserver import HTTPServer
from tornado.web import *
from concurrent.futures import ProcessPoolExecutor
  
  
class XpathHandler(RequestHandler):
 
    @asynchronous
    def get(self):
        #http://127.0.0.1:8888/xpath/?file=xml_file.orx&xpath=.//
        file_name = self.get_argument('file')
        xpath = self.get_argument('xpath')
        with ProcessPoolExecutor(max_workers=1) as executor:
            future = executor.submit(XpathHandler.parse_xml, file_name, xpath)
            future.add_done_callback(self.callback)
 
    @staticmethod
    def parse_xml(self, file_name, xpath):
        output = BytesIO()
        tree = ET.parse(file_name)
        root = tree.getroot()
        finded_elements = root.findall(xpath)
        resulted_element = ET.Element('result')
        resulted_element.extend(finded_elements)
        tree._setroot(resulted_element)
        tree.write(output, encoding='utf-8', xml_declaration=True)
        result = output.getvalue().decode()
        output.close()
        return result
 
    def callback(self, future):
        self.write(str(future.result()))
        self.finish()
 
if __name__ == '__main__':
    application = Application([
        (r"/xpath/", XpathHandler)],
        debug=True,
    )
    http_server = HTTPServer(application)
    http_server.listen(8888, 'localhost')
    IOLoop.instance().start()

http://127.0.0.1:8888/xpath/?file=light_xml.xml&xpath=.//

Traceback (most recent call last):
  File "C:\Python33\lib\multiprocessing\queues.py", line 245, in _feed
    send(obj)
  File "C:\Python33\lib\multiprocessing\connection.py", line 206, in send
    ForkingPickler(buf, pickle.HIGHEST_PROTOCOL).dump(obj)
_pickle.PicklingError: Can't pickle <class 'function'>: attribute lookup builtins.function failed