Home/ Questions/Q 3272440
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-17T18:49:34+00:00

I’m writing a simple browser-based front end that should be able to launch a

  • 0

I’m writing a simple browser-based front end that should be able to launch a background task and then get progress from it. I want the browser to receive a response saying whether the task launched successfully, and then poll to determine when it is done. However, the presence of a background task seems to be stopping the XMLHttpRequest response from being sent immediately, so I can’t report the success of launching the process. Consider the following (simplified) code:

import SocketServer
import SimpleHTTPServer
import multiprocessing
import time

class MyProc(multiprocessing.Process):
    def run(self):
        print 'Starting long process..'
        for i in range(100): time.sleep(1)
        print 'Done long process'

class Page(SimpleHTTPServer.SimpleHTTPRequestHandler):
    def do_GET(self):
        if self.path == '/':
            print >>self.wfile, "<html><body><a href='/run'>Run</a></body></html>"
        if self.path == '/run':
            self.proc = MyProc()
            print 'Starting..'
            self.proc.start()
            print 'After start.'
            print >>self.wfile, "Process started."

httpd = SocketServer.TCPServer(('', 8000), Page)
httpd.serve_forever()

When I run this, and browse to http://localhost:8000, I get a button named “Run”. When I click on it, the terminal displays:

Starting..
After start.

However the browser view does not change.. in fact the cursor is spinning. Only when I press Ctrl-C in the terminal to interrupt the program, then the browser is update with the message Process started.

The message After start is clearly being printed. Therefore I can assume that do_GET is returning after starting the process. Yet, the browser doesn’t get a response until after I interrupt the long-running process. I have to conclude there is something blocking between do_GET and the response being sent, which is inside SimpleHTTPServer.

I’ve also tried this with threads and subprocess.Popen but ran into similar problems. Any ideas?

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    In addition to Steve’s and my comments above, here is a solution that works for me.

    The method to determine a content-length is a bit ugly. If you don’t specify one, the browser may still show a spinning cursor although the content is shown. Closing the self.wfile instead could also work.

    from cStringIO import StringIO

class Page(SimpleHTTPServer.SimpleHTTPRequestHandler):
    def do_GET(self):
        out = StringIO()
        self.send_response(200)
        self.send_header("Content-type", "text/html")
        if self.path == '/':
            out.write("<html><body><a href='/run'>Run</a></body></html>\n")
        elif self.path == '/run':
            self.proc = MyProc()
            print 'Starting..'
            self.proc.start()
            print 'After start.'
            out.write("<html><body><h1>Process started</h1></body></html>\n")
        text = out.getvalue()
        self.send_header("Content-Length", str(len(text)))
        self.end_headers()
        self.wfile.write(text)
    • 0