How to determine from a python application if X server/X forwarding is running?

Question

I'm writing a linux application which uses PyQt4 for GUI and which will only be used during remote sessions
(ssh -XY / vnc).

So sometimes it may occur that a user will forget to run ssh with X forwarding parameters or X forwarding will be unavailable for some reason. In this case the application crashes badly (unfortunately I am force to use an old C++ library wrapped into python and it completely messes user's current session if the application crashes).

I cannot use something else so my idea is to check if X forwarding is available before loading that library. However I have no idea how to do that.

I usually use xclock to check if my session has X forwarding enabled, but using xclock sounds like a big workaround.

ADDED
If possible I would like to use another way than creating an empty PyQt window and catching an exception.

Answer 1

Check to see that the $DISPLAY environment variable is set - if they didn't use ssh -X, it will be empty (instead of containing something like localhost:10).

Answer 2

As mentioned before, you can check the DISPLAY environment variable:

>>> os.environ['DISPLAY']
'localhost:10.0'

If you're so inclined, you could actually connect to the display port to see that sshd is listening on it:

import os
import socket

def tcp_connect_to_display():
        # get the display from the environment
        display_env = os.environ['DISPLAY']

        # parse the display string
        display_host, display_num = display_env.split(':')
        display_num_major, display_num_minor = display_num.split('.')

        # calculate the port number
        display_port = 6000 + int(display_num_major)

        # attempt a TCP connection
        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        try:
                sock.connect((display_host, display_port))
        except socket.error:
                return False
        finally:
            sock.close()
        return True

This relies on standard X configuration using ports 6000 + display number.