A module which lets you create a pygtk application which will start the running instance if being executed again. Based on http://askubuntu.com/a/171996/98448

activate_app_if_running.py

"""
Allow an application to activate a running instance of itself instead of
starting another instance.
"""

import sys

import gtk
import dbus.service
from dbus.mainloop.glib import DBusGMainLoop

def _get_path(app_id):
    return '/' + app_id.replace('.', '/')

def listen_for_activation(app_id, window):
    """
    Listen for 'activate' events. If one is sent, activate 'window'.
    """
    class MyDBUSService(dbus.service.Object):                                                                                                       
        def __init__(self, window):
            self.window = window
            
            bus_name = dbus.service.BusName(app_id, bus=dbus.SessionBus())                                                     
            dbus.service.Object.__init__(self, bus_name, _get_path(app_id))
    
        @dbus.service.method(app_id)                                                                         
        def activate(self):
            print "The process was activated by another instance."
            self.window.present()
    
    DBusGMainLoop(set_as_default=True)
    _myservice = MyDBUSService(window)

def activate_if_already_running(app_id):
    """
    Activate the existing window if it's already running. Return True if found
    an existing window, and False otherwise.
    """
    bus = dbus.SessionBus()
    try:
        programinstance = bus.get_object(app_id, _get_path(app_id))
        activate = programinstance.get_dbus_method('activate', app_id)
    except dbus.exceptions.DBusException:
        return False
    else:
        print "A running process was found. Activating it."
        activate()
        return True
    finally:
        bus.close()

def test():
    APP_ID = 'com.example.myapp'

    activated = activate_if_already_running(APP_ID)
    if activated:
        sys.exit(0)
        
    w = gtk.Window()
    b = gtk.Button("Hello!")
    b.set_size_request(200, 200)
    w.add(b)
    w.connect('delete-event', gtk.main_quit)
    w.show_all()
    
    listen_for_activation(APP_ID, w)
        
    gtk.main()

if __name__ == '__main__':
    test()

Hi, can I modify your code and use it in my application? If yes, what license is it on?