Notification of gnome-screensaver state changes through D-Bus

screen_locked.py

#!/usr/bin/env python

import gobject, dbus
from dbus.mainloop.glib import DBusGMainLoop

def screensaver_state_changed(isActive):
	print "locked" if isActive else "unlocked"

DBusGMainLoop(set_as_default=True)
sess_bus = dbus.SessionBus()

sess_bus.add_signal_receiver(screensaver_state_changed, path='/org/gnome/ScreenSaver', dbus_interface='org.gnome.ScreenSaver', signal_name='ActiveChanged')
gobject.MainLoop().run()