You must not access self.set_label or any other GTK stuff while running in a thread. You can surround such calls with gtk.threads_enter() and gtk.threads_leave() to use them in a thread, but have to be very very careful, because PyGTK + threads is an evil mix that deadlocks all too easily. My advice: never ever use threads in PyGTK unless absolutely necessary. Async IO via gobject means is usually the better choice with PyGTK.