Question

I have a two-thread application: GUI, and some background work. I’m trying to send requests to the main thread to do GUI updates (move a progress bar), but it doesn’t seem to work. I’ve boiled it down to a really minimal example:

import pygtk
pygtk.require('2.0')
import glib
import gtk

import threading
import sys
import time

def idle():
    sys.stderr.write('Hello from another world.\n')
    sys.stderr.flush()
    gtk.main_quit()

def another_thread():
    time.sleep(1)
    glib.idle_add(idle)

thread = threading.Thread(target=another_thread)
thread.start()
gtk.main()

This should, I thought, print something to standard error from the main/GUI thread, but nothing happens. And it doesn’t quit, either, so gtk.main_quit isn’t being called.

Also, adding more output to stderr acts weirdly. If I change the thread’s function to:

sys.stderr.write('----\n')
sys.stderr.write('----\n')
sys.stderr.flush()
sys.stderr.write('After.\n')
sys.stderr.flush()

I see 1, sometimes 2 lines out output. It looks like some kind of race condition with the main thread entering gtk.main, but I don’t know why this would be.

Answer 1

The following makes the above work for me:

A call to gtk.gdk.threads_init() before doing anything else: before starting threads, before entering gtk.main(). I think in reality, this only needs to be called before entering gtk.main(), but it is easy enough to call it while everything is single threaded and simple.

In the idle callback (idle, in the example), a call to gtk.gdk.threads_enter() before GTK stuff (easy to do just at the top of the function), and a call to gtk.gdk.threads_leave(), before the end of the function.

gtk.gdk.threads_init() seems to also tell PyGTK not hold the GIL when it goes to sleep – I think I was missing some output from the aux. thread in the example just because the sleeping main thread (sleeping in gtk.main()) was still holding the GIL. gtk.gdk.threads_init(), as far as I can tell, instills good mojo into PyGTK and GTK+. Because of this, gtk.gdk.threads_init() is needed even if you launch a thread that doesn’t touch GTK+, doesn’t do anything with glib, gobject, etc., just does basic computation.