from time import sleep
import gtk
import pygtk
pygtk.require("2.0")
from threading import Thread
import gobject
class PyApp(gtk.Window): 
    def __init__(self):
        super(PyApp, self).__init__()
        self.connect("destroy", gtk.main_quit)       
        self.set_size_request(250, 100)
        self.set_position(gtk.WIN_POS_CENTER)
        self.set_title("Test")
        btn = gtk.Button("Click Here")
        btn.connect("clicked", self.on_click)
        self.add(btn)
        self.show_all()
    def decorator_threaded(func):
        def wrapper(*args, **kwargs):
            gtk.gdk.threads_enter()
            thread = Thread(target=func, args=args, kwargs=kwargs)
            thread.start()
            return thread
        return wrapper
    @decorator_threaded
    def running_on_another_thread(self):
        sleep(2) # Heavy task
        gobject.idle_add(self.error_message)
    def on_click(self, widget):
        self.running_on_another_thread()
    def error_message(self):
        md = gtk.MessageDialog(self, 
            gtk.DIALOG_DESTROY_WITH_PARENT, gtk.MESSAGE_ERROR, 
            gtk.BUTTONS_CLOSE, "Error")
        md.run()
        md.destroy()
PyApp()
gtk.gdk.threads_init()
gtk.main()

import threading
import time
import gi
gi.require_version("Gtk", "3.0")
from gi.repository import GLib, Gtk
def app_main():
    win = Gtk.Window(default_height=100, default_width=250)
    win.connect("destroy", Gtk.main_quit)
    win.set_position(Gtk.WindowPosition.CENTER)
    def error_message():
        md = Gtk.MessageDialog(
            transient_for=win,
            flags=0,
            message_type=Gtk.MessageType.ERROR,
            buttons=Gtk.ButtonsType.CLOSE,
            text="Error",
        md.run()
        md.destroy()
    def example_target():
        time.sleep(2) # Heavy task
        GLib.idle_add(error_message)
    win.show_all()
    thread = threading.Thread(target=example_target)
    thread.daemon = True
    thread.start()
if __name__ == "__main__":
    app_main()
    Gtk.main()