Multi Threading#

Operations which could potentially block should not be executed in the main loop. The main loop is in charge of input processing and drawing and blocking it results in the user interface freezing. For the user this means not getting any feedback and not being able to pause or abort the operation which causes the problem.

Such an operation might be:

  • Loading external resources like an image file on the web

  • Searching the local file system

  • Writing, reading and copying files

  • Calculations where the runtime depends on some external factor

The following examples show

  • how Python threads, running in parallel to GTK, can interact with the UI

  • how to use and control asynchronous I/O operations in glib

This page will discuss multi-threading. A separate page discusses :doc:asynchronous.

This example uses a Python thread to execute code in the background while still showing feedback on the progress in a window.

import threading
import time
import gi

gi.require_version('Gtk', '4.0')
from gi.repository import GLib, Gtk, GObject


class Application(Gtk.Application):

    def do_activate(self):
        window = Gtk.ApplicationWindow(application=self)
        self.progress = Gtk.ProgressBar(show_text=True)

        window.set_child(self.progress)
        window.present()

        thread = threading.Thread(target=self.example_target)
        thread.daemon = True
        thread.start()

    def update_progress(self, i):
        self.progress.pulse()
        self.progress.set_text(str(i))
        return False

    def example_target(self):
        for i in range(50):
            GLib.idle_add(self.update_progress, i)
            time.sleep(0.2)


app = Application()
app.run()

The example shows a simple window containing a progress bar. After everything is set up it constructs a Python thread, passes it a function to execute, starts the thread and the GTK main loop. After the main loop is started it is possible to see the window and interact with it.

In the background example_target() gets executed and calls GLib.idle_add() and time.sleep() in a loop. In this example time.sleep() represents the blocking operation. GLib.idle_add() takes the update_progress() function and arguments that will get passed to the function and asks the main loop to schedule its execution in the main thread. This is needed because GTK isn’t thread safe; only one thread, the main thread, is allowed to call GTK code at all times.

Threads: FAQ#

  • I’m porting code from pygtk (GTK 2) to PyGObject (GTK 3). Has anything changed regarding threads?

    Short answer: No.

    Long answer: gtk.gdk.threads_init(), gtk.gdk.threads_enter() and gtk.gdk.threads_leave() are now Gdk.threads_init(), Gdk.threads_enter() and Gdk.threads_leave(). gobject.threads_init() can be removed.

  • I’m using Gdk.threads_init() and want to get rid of it. What do I need to do?

    • Remove any Gdk.threads_init(), Gdk.threads_enter() and Gdk.threads_leave() calls. In case they get executed in a thread, move the GTK code into its own function and schedule it using GLib.idle_add(). Be aware that the newly created function will be executed some time later, so other stuff can happen in between.

    • Replace any call to Gdk.threads_add_*() with their GLib counterpart. For example GLib.idle_add() instead of Gdk.threads_add_idle().

  • What about signals and threads?

    Signals get executed in the context they are emitted from. In which context the object is created or where connect() is called from doesn’t matter. In GStreamer, for example, some signals can be called from a different thread, see the respective signal documentation for when this is the case. In case you connect to such a signal you have to make sure to not call any GTK code or use GLib.idle_add() accordingly.

  • What if I need to call GTK code in signal handlers emitted from a thread?

    In case you have a signal that is emitted from another thread and you need to call GTK code during and not after signal handling, you can push the operation with an threading.Event object to the main loop and wait in the signal handler until the operation gets scheduled and the result is available. Be aware that if the signal is emitted from the main loop this will deadlock. See the following example

    # [...]

toggle_button = Gtk.ToggleButton()

def signal_handler_in_thread():

    def function_calling_gtk(event, result):
        result.append(toggle_button.get_active())
        event.set()

    event = threading.Event()
    result = []
    GLib.idle_add(function_calling_gtk, event, result)
    event.wait()
    toggle_button_is_active = result[0]
    print(toggle_button_is_active)

# [...]

  • What about the Python GIL ?

    Similar to I/O operations in Python, all PyGObject calls release the GIL during their execution and other Python threads can be executed during that time.

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