Threads & Concurrency¶
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
The first 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 from gi.repository import GLib, Gtk, GObject def app_main(): win = Gtk.Window(default_height=50, default_width=300) win.connect("destroy", Gtk.main_quit) progress = Gtk.ProgressBar(show_text=True) win.add(progress) def update_progess(i): progress.pulse() progress.set_text(str(i)) return False def example_target(): for i in range(50): GLib.idle_add(update_progess, i) time.sleep(0.2) win.show_all() thread = threading.Thread(target=example_target) thread.daemon = True thread.start() if __name__ == "__main__": app_main() Gtk.main()
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
time.sleep() in a loop. In this example
time.sleep() represents the blocking operation.
update_progess() 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.
I’m porting code from pygtk (GTK 2) to PyGObject (GTK 3). Has anything changed regarding threads?
Short answer: No.
gobject.threads_init()can be removed.
Gdk.threads_init()and want to get rid of it. What do I need to do?
- Remove any
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
- Remove any
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
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.Eventobject 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 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.
In addition to functions for blocking I/O glib also provides corresponding
asynchronous versions, usually with the same name plus a
These functions do the same operation as the synchronous ones but don’t block
during their execution. Instead of blocking they execute the operation in the
background and call a callback once the operation is finished or got canceled.
The following example shows how to download a web page and display the source in a text field. In addition it’s possible to abort the running operation.
import time from gi.repository import Gio, GLib, Gtk class DownloadWindow(Gtk.Window): def __init__(self): super(DownloadWindow, self).__init__( default_width=500, default_height=400, title="Async I/O Example") self.cancellable = Gio.Cancellable() self.cancel_button = Gtk.Button(label="Cancel") self.cancel_button.connect("clicked", self.on_cancel_clicked) self.cancel_button.set_sensitive(False) self.start_button = Gtk.Button(label="Load") self.start_button.connect("clicked", self.on_start_clicked) textview = Gtk.TextView() self.textbuffer = textview.get_buffer() scrolled = Gtk.ScrolledWindow() scrolled.add(textview) box = Gtk.Box(orientation=Gtk.Orientation.VERTICAL, spacing=6, border_width=12) box.pack_start(self.start_button, False, True, 0) box.pack_start(self.cancel_button, False, True, 0) box.pack_start(scrolled, True, True, 0) self.add(box) def append_text(self, text): iter_ = self.textbuffer.get_end_iter() self.textbuffer.insert(iter_, "[%s] %s\n" % (str(time.time()), text)) def on_start_clicked(self, button): button.set_sensitive(False) self.cancel_button.set_sensitive(True) self.append_text("Start clicked...") file_ = Gio.File.new_for_uri( "http://python-gtk-3-tutorial.readthedocs.org/") file_.load_contents_async( self.cancellable, self.on_ready_callback, None) def on_cancel_clicked(self, button): self.append_text("Cancel clicked...") self.cancellable.cancel() def on_ready_callback(self, source_object, result, user_data): try: succes, content, etag = source_object.load_contents_finish(result) except GLib.GError as e: self.append_text("Error: " + e.message) else: content_text = content[:100].decode("utf-8") self.append_text("Got content: " + content_text + "...") finally: self.cancellable.reset() self.cancel_button.set_sensitive(False) self.start_button.set_sensitive(True) if __name__ == "__main__": win = DownloadWindow() win.show_all() win.connect("destroy", Gtk.main_quit) Gtk.main()
The example uses the asynchronous version of
load the content of an URI pointing to a web page, but first we look at the
simpler blocking alternative:
file = Gio.File.new_for_uri("http://python-gtk-3-tutorial.readthedocs.org/") try: status, contents, etag_out = file.load_contents(None) except GLib.GError: print("Error!") else: print(contents)
In the asynchronous variant we need two more things:
Gio.Cancellable, which we can use during the operation to abort or cancel it.
- And a
Gio.AsyncReadyCallback()callback function, which gets called once the operation is finished and we can collect the result.
The window contains two buttons for which we register
on_start_clicked()signal handler calls
on_cancel_clicked()signal handler calls
Gio.Cancellable.cancel()to cancel the running operation.
Once the operation is finished, either because the result is available, an
error occurred or the operation was canceled,
on_ready_callback() will be
called with the
Gio.File instance and a
instance which holds the result.
To get the result we now have to call
which returns the same things as
Gio.File.load_contents() except in
this case the result is already there and it will return immediately
After all this is done we call
Gio.Cancellable.reset() so the
Gio.Cancellable can be re-used for new operations and we can click
the “Load” button again. This works since we made sure that only one
operation can be active at any time by deactivating the “Load” button using