Asynchronous Programming — PyGObject

Skip to main content
`Ctrl`+`K`

# Asynchronous Programming

Asynchronous programming is an essential paradigm for handling tasks like I/O operations, ensuring that your application remains responsive. GLib supports asynchronous operations alongside its synchronous counterparts. These asynchronous functions typically have a `_async` suffix and execute tasks in the background, invoking a callback or returning a future-like object upon completion or cancellation.

PyGObject offers two primary ways to implement asynchronous programming:

1. Using Python’s `asyncio` module (available since PyGObject 3.50)

2. Using callbacks (the traditional approach)

## Asynchronous Programming with `asyncio`

Attention

Asyncio support for PyGObject is experimental. Feel free to explore its integration, but note that the API is subject to change as potential issues are addressed.

### Overview of asyncio Integration

PyGObject integrates seamlessly with Python’s `asyncio` module by providing:

1. An `asyncio` event loop implementation, enabling normal operation of Python’s asynchronous code. 2. Awaitable objects for Gio’s asynchronous functions, allowing `await` syntax within Python coroutines.

### Event Loop Integration

To use the `asyncio` event loop with GLib, set up the GLib-based event loop policy:

    import asyncio
    from gi.events import GLibEventLoopPolicy

    # Set up the GLib event loop
    policy = GLibEventLoopPolicy()
    asyncio.set_event_loop_policy(policy)

Now, fetch the event loop and submit tasks:

    loop = policy.get_event_loop()

    async def do_some_work():
        await asyncio.sleep(2)
        print("Done working!")

    task = loop.create_task(do_some_work())

Note

Keep a reference to the tasks you create, as `asyncio` only maintains weak references to them.

### Gio Asynchronous Function Integration

If the callback parameter of a Gio asynchronous function is omitted, PyGObject automatically returns an awaitable object (similar to `asyncio.Future`). This allows you to use `await` and cancel operations from within a coroutine.

    loop = policy.get_event_loop()

    async def list_files():
        f = Gio.file_new_for_path("/")
        for info in await f.enumerate_children_async(
            "standard::*", 0, GLib.PRIORITY_DEFAULT
        ):
            print(info.get_display_name())

    task = loop.create_task(list_files())

### Example: Download Window with Async Feedback

Here is a full example illustrating asynchronous programming with `asyncio` for a download window with async feedback.

    import asyncio
    import time
    import gi

    gi.require_version("Gtk", "4.0")
    from gi.repository import Gio, GLib, Gtk
    from gi.events import GLibEventLoopPolicy

    class DownloadWindow(Gtk.ApplicationWindow):
        def __init__(self, *args, **kwargs):
            super().__init__(
                *args,
                **kwargs,
                default_width=500,
                default_height=400,
                title="Async I/O Example",
            )

            self.background_task = None

            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(vexpand=True)
            self.textbuffer = textview.get_buffer()
            scrolled = Gtk.ScrolledWindow()
            scrolled.set_child(textview)

            box = Gtk.Box(
                orientation=Gtk.Orientation.VERTICAL,
                spacing=6,
                margin_start=12,
                margin_end=12,
                margin_top=12,
                margin_bottom=12,
            )
            box.append(self.start_button)
            box.append(self.cancel_button)
            box.append(scrolled)

            self.set_child(box)

        def append_text(self, text):
            iter_ = self.textbuffer.get_end_iter()
            self.textbuffer.insert(iter_, f"[{time.time()}] {text}\n")

        def on_start_clicked(self, button):
            button.set_sensitive(False)
            self.cancel_button.set_sensitive(True)
            self.append_text("Start clicked...")

            self.background_task = asyncio.create_task(self.download())

        def on_cancel_clicked(self, button):
            self.append_text("Cancel clicked...")
            self.background_task.cancel()

        async def download(self):
            file_ = Gio.File.new_for_uri("https://pygobject.gnome.org/")

            try:
                _success, content, _etag = await file_.load_contents_async()
            except GLib.GError as e:
                self.append_text(f"Error: {e.message}")
            else:
                content_text = content[:100].decode("utf-8")
                self.append_text(f"Got content: {content_text}...")
            finally:
                self.cancel_button.set_sensitive(False)
                self.start_button.set_sensitive(True)

    class Application(Gtk.Application):
        def do_activate(self):
            window = DownloadWindow(application=self)
            window.present()

    def main():
        asyncio.set_event_loop_policy(GLibEventLoopPolicy())
        app = Application()
        app.run()

    if __name__ == "__main__":
        main()

### Key Considerations

- Async tasks use `GLib.PRIORITY_DEFAULT`. For background tasks, consider using a lower priority to avoid affecting the responsiveness of your GTK UI. Please see the PRIORITY_* GLib Constants <https://docs.gtk.org/glib/index.html#constants> for other settings.

- Prefer starting your application using `Gio.Application` or `Gtk.Application` instead of `asyncio.run()`, which is incompatible.

## Asynchronous Programming with Callbacks

The traditional callback approach is a robust alternative for asynchronous programming in PyGObject. Consider this example of downloading a web page and displaying its source in a text field. The operation can also be canceled mid-execution.

    import time
    import gi

    gi.require_version("Gtk", "4.0")
    from gi.repository import Gio, GLib, Gtk

    class DownloadWindow(Gtk.ApplicationWindow):
        def __init__(self, *args, **kwargs):
            super().__init__(
                *args,
                **kwargs,
                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(vexpand=True)
            self.textbuffer = textview.get_buffer()
            scrolled = Gtk.ScrolledWindow()
            scrolled.set_child(textview)

            box = Gtk.Box(
                orientation=Gtk.Orientation.VERTICAL,
                spacing=6,
                margin_start=12,
                margin_end=12,
                margin_top=12,
                margin_bottom=12,
            )
            box.append(self.start_button)
            box.append(self.cancel_button)
            box.append(scrolled)

            self.set_child(box)

        def append_text(self, text):
            iter_ = self.textbuffer.get_end_iter()
            self.textbuffer.insert(iter_, f"[{time.time()}] {text}\n")

        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("https://pygobject.gnome.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:
                _success, content, _etag = source_object.load_contents_finish(result)
            except GLib.GError as e:
                self.append_text(f"Error: {e.message}")
            else:
                content_text = content[:100].decode("utf-8")
                self.append_text(f"Got content: {content_text}...")
            finally:
                self.cancellable.reset()
                self.cancel_button.set_sensitive(False)
                self.start_button.set_sensitive(True)

    class Application(Gtk.Application):
        def do_activate(self):
            window = DownloadWindow(application=self)
            window.present()

    def main():
        app = Application()
        app.run()

    if __name__ == "__main__":
        main()

### Synchronous Comparison

> Before exploring the asynchronous method, let’s review the simpler blocking approach:

    file = Gio.File.new_for_uri(
        "https://developer.gnome.org/documentation/tutorials/beginners.html"
    )
    try:
        status, contents, etag_out = file.load_contents(None)
    except GLib.GError:
        print("Error!")
    else:
        print(contents)

### Asynchronous Workflow with Callbacks

For asynchronous tasks, you’ll need:

1. A `Gio.Cancellable` to allow task cancellation.

2. A `Gio.AsyncReadyCallback` function to handle the result upon task completion.

The example setup includes:

- Start Button: The handler calls `Gio.File.load_contents_async()` with a cancellable and a callback function.

- Cancel Button: The handler calls `Gio.Cancellable.cancel()` to stop the operation.

Once the operation completes—whether successfully, due to an error, or because it was canceled—the `on_ready_callback()` function is invoked. This callback receives two arguments: the `Gio.File` instance and a `Gio.AsyncResult` instance containing the operation’s result.

To retrieve the result, call `Gio.File.load_contents_finish()`. This method behaves like `Gio.File.load_contents()`, but since the operation has already completed, it returns immediately without blocking.

After handling the result, call `Gio.Cancellable.reset()` to prepare the `Gio.Cancellable` for reuse in future operations. This ensures that the “Load” button can be clicked again to initiate another task. The application enforces that only one operation is active at a time by disabling the “Load” button during an ongoing task using `Gtk.Widget.set_sensitive()`.