Running tkinter and asyncio event loops in separate threads

event_thread.py

import asyncio
import concurrent.futures
import threading
from tkinter import Event, Tk
from tkinter.ttk import Button
from typing import Self


class EventThread(threading.Thread):
    """Runs an asyncio event loop in a separate thread.

    Starting and stopping can be done with :meth:`start()` and :meth:`stop()`,
    but for reliability, you should use the class in a context manager instead::

        >>> with EventThread() as event_thread:
        ...     loop = event_thread.loop
        ...     future = asyncio.run_coroutine_threadsafe(my_task(), loop)
        ...     result = future.result()
        >>> # Event loop will be cleaned up before exiting

    """

    def __init__(self, *args, **kwargs) -> None:
        super().__init__(*args, **kwargs)
        self.loop_fut = concurrent.futures.Future()
        self.stop_fut = concurrent.futures.Future()
        self.finished_fut = concurrent.futures.Future()

    def __enter__(self) -> Self:
        self.start()
        return self

    def __exit__(self, exc_type, exc_val, tb) -> None:
        self.stop()
        self.join()

    @property
    def loop(self) -> asyncio.AbstractEventLoop:
        return self.loop_fut.result()

    def run(self) -> None:
        try:
            asyncio.run(self._run_forever())
        finally:
            self.finished_fut.set_result(None)

    def stop(self) -> None:
        try:
            self.stop_fut.set_result(None)
        except concurrent.futures.InvalidStateError:
            pass

    async def _run_forever(self) -> None:
        self.loop_fut.set_result(asyncio.get_running_loop())
        await asyncio.wrap_future(self.stop_fut)


class TkApp(Tk):
    def __init__(self, event_thread: EventThread) -> None:
        super().__init__()
        self.event_thread = event_thread
        self._connect_lifetime(event_thread)
        self.bind("<<Destroy>>", self._on_destroy)

    def destroy(self) -> None:
        # asyncio tasks that make any tkinter calls, including event_generate(),
        # require the tkinter mainloop to still be running. As such, we need to
        # make sure the event loop closes before tkinter does.
        self.event_thread.stop()

    def _connect_lifetime(self, event_thread: EventThread) -> None:
        event_callback = lambda fut: self.event_generate("<<Destroy>>")
        event_thread.finished_fut.add_done_callback(event_callback)

    def _on_destroy(self, event: Event) -> None:
        return super().destroy()


# Example usage:
async def some_task():
    print("Running a task...")
    try:
        await asyncio.sleep(5)
    except asyncio.CancelledError:
        print("Task was cancelled!")
        raise
    else:
        print("Task was completed!")


def start_task(app: Tk, event_thread: EventThread):
    coro = some_task()
    fut = asyncio.run_coroutine_threadsafe(coro, event_thread.loop)
    fut.add_done_callback(lambda fut: app.event_generate("<<TaskFinished>>"))

    # Upon completion, callback will run in the event loop's thread.
    # Whatever you do should be thread-safe, such as pushing the result
    # to a queue and generating an event.
    #
    # Sidenote, if you want to push to an asyncio queue from tkinter you must
    # do that thread-safely as well, otherwise waiting tasks won't wake up:
    #
    #     >>> asyncio.run_coroutine_threadsafe(queue.put("Hello world!"), event_thread.loop)
    #     # or:
    #     >>> event_thread.loop.call_soon_threadsafe(queue.put_nowait, "Hello world!")


def on_task_finished(event: Event):
    print("Tkinter received a TaskFinished event")


def main():
    with EventThread() as event_thread:
        app = TkApp(event_thread)
        app.geometry("320x240")
        app.grid_columnconfigure(0, weight=1)
        app.grid_rowconfigure(0, weight=1)

        command = lambda: start_task(app, event_thread)
        button = Button(app, text="Start a task", command=command)
        button.grid(sticky="nesw", padx=20, pady=20)

        app.bind("<<TaskFinished>>", on_task_finished)

        try:
            app.mainloop()
        except BaseException:
            # This is probably a KeyboardInterrupt. If we exit now, the context
            # manager will stop our tasks, but it may trigger callbacks
            # that perform tkinter calls. To make sure they don't fail,
            # allow tkinter to run for just a bit longer.
            app.destroy()
            app.mainloop()
            raise


if __name__ == "__main__":
    try:
        main()
    except KeyboardInterrupt:
        print("KeyboardInterrupt caught")

exiting_window.txt

Running a task...
Running a task...
Task was completed!
Tkinter received a TaskFinished event
Task was cancelled!
Tkinter received a TaskFinished event

keyboard_interrupt.txt

Running a task...
Running a task...
Running a task...
Task was cancelled!
Task was cancelled!
Task was cancelled!
Tkinter received a TaskFinished event
Tkinter received a TaskFinished event
Tkinter received a TaskFinished event
KeyboardInterrupt caught

License

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