Async examples#

The Python language provides keywords for asynchronous operations, i.e., “async” to define coroutines or “await” to schedule asynchronous calls in the event loop (see PEP 492). It is up to packages to implement an event loop, support for these keywords, and more.

One such package is trio. Since both an async package and Qt itself work with event loops, special care must be taken to ensure that both event loops work with each other. trio offers a dedicated low-level API for more complicated use cases such as this. Specifically, there exists a function start_guest_run that enables running the Trio event loop as a “guest” inside another event loop - Qt’s in our case.

Based on this functionality, two examples for async usage with Qt have been implemented: eratosthenes and minimal:

Async example: Minimal
  • eratosthenes is a more extensive example that visualizes the Sieve of Eratosthenes algorithm. This algorithm per se is not one that is particularly suitable for asynchronous operations as it’s not I/O-heavy, but synchronizing coroutines to a configurable tick allows for a good visualization.

  • minimal is a minimal example featuring a button that triggers an asynchronous coroutine with a sleep. It is designed to highlight which boilerplate code is essential for an async program with Qt and offers a starting point for more complex programs.

Both examples feature:

  1. A window class.

  2. An AsyncHelper class containing start_guest_run plus helpers and callbacks necessary for its invocation. The entry point for the Trio guest run is provided as an argument from outside, which can be any async function.

While eratosthenes offloads the asynchronous logic that will run in trio’s event loop into a separate class, minimal demonstrates that async functions can be integrated into any class, including subclasses of Qt classes.

# Copyright (C) 2022 The Qt Company Ltd.
# SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause

from PySide6.QtCore import (Qt, QEvent, QObject, Signal, Slot)
from PySide6.QtWidgets import (QApplication, QLabel, QMainWindow, QPushButton, QVBoxLayout, QWidget)

import outcome
import signal
import sys
import traceback
import trio

class MainWindow(QMainWindow):

    def __init__(self, async_signal):

        self.async_signal = async_signal

        widget = QWidget()

        layout = QVBoxLayout(widget)

        self.text = QLabel("The answer is 42.")
        layout.addWidget(self.text, alignment=Qt.AlignmentFlag.AlignCenter)

        async_trigger = QPushButton(text="What is the question?")
        layout.addWidget(async_trigger, alignment=Qt.AlignmentFlag.AlignCenter)

    def async_start(self):

    async def set_text(self):
        await trio.sleep(1)
        self.text.setText("What do you get if you multiply six by nine?")

class AsyncHelper(QObject):

    trigger_signal = Signal()

    class ReenterQtObject(QObject):
        """ This is a QObject to which an event will be posted, allowing
            Trio to resume when the event is handled. event.fn() is the
            next entry point of the Trio event loop. """
        def event(self, event):
            if event.type() == QEvent.User + 1:
                return True
            return False

    class ReenterQtEvent(QEvent):
        """ This is the QEvent that will be handled by the ReenterQtObject.
            self.fn is the next entry point of the Trio event loop. """
        def __init__(self, fn):
            super().__init__(QEvent.Type(QEvent.User + 1))
            self.fn = fn

    def __init__(self, entry=None):
        self.reenter_qt = self.ReenterQtObject()
        self.entry = entry

    def set_entry(self, entry):
        self.entry = entry

    def launch_guest_run(self):
        """ To use Trio and Qt together, one must run the Trio event
            loop as a "guest" inside the Qt "host" event loop. """
        if not self.entry:
            raise Exception("No entry point for the Trio guest run was set.")

    def next_guest_run_schedule(self, fn):
        """ This function serves to re-schedule the guest (Trio) event
            loop inside the host (Qt) event loop. It is called by Trio
            at the end of an event loop run in order to relinquish back
            to Qt's event loop. By posting an event on the Qt event loop
            that contains Trio's next entry point, it ensures that Trio's
            event loop will be scheduled again by Qt. """
        QApplication.postEvent(self.reenter_qt, self.ReenterQtEvent(fn))

    def trio_done_callback(self, outcome_):
        """ This function is called by Trio when its event loop has
            finished. """
        if isinstance(outcome_, outcome.Error):
            error = outcome_.error
            traceback.print_exception(type(error), error, error.__traceback__)

if __name__ == "__main__":
    app = QApplication(sys.argv)
    async_helper = AsyncHelper()
    main_window = MainWindow(async_helper.trigger_signal)

    # This establishes the entry point for the Trio guest run. It varies
    # depending on how and when its event loop is to be triggered, e.g.,
    # at a specific moment like a button press (as here) or rather from
    # the beginning.

    signal.signal(signal.SIGINT, signal.SIG_DFL)