Async “Minimal” Example#
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.
The best-known package for this is asyncio. asyncio offers an API that allows for the asyncio event loop to be replaced by a custom implementation. Such an implementation is available with the QtAsyncio module. It is based on Qt and uses Qt’s event loop in the backend.
trio is another popular package that offers a dedicated low-level API for more complex use cases. 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, standing in contrast to asyncio’s approach.
Based on this functionality, two examples for async usage with Qt have been implemented: eratosthenes and 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.
While eratosthenes offloads the asynchronous logic that will run in trio’s/asyncio’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):
start_signal = Signal()
def __init__(self):
super().__init__()
widget = QWidget()
self.setCentralWidget(widget)
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?")
async_trigger.clicked.connect(self.async_start)
layout.addWidget(async_trigger, alignment=Qt.AlignmentFlag.AlignCenter)
@Slot()
def async_start(self):
self.start_signal.emit()
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):
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.Type.User + 1:
event.fn()
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.Type.User + 1))
self.fn = fn
def __init__(self, worker, entry):
super().__init__()
self.reenter_qt = self.ReenterQtObject()
self.entry = entry
self.worker = worker
if hasattr(self.worker, "start_signal") and isinstance(self.worker.start_signal, Signal):
self.worker.start_signal.connect(self.launch_guest_run)
@Slot()
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.")
trio.lowlevel.start_guest_run(
self.entry,
run_sync_soon_threadsafe=self.next_guest_run_schedule,
done_callback=self.trio_done_callback,
)
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)
main_window = MainWindow()
async_helper = AsyncHelper(main_window, main_window.set_text)
main_window.show()
signal.signal(signal.SIGINT, signal.SIG_DFL)
app.exec()
# Copyright (C) 2022 The Qt Company Ltd.
# SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause
from PySide6.QtCore import Qt
from PySide6.QtWidgets import (QApplication, QLabel, QMainWindow, QPushButton, QVBoxLayout, QWidget)
import PySide6.QtAsyncio as QtAsyncio
import asyncio
import sys
class MainWindow(QMainWindow):
def __init__(self):
super().__init__()
widget = QWidget()
self.setCentralWidget(widget)
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?")
async_trigger.clicked.connect(lambda: asyncio.ensure_future(self.set_text()))
layout.addWidget(async_trigger, alignment=Qt.AlignmentFlag.AlignCenter)
async def set_text(self):
await asyncio.sleep(1)
self.text.setText("What do you get if you multiply six by nine?")
if __name__ == "__main__":
app = QApplication(sys.argv)
main_window = MainWindow()
main_window.show()
QtAsyncio.run()