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.
The best-known package for this is asyncio. 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. asyncio offers a function stop that allows stopping an event loop without closing it. If it is called while a loop is running through run_forever, the loop will run the current batch of callbacks and then exit. New callbacks wil be scheduled the next time run_forever is called.
This approach is highly experimental and does not represent the state of the art of integrating Qt with asyncio. Instead it should rather be regarded more as a proof of concept to contrast asyncio with other async packages such as trio, which 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:
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:
A window class.
An AsyncHelper class containing start_guest_run plus helpers and callbacks necessary for its invocation. The entry point for the Trio/asyncio 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/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, QEvent, QObject, Signal, Slot)
from PySide6.QtWidgets import (QApplication, QLabel, QMainWindow, QPushButton, QVBoxLayout, QWidget)
import asyncio
import signal
import sys
class MainWindow(QMainWindow):
start_signal = Signal()
done_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 asyncio.sleep(1)
self.text.setText("What do you get if you multiply six by nine?")
self.done_signal.emit()
class AsyncHelper(QObject):
class ReenterQtObject(QObject):
""" This is a QObject to which an event will be posted, allowing
asyncio to resume when the event is handled. event.fn() is
the next entry point of the asyncio 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 asyncio 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.loop = asyncio.new_event_loop()
self.done = False
self.worker = worker
if hasattr(self.worker, "start_signal") and isinstance(self.worker.start_signal, Signal):
self.worker.start_signal.connect(self.on_worker_started)
if hasattr(self.worker, "done_signal") and isinstance(self.worker.done_signal, Signal):
self.worker.done_signal.connect(self.on_worker_done)
@Slot()
def on_worker_started(self):
""" To use asyncio and Qt together, one must run the asyncio
event loop as a "guest" inside the Qt "host" event loop. """
if not self.entry:
raise Exception("No entry point for the asyncio event loop was set.")
asyncio.set_event_loop(self.loop)
self.loop.create_task(self.entry())
self.loop.call_soon(self.next_guest_run_schedule)
self.done = False # Set this explicitly as we might want to restart the guest run.
self.loop.run_forever()
@Slot()
def on_worker_done(self):
""" When all our current asyncio tasks are finished, we must end
the "guest run" lest we enter a quasi idle loop of switching
back and forth between the asyncio and Qt loops. We can
launch a new guest run by calling launch_guest_run() again. """
self.done = True
def continue_loop(self):
""" This function is called by an event posted to the Qt event
loop to continue the asyncio event loop. """
if not self.done:
self.loop.call_soon(self.next_guest_run_schedule)
self.loop.run_forever()
def next_guest_run_schedule(self):
""" This function serves to pause and re-schedule the guest
(asyncio) event loop inside the host (Qt) event loop. It is
registered in asyncio as a callback to be called at the next
iteration of the event loop. When this function runs, it
first stops the asyncio event loop, then by posting an event
on the Qt event loop, it both relinquishes to Qt's event
loop and also schedules the asyncio event loop to run again.
Upon handling this event, a function will be called that
resumes the asyncio event loop. """
self.loop.stop()
QApplication.postEvent(self.reenter_qt, self.ReenterQtEvent(self.continue_loop))
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()