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:
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 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, QTimer, Signal, Slot)
from PySide6.QtGui import (QColor, QFont, QPalette)
from PySide6.QtWidgets import (QApplication, QGridLayout, QLabel, QMainWindow, QVBoxLayout, QWidget)
import outcome
import signal
import sys
import traceback
import trio
from random import randint
class MainWindow(QMainWindow):
set_num = Signal(int, QColor)
def __init__(self, rows, cols):
super().__init__()
self.rows = rows
self.cols = cols
widget_central = QWidget()
self.setCentralWidget(widget_central)
layout_outer = QVBoxLayout(widget_central)
self.widget_outer_text = QLabel()
font = QFont()
font.setPointSize(14)
self.widget_outer_text.setFont(font)
layout_outer.addWidget(self.widget_outer_text, alignment=Qt.AlignmentFlag.AlignCenter)
widget_inner_grid = QWidget()
layout_outer.addWidget(widget_inner_grid, alignment=Qt.AlignmentFlag.AlignCenter)
self.layout_inner_grid = QGridLayout(widget_inner_grid)
k = 1
for i in range(self.rows):
for j in range(self.cols):
box = QLabel(f"{k}")
self.layout_inner_grid.addWidget(box, i, j, Qt.AlignmentFlag.AlignCenter)
k += 1
self.set_num.connect(self.set_num_handler)
@Slot(int, QColor)
def set_num_handler(self, i, color):
row = int((i - 1) / self.cols)
col = (i - 1) - (row * self.cols)
widget = self.layout_inner_grid.itemAtPosition(row, col).widget()
font = QFont()
font.setWeight(QFont.Bold)
palette = QPalette()
palette.setColor(QPalette.WindowText, color)
widget.setFont(font)
widget.setPalette(palette)
class Eratosthenes():
""" This Sieve of Eratosthenes runs on a configurable tick (default
0.1 seconds). At each tick, a new subroutine will be created
that will check multiples of the next prime number. Each of
these subroutines also operates on the same tick. """
def __init__(self, num, window, tick=0.1):
self.num = num
self.sieve = [True] * self.num
self.base = 0
self.window = window
self.tick = tick
self.coroutines = []
self.done = False
self.nursery = None
async def start(self):
async with trio.open_nursery() as self.nursery:
self.nursery.start_soon(self.update_text)
while self.base <= self.num / 2:
await trio.sleep(self.tick)
for i in range(self.base + 1, self.num):
if self.sieve[i]:
self.base = i
break
self.nursery.start_soon(self.mark_number, self.base + 1)
while sum(self.coroutines) > 0:
await trio.sleep(self.tick)
self.done = True
async def mark_number(self, base):
id = len(self.coroutines)
self.coroutines.append(1)
color = QColor(randint(64, 192), randint(64, 192), randint(64, 192))
for i in range(2 * base, self.num + 1, base):
if self.sieve[i - 1]:
self.sieve[i - 1] = False
self.window.set_num.emit(i, color)
await trio.sleep(self.tick)
self.coroutines[id] = 0
async def update_text(self):
while not self.done:
await trio.sleep(self.tick)
if int(trio.lowlevel.current_clock().current_time() + self.tick) % 2:
text = "⚙️ ...Calculating prime numbers... ⚙️"
else:
text = "👩💻 ...Hacking the universe... 👩💻"
self.window.widget_outer_text.setText(text)
self.window.widget_outer_text.setText(
"🥳 Congratulations! You found all the prime numbers and solved mathematics. 🥳"
)
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:
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.User + 1))
self.fn = fn
def __init__(self, entry=None):
super().__init__()
self.reenter_qt = self.ReenterQtObject()
self.entry = entry
def set_entry(self, entry):
self.entry = entry
@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__":
rows = 40
cols = 40
num = rows * cols
app = QApplication(sys.argv)
main_window = MainWindow(rows, cols)
eratosthenes = Eratosthenes(num, main_window)
async_helper = AsyncHelper(entry=eratosthenes.start)
# 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.,
# from the beginning (as here) or rather at a specific moment like
# a button press.
QTimer.singleShot(0, async_helper.launch_guest_run)
main_window.show()
signal.signal(signal.SIGINT, signal.SIG_DFL)
app.exec()