Async “Eratosthenes” 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:

Async example: Eratosthenes
  • 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.

Download this example

# 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(QObject):

    """ 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):
        super().__init__()
        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):

    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__":
    rows = 40
    cols = 40
    num = rows * cols

    app = QApplication(sys.argv)
    main_window = MainWindow(rows, cols)
    eratosthenes = Eratosthenes(num, main_window)
    async_helper = AsyncHelper(eratosthenes, 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()
# Copyright (C) 2022 The Qt Company Ltd.
# SPDX-License-Identifier: LicenseRef-Qt-Commercial OR BSD-3-Clause

from PySide6.QtCore import (Qt, QObject, Signal, Slot)
from PySide6.QtGui import (QColor, QFont, QPalette)
from PySide6.QtWidgets import (QApplication, QGridLayout, QLabel, QMainWindow, QVBoxLayout, QWidget)

import PySide6.QtAsyncio as QtAsyncio

import asyncio
import sys
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(QObject):

    """ 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):
        super().__init__()
        self.num = num
        self.sieve = [True] * self.num
        self.base = 0
        self.window = window
        self.tick = tick
        self.coroutines = []
        self.done = False
        self.loop = None

    def get_tick(self):
        return self.loop.time() + self.tick

    async def start(self):
        self.loop = asyncio.get_event_loop()
        asyncio.create_task(self.update_text())
        while self.base <= self.num / 2:
            await asyncio.sleep(self.tick)
            for i in range(self.base + 1, self.num):
                if self.sieve[i]:
                    self.base = i
                    break
            asyncio.create_task(self.mark_number(self.base + 1))
        while sum(self.coroutines) > 0:
            await asyncio.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 asyncio.sleep(self.tick)
        self.coroutines[id] = 0

    async def update_text(self):
        while not self.done:
            await asyncio.sleep(self.tick)
            if int(self.loop.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. 🥳"
        )


if __name__ == "__main__":
    rows = 40
    cols = 40
    num = rows * cols

    app = QApplication(sys.argv)
    main_window = MainWindow(rows, cols)
    eratosthenes = Eratosthenes(num, main_window)

    main_window.show()

    QtAsyncio.run(eratosthenes.start())