PyQt5 QThread Issue

Question

I am trying to get the basics of QT5 threading. This is my first attempt, combined from various sources:

import sys
from time import sleep

from PyQt5.QtWidgets import QApplication, QWidget, QPushButton, QGridLayout

from PyQt5.QtCore import QThread, QObject


'''

Traceback (most recent call last):
  File "threads.py", line 68, in <module>
    main(sys.argv)
  File "threads.py", line 63, in main
    window = Window()
  File "threads.py", line 15, in __init__
    self.initUi()
  File "threads.py", line 28, in initUi
    self.worker.moveToThread(self.thread)
AttributeError: 'NoneType' object has no attribute 'moveToThread'
Press any key to continue . . .

'''



class Window(QWidget):

    def __init__(self):

        super().__init__()
        self.initUi()

        self.low = 0
        self.high = 100

        self.show()


    def initUi(self):


        self.thread = QThread()
        self.worker = Worker(self)
        self.worker.moveToThread(self.thread)
        self.thread.start()

        self.button = QPushButton(
                'Start long running task')

        self.layout = QGridLayout()        
        self.layout.addWidget(self.button, 0, 0)

        self.setLayout(self.layout)



def Worker(QObject):

    def __init__(self, parent):
        super(Worker, self).__init__(parent)
        do_work()

    def do_work(self):

        for _ in range(20):
            print('running . . .')
            sleep(2)



def main(args):

    app = QApplication(args)
    window = Window()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main(sys.argv)

I have included the error I get in the code snippet. From online articles i learned that in PyQt5 I shouldn't subclass QThread.

Answer 1

stumbled upon this question while reviewing QThreads , amazing how fast I forget stuff, that's why I am posting an upgraded version of @ellyansec answer, need to keep it to avoid having to start from scratch next time too. I am sure the way I stop the QThreads is suboptimal , please feel free to make it better, my code:

import sys

from PyQt5.QtWidgets import QApplication, QWidget, QPushButton, QGridLayout

from PyQt5.QtCore import QThread, QObject, pyqtSlot, pyqtSignal

import threading


class Window(QWidget):

    button_pushed = pyqtSignal()

    def __init__(self):

        super().__init__()
        self.initUi()

        self.low = 0
        self.high = 100

        self.show()

    def initUi(self):

        self.thread = QThread()
        self.worker = Worker()
        self.worker.moveToThread(self.thread)

        self.button_pushed.connect(self.worker.do_work)

        self.thread.finished.connect(self.thread.deleteLater)

        self.thread.start()

        self.button = QPushButton(
            'Start long running task')

        self.button_2 = QPushButton('Stop QThread')

        self.button.clicked.connect(self.start)

        self.button_2.clicked.connect(self.stop_thread)

        self.layout = QGridLayout()
        self.layout.addWidget(self.button, 0, 0)

        self.layout.addWidget(self.button_2, 50, 0)
        self.setLayout(self.layout)

        self.cnt = 0

    def start(self):

        self.cnt += 1

        print('button pushed ' + str(self.cnt) + '  time ')

        self.button_pushed.emit()

    def stop_thread(self):

        self.thread.quit()
        self.thread.wait()


class Worker(QObject):
    def __init__(self, parent=None):
        QObject.__init__(self, parent=parent)

    @pyqtSlot()
    def do_work(self):
        for k in range(3):
            print('running . . .' + str(threading.get_ident())+'.......'+str(k+1))
            QThread.sleep(2)

        print('done !!!')


def main(args):

    app = QApplication(args)
    window = Window()
    window.show()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main(sys.argv)

Answer 2

You have 2 problems, the first is that the worker must be a class for it changes:

def Worker(QObject):

to

class Worker(QObject):

The other problem is that you must call do_work by means of the instance, ie self, for it changes:

do_work()

to:

self.do_work()

In the following part I show a complete example:

import sys

from PyQt5.QtWidgets import QApplication, QWidget, QPushButton, QGridLayout

from PyQt5.QtCore import QThread, QObject


class Window(QWidget):

    def __init__(self):

        super().__init__()
        self.initUi()

        self.low = 0
        self.high = 100

        self.show()


    def initUi(self):


        self.thread = QThread()
        self.worker = Worker()
        self.worker.moveToThread(self.thread)

        self.thread.started.connect(self.worker.do_work)
        self.thread.finished.connect(self.thread.deleteLater)

        self.button = QPushButton(
                'Start long running task')

        self.button.clicked.connect(self.thread.start)

        self.layout = QGridLayout()        
        self.layout.addWidget(self.button, 0, 0)
        self.setLayout(self.layout)



class Worker(QObject):
    def __init__(self, parent=None):
        QObject.__init__(self, parent=parent)

    def do_work(self):
        for _ in range(20):
            print('running . . .')
            QThread.sleep(2)



def main(args):

    app = QApplication(args)
    window = Window()
    window.show()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main(sys.argv)