How to Use QThreads Properly in PyQt: Best Practices and Example Usage?

How to Use QThread Properly in PyQt

QThread Background

QThreads allow you to create and execute tasks in separate threads, parallelizing operations that might freeze the user interface. However, the blog post https://mayaposch.wordpress.com/2011/11/01/how-to-really-truly-use-qthreads-the-full-explanation/ recommends against re-implementing the run method to utilize QThreads.

Correct QThread Usage

Instead of manipulating the run method, consider the following best practices for QThread usage in PyQt:

Example Usage

The code below provides an example of a separate worker thread that communicates with the GUI using signals:

<code class="python">from PyQt4 import QtGui, QtCore
import sys
import random

class Example(QtCore.QObject):

    signalStatus = QtCore.pyqtSignal(str)

    def __init__(self, parent=None):
        super(self.__class__, self).__init__(parent)

        # Create a gui object.
        self.gui = Window()

        # Create a new worker thread.
        self.createWorkerThread()

        # Make any cross object connections.
        self._connectSignals()

        self.gui.show()


    def _connectSignals(self):
        self.gui.button_cancel.clicked.connect(self.forceWorkerReset)
        self.signalStatus.connect(self.gui.updateStatus)
        self.parent().aboutToQuit.connect(self.forceWorkerQuit)


    def createWorkerThread(self):

        # Setup the worker object and the worker_thread.
        self.worker = WorkerObject()
        self.worker_thread = QtCore.QThread()
        self.worker.moveToThread(self.worker_thread)
        self.worker_thread.start()

        # Connect any worker signals
        self.worker.signalStatus.connect(self.gui.updateStatus)
        self.gui.button_start.clicked.connect(self.worker.startWork)


    def forceWorkerReset(self):      
        if self.worker_thread.isRunning():
            print('Terminating thread.')
            self.worker_thread.terminate()

            print('Waiting for thread termination.')
            self.worker_thread.wait()

            self.signalStatus.emit('Idle.')

            print('building new working object.')
            self.createWorkerThread()


    def forceWorkerQuit(self):
        if self.worker_thread.isRunning():
            self.worker_thread.terminate()
            self.worker_thread.wait()


class WorkerObject(QtCore.QObject):

    signalStatus = QtCore.pyqtSignal(str)

    def __init__(self, parent=None):
        super(self.__class__, self).__init__(parent)

    @QtCore.pyqtSlot()        
    def startWork(self):
        for ii in range(7):
            number = random.randint(0,5000**ii)
            self.signalStatus.emit('Iteration: {}, Factoring: {}'.format(ii, number))
            factors = self.primeFactors(number)
            print('Number: ', number, 'Factors: ', factors)
        self.signalStatus.emit('Idle.')

    def primeFactors(self, n):
        i = 2
        factors = []
        while i * i <= n:
            if n % i:
                i += 1
            else:
                n //= i
                factors.append(i)
        if n > 1:
            factors.append(n)
        return factors


class Window(QtGui.QWidget):

    def __init__(self):
        QtGui.QWidget.__init__(self)
        self.button_start = QtGui.QPushButton('Start', self)
        self.button_cancel = QtGui.QPushButton('Cancel', self)
        self.label_status = QtGui.QLabel('', self)

        layout = QtGui.QVBoxLayout(self)
        layout.addWidget(self.button_start)
        layout.addWidget(self.button_cancel)
        layout.addWidget(self.label_status)

        self.setFixedSize(400, 200)

    @QtCore.pyqtSlot(str)
    def updateStatus(self, status):
        self.label_status.setText(status)


if __name__=='__main__':
    app = QtGui.QApplication(sys.argv)
    example = Example(app)
    sys.exit(app.exec_())</code>

In this example, the GUI and worker thread are separate objects that communicate through signals. The GUI listens for status updates, while the worker thread performs calculations and sends back progress.

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