Cannot set parent error with QThread when using QMessageBox

Question

I am trying to run a progress bar on a thread and a function on another thread. The following is my approach and it is working fine, until I add a QMessageBox. I created two new classes for QThread, one handles the progress bar, another my function. They are being called when the button is pressed using the onButtonClicked function

from PyQt5 import QtCore, QtGui, QtWidgets
from PyQt5.QtCore import QThread, pyqtSignal
from PyQt5.QtWidgets import QApplication, QWidget, QPushButton, QMessageBox, QLineEdit, QProgressBar, QLabel, QFileDialog, QCheckBox, QMenuBar, QStatusBar

import time

TIME_LIMIT = 100
class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(800, 600)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.msg = QMessageBox()
        self.label = QtWidgets.QLabel(self.centralwidget)
        self.label.setGeometry(QtCore.QRect(300, 60, 47, 13))
        self.label.setObjectName("label")
        self.lineEdit = QtWidgets.QLineEdit(self.centralwidget)
        self.lineEdit.setGeometry(QtCore.QRect(270, 100, 113, 20))
        self.lineEdit.setObjectName("lineEdit")
        self.pushButton = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton.setGeometry(QtCore.QRect(290, 150, 75, 23))
        self.pushButton.setObjectName("pushButton")
        self.progressBar = QtWidgets.QProgressBar(self.centralwidget)
        self.progressBar.setGeometry(QtCore.QRect(280, 210, 118, 23))
        self.progressBar.setProperty("value", 24)
        self.progressBar.setObjectName("progressBar")
        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(MainWindow)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 800, 21))
        self.menubar.setObjectName("menubar")
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)
        self.Actionlistenr()

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.label.setText(_translate("MainWindow", "TextLabel"))
        self.pushButton.setText(_translate("MainWindow", "PushButton"))

    def Actionlistenr(self):
        self.pushButton.clicked.connect(self.onButtonClick)


    def test(self):
        if self.lineEdit.text() == "":
            self.msg.setIcon(QMessageBox.Critical)
            self.msg.setText("Please select a document first!")
            self.msg.setWindowTitle("Error")
            return self.msg.exec()
            # If this was just a regular print statement,
            # then it would work, or any other statement that
            # does not involve a QMessageBox

    def onButtonClick(self):
        self.calc = External()        
        self.calc.countChanged.connect(self.onCountChanged)
        self.calc.start()
        self.calc2 = External2(self)
        self.calc2.start()


    def onCountChanged(self, value):
        self.progressBar.setValue(value)


class External(QThread):
    """
    Runs a counter thread.
    """
    countChanged = pyqtSignal(int)

    def run(self):
        count = 0
        while count < TIME_LIMIT:
            count +=1
            time.sleep(1)
            self.countChanged.emit(count)

class External2(QThread, object):    
    """
    Runs a counter thread.
    """
    def __init__(self, outer_instance):
        super().__init__()
        self.outer_instance = outer_instance       


    def run(self):
       self.outer_instance.test()


if __name__ == "__main__":
    import sys
    app = QtWidgets.QApplication(sys.argv)
    MainWindow = QtWidgets.QMainWindow()
    ui = Ui_MainWindow()
    ui.setupUi(MainWindow)
    MainWindow.show()
    sys.exit(app.exec_())

I am getting QObject::setParent: Cannot set parent, new parent is in a different thread Error when doing this, only when I add a QMessageBox in my test function. I am assuming that this is happening because QMessagebox is running on the main thread, not my External2() class, how can I fix this?

eyllanesc · Accepted Answer

The first thing you have to do is verify if the requirements are met (in this case the QLineEdit is not empty) for the heavy duty to start. In these cases I prefer to use the worker thread approach. To launch the heavy task, the method must be invoked asynchronously, for example using QTimer.singleShot(), and to pass the additional arguments use functools.partial(), you must also use @pyqtSlot to be sure that the tasks are executed in the thread Right.

On the other hand you should not modify the class generated by Qt Designer⁽¹⁾ but create another class that inherits from the widget and use the first one to fill it.

from PyQt5 import QtCore, QtGui, QtWidgets
from functools import partial
import time

TIME_LIMIT = 100


class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(800, 600)
        self.centralwidget = QtWidgets.QWidget(MainWindow)
        self.centralwidget.setObjectName("centralwidget")
        self.label = QtWidgets.QLabel(self.centralwidget)
        self.label.setGeometry(QtCore.QRect(300, 60, 47, 13))
        self.label.setObjectName("label")
        self.lineEdit = QtWidgets.QLineEdit(self.centralwidget)
        self.lineEdit.setGeometry(QtCore.QRect(270, 100, 113, 20))
        self.lineEdit.setObjectName("lineEdit")
        self.pushButton = QtWidgets.QPushButton(self.centralwidget)
        self.pushButton.setGeometry(QtCore.QRect(290, 150, 75, 23))
        self.pushButton.setObjectName("pushButton")
        self.progressBar = QtWidgets.QProgressBar(self.centralwidget)
        self.progressBar.setGeometry(QtCore.QRect(280, 210, 118, 23))
        self.progressBar.setProperty("value", 24)
        self.progressBar.setObjectName("progressBar")
        MainWindow.setCentralWidget(self.centralwidget)
        self.menubar = QtWidgets.QMenuBar(MainWindow)
        self.menubar.setGeometry(QtCore.QRect(0, 0, 800, 21))
        self.menubar.setObjectName("menubar")
        MainWindow.setMenuBar(self.menubar)
        self.statusbar = QtWidgets.QStatusBar(MainWindow)
        self.statusbar.setObjectName("statusbar")
        MainWindow.setStatusBar(self.statusbar)

        self.retranslateUi(MainWindow)
        QtCore.QMetaObject.connectSlotsByName(MainWindow)

    def retranslateUi(self, MainWindow):
        _translate = QtCore.QCoreApplication.translate
        MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
        self.label.setText(_translate("MainWindow", "TextLabel"))
        self.pushButton.setText(_translate("MainWindow", "PushButton"))


class MainWindow(QtWidgets.QMainWindow, Ui_MainWindow):
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)
        self.setupUi(self)
        self.msg = QtWidgets.QMessageBox()

        self.Actionlistenr()

        thread = QtCore.QThread(self)
        thread.start()
        self.m_worker = Worker()
        self.m_worker.countChanged.connect(self.progressBar.setValue)
        self.m_worker.moveToThread(thread)

    def Actionlistenr(self):
        self.pushButton.clicked.connect(self.onButtonClick)

    def request_information(self):
        filename = self.lineEdit.text()
        if filename:
            wrapper = partial(self.m_worker.task, filename)
            QtCore.QTimer.singleShot(0, wrapper)
        else:
            self.msg.setIcon(QtWidgets.QMessageBox.Critical)
            self.msg.setText("Please select a document first!")
            self.msg.setWindowTitle("Error")
            self.msg.exec_()

    @QtCore.pyqtSlot()
    def onButtonClick(self):
        self.request_information()


class Worker(QtCore.QObject):
    countChanged = QtCore.pyqtSignal(int)

    @QtCore.pyqtSlot(str)
    def task(self, filename):
        # execute heavy task here
        print("start")
        print(filename)
        count = 0
        while count < TIME_LIMIT:
            count += 1
            time.sleep(1)
            self.countChanged.emit(count)
        print("finished")


if __name__ == "__main__":
    import sys

    app = QtWidgets.QApplication(sys.argv)
    w = MainWindow()
    w.show()
    sys.exit(app.exec_())

^{(1) http://pyqt.sourceforge.net/Docs/PyQt5/designer.html}

Cannot set parent error with QThread when using QMessageBox

Answers (2)

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