Python sending signals between two QThreads

Question

I am having trouble sending a signal from one qthread to another. A similar extract of code I am using is below:

class auth(QtCore.QThread):
    authenticate = pyqtSignal()
    serverDown = pyqtSignal()
    accntonline = pyqtSignal()

    def __init__(self, parent=None):
        super(auth, self).__init__(parent)
    def run(self):
        self.authenticate.emit()

class Threadclass(QtCore.QThread):
    authenticator = "n"
    def __init__(self, parent=None):

        super(Threadclass, self).__init__(parent)
        #setting first values avoiding errors

        self.a = auth(self)

        self.a.authenticate.connect(self.setauthentication)

    #All small functions setting values from box.

    def setauthentication(self):
        print("awdawdawda")
        self.authenticator = "y"

When I run the thread auth the variable self.authenticator does not change. it stays at "n". all help appreciated

Answer 1

If you override the run method of a QThread, it will not start its own event-loop unless you explicitly call its exec method. The event-loop is required for processing cross-thread signals. Because of this, it's usually better to create a worker object and move it to the thread, rather than overriding run.

Here is a demo script that shows how to use worker objects with threads:

import sys
from PyQt5 import QtWidgets, QtCore

class AuthWorker(QtCore.QObject):
    authenticate = QtCore.pyqtSignal()

    def __init__(self):
        super(AuthWorker, self).__init__()

    def run(self):
        QtCore.QThread.sleep(0.5)
        self.authenticate.emit()

class Worker(QtCore.QObject):
    finished = QtCore.pyqtSignal(str)
    authenticator = "n"

    def __init__(self, parent=None):
        super(Worker, self).__init__(parent)
        self.auth = AuthWorker()
        self.auth_thread = QtCore.QThread()
        self.auth.moveToThread(self.auth_thread)
        self.auth.authenticate.connect(self.setauthentication)
        self.auth_thread.started.connect(self.auth.run)

    def setauthentication(self):
        self.authenticator = "y"

    def run(self):
        self.auth_thread.start()
        QtCore.QThread.sleep(1)
        self.finished.emit('auth: %s' % self.authenticator)

class Window(QtWidgets.QWidget):
    def __init__(self):
        super(Window, self).__init__()
        self.button = QtWidgets.QPushButton('Test', self)
        self.button.clicked.connect(self.handleButton)
        self.edit = QtWidgets.QLineEdit(self)
        self.edit.setReadOnly(True)
        layout = QtWidgets.QVBoxLayout(self)
        layout.addWidget(self.edit)
        layout.addWidget(self.button)
        self.thread = QtCore.QThread()
        self.worker = Worker()
        self.worker.moveToThread(self.thread)
        self.worker.finished.connect(self.handleFinished)
        self.thread.started.connect(self.worker.run)

    def handleFinished(self, text):
        self.thread.quit()
        self.edit.setText(text)

    def handleButton(self):
        if not self.thread.isRunning():
            self.edit.clear()
            self.thread.start()

if __name__ == '__main__':

    app = QtWidgets.QApplication(sys.argv)
    window = Window()
    window.setGeometry(600, 100, 200, 100)
    window.show()
    sys.exit(app.exec_())