PyQt5 QObject: Cannot create children for a parent that is in a different thread

Question

I am working in a menu system tray with PyQt5. I am very new with PyQt5, and what I want to do is to trigger an action without the menu being blocked (multithreading). After

Answer 1

Since this is the google top answer for this error and it took me longer than expected to work this out properly, I will share my very simple solution for Python 3 and PyQt 5 (if you change some imports it should work in PyQt4 too I guess).

The situation I had was a systray icon with a right-click menu, that should be re-built when a different thread requests it. You can of course apply this to other problems where you want to communicate through thread limits.

import time
import sys
import threading
from PyQt5 import QtGui
from PyQt5 import QtWidgets
from PyQt5 import QtCore



class SystemTrayIcon(QtWidgets.QSystemTrayIcon):
    def __init__(self, icon=None, parent=None):
        icon = QtGui.QIcon(QtWidgets.QApplication.style().standardPixmap(QtWidgets.QStyle.SP_MediaPlay))
        QtWidgets.QSystemTrayIcon.__init__(self, icon, parent)

        self.menu = QtWidgets.QMenu(parent)
        self.setContextMenu(self.menu)

        self.build_menu()
        self.show()

        # see http://pyqt.sourceforge.net/Docs/PyQt5/signals_slots.html for more information
        self.signal = MySignal()
        self.signal.sig_no_args.connect(self.build_menu)
        self.signal.sig_with_str.connect(self.print_string)


    def build_menu(self):
        ''' This function should be called in order to rebuild 
        the right-click menu for the systray icon'''
        global list_dict_streams
        self.menu.clear()

        exitAction = self.menu.addAction("Exit")
        exitAction.triggered.connect(self._exit)

        for x in list_dict_streams :
            self.menu.addAction(x)


    def print_string(self, str):
        print(str)


    def _exit(self):
        QtCore.QCoreApplication.exit()



class MySignal(QtCore.QObject):
    ''' Why a whole new class? See here: 
    https://stackoverflow.com/a/25930966/2441026 '''
    sig_no_args = QtCore.pyqtSignal()
    sig_with_str = QtCore.pyqtSignal(str)


list_dict_streams = ["1"]
def work_thread(trayIcon):
    ''' Will add one menu item to the systray menu every 5 seconds
    and will send a signal with a string '''
    global list_dict_streams

    while True:
        trayIcon.signal.sig_no_args.emit()
        trayIcon.signal.sig_with_str.emit("String emitted")
        list_dict_streams.append(str(len(list_dict_streams)+1))
        time.sleep(5)


def main():
    app = QtWidgets.QApplication(sys.argv)
    trayIcon = SystemTrayIcon()

    t = threading.Thread(target=work_thread, args=(trayIcon,))
    t.daemon = True     # otherwise the 'Exit' from the systray menu will not work
    t.start()

    sys.exit(app.exec_())


if __name__ == '__main__':
    main()

Basically you have to create a new class MySignal(QtCore.QObject) why. I created a class with two examples - one that sends no arguments along another one that you can pass a string. You can of course define other arguments. Then in your target thread you create a new instance of this class and connect the functions from that class, to the functions inside your target (the systray icon in my case). After that you can now call the emit(...) functions like I do in the while-loop.
Now Qt is happy as you just emit a signal compared to when you would call trayIcon.build_menu() directly from a different thread.