from PySide2.QtCore import QObject, Signal, Slot, QThread, QTimer
from PySide2.QtWidgets import QApplication


class ThreadPrinter(QObject):

    def __init__(self):
        QObject.__init__(self)
        # 'sig' is assigned outside of __init__
        self.sig.connect(lambda: print('Never called.'))

    def undecorated_print_thread(self):
        print(f'Current: {QThread.currentThread()}')

    @Slot()
    def decorated_print_thread(self):
        print(f'Current: {QThread.currentThread()}')


class Signaller(QObject):
    sig_undecorated = Signal()
    sig_decorated = Signal()

    def __init__(self):
        QObject.__init__(self)
        self.thread = QThread()
        self.thread_printer = ThreadPrinter()
        self.thread_printer.moveToThread(self.thread)
        self.thread.start()

        self.sig_decorated.connect(self.thread_printer.decorated_print_thread)
        self.sig_undecorated.connect(self.thread_printer.undecorated_print_thread)

    def run(self):
        print(f'Main thread: {QApplication.instance().thread()}')
        # Trigger ThreadPrinter.decorated_print_thread
        self.sig_decorated.emit()

        # Trigger ThreadPrinter.undecorated_print_thread
        self.sig_undecorated.emit()


if __name__ == '__main__':
    app = QApplication()
    ThreadPrinter.sig = Signal()
    signaller = Signaller()
    timer = QTimer.singleShot(0, signaller.run)
    app.exec_()