Not running class destructor when i close app in python and pyqt

Question

When I get out of program(sys.exit(app.exec_())),Main Form closes, but there are two problems:
1 - Destructor of MainForm class not run
2 - Thread is still running too
I want when i close app, destructor of MainForm is runed and all threads are killed too

    class MainForm(QMainWindow,Ui_MainWindow):
        def __init__(self,parent=None):
            super(MainForm,self).__init__(parent)
            self.setupUi(self)
            #...
        def init_main_form(self):
            #...
            self.show_time()
        def show_time(self):
            self.label_9.setText(u"{}:{}:{}".format(str(datetime.datetime.now().hour),str(datetime.datetime.now().minute),str(datetime.datetime.now().second)))
            self.label_9.resize(self.label_9.width()+len(self.label_9.text())*3,self.label_9.height())
            b = threading.Timer(1,self.show_time)
            #b.setName('localtime')
            #self.thread_list.append(b)
            b.start()
        def __del__(self):
            print("app is closed")
            for tr in threading.enumerate():
                if tr.isAlive():
                    tr._Thread__stop()
                    # or tr.finished
                    # or tr.terminate()
    def main():
        app = QApplication(sys.argv)
        main_form = MainForm()
        main_form.show()
        sys.exit(app.exec_())

    if __name__ == '__main__':
        main()

Answer 1

While you're at it please remember that per documentation __del__ is not guaranteed to run when the interpreter exits.

__del__ is even trickier in other implementations of Python (like Jython). Your application shouldn't rely on it executing to run correctly.