In PyQt, what is the best way to share data between the main window and a thread

Widgets are not thread safe, see Threads and QObjects:

Although QObject is reentrant, the GUI classes, notably QWidget and all its subclasses, are not reentrant. They can only be used from the main thread.

And see more definitions here: Reentrancy and Thread-Safety

You should only use widgets in the main thread, and use signal and slots to communicate with other threads.

I don't think a global variable would work, but I honestly don't know why.

How to use signals in this example:

#in mainself.worker = Worker(self.spinbox.value())self.worker.beep.connect(self.update)self.spinbox.valueChanged.connect(self.worker.update_value)class Worker(QtCore.QThread):    beep=QtCore.pyqtSignal(int)    def __init__(self,sleep_time):        super(Worker, self).__init__()        self.running = False        self.sleep_time=sleep_time    def run(self):        self.running = True        i = 0        while self.running:            i += 1            self.beep.emit(i)            time.sleep(self.sleep_time)    def stop(self):        self.running = False    def update_value(self,value):        self.sleep_time=value

NB: I use the new style signal and slots