Um dein Beispiel zu verwenden:

class Test(Thread):

def __init__(self):
self.is_running = False
Thread.__init__(self)

def run(self):
self.is_running = True
i = 0
j = 0
while self.is_running:
i = i+1
j = j+2

def ausstieg(self):
self.is_running = False

class GUI(Frame):

def create ...