How to make sure the QApplication and its QThreads are all closed

The code below creates a single QDialog which starts a QThread that gets a function that is long to calculate. QDialog's closeEvent() method was modified to terminate the thread that was started.

How to make sure the thread is terminated only if it completed a task it was working on? What is the difference between stopping a thread with its quit() and its terminate() method? Should the thread always be terminated before the main application window is closed? Why on Mac OS X the Python process remains to be listed in Activity Monitor even after the main dialog is closed and the thread is terminated?

import threading
import Queue as Queue
import datetime

global queue
queue = Queue.Queue()


class Thread(QThread):
    def __init__(self, queue, parent):
        QThread.__init__(self, parent)
        self.queue = queue

    def run(self):
        while True:
            task = queue.get()
            output = task()
            queue.task_done()


def longToCalculate():
    for i in range(30000000):
        i += i
        if not i % 100000:
            print '%s ...still calculating ' % datetime.datetime.now()
    print 'calculation completed'
    return i


class Dialog(QDialog):
    def __init__(self, parent=None):
        super(Dialog, self).__init__(parent)

    def closeEvent(self, event):
        # self.thread.quit()
        self.thread.terminate()
        event.accept()


class Dialog(QDialog):
    def __init__(self, parent=None):
        QDialog.__init__(self, parent)
        self.queue = Queue.Queue()
        self.thread = Thread(queue=self.queue, parent=self)
        self.thread.start()
        queue.put(longToCalculate)

if __name__ == '__main__':
    app = QApplication([])
    dialog = Dialog()
    dialog.show()
    qApp.exec_()