与Queue一起使用时，“线程”列表变量的用途是什么

Question

If a list variable threads is not declared (or if it is declared outside of class or declared as a local) the code throws:

QThread: Destroyed while thread is still running
[Finished in 4.4s with exit code -11]

Yet, it appears it doesn't do anything other than allowing to append Thread instance to itself. What is the purpose of this variable? Why it is needed and where I could read more about it?

from PyQt4 import QtCore, QtGui
import threading
import Queue as queue
app = QtGui.QApplication([])

class SimpleThread(QtCore.QThread):
    def __init__(self, queue, parent=None):
        QtCore.QThread.__init__(self, parent)      
        self.queue=queue        
    def run(self):
        while True:
            arg=self.queue.get() 
            self.fun(arg)    
            self.queue.task_done()
    def fun(self, arg):
        print 'func: %s'%arg
        return arg+1

class AppWindow(QtGui.QMainWindow):
    def __init__(self):
        super(AppWindow, self).__init__()
        mainWidget=QtGui.QWidget()
        self.setCentralWidget(mainWidget)
        mainLayout = QtGui.QVBoxLayout()
        mainWidget.setLayout(mainLayout)  
        button=QtGui.QPushButton('Process')
        button.clicked.connect(self.process)
        mainLayout.addWidget(button)
    def process(self):
        MAX_CORES=2
        self.queue=queue.Queue()
        self.threads=[]
        for i in range(1, MAX_CORES):
            thread=SimpleThread(self.queue)
            self.threads.append(thread)
            thread.start()  

        for arg in [1,2,3]:
            self.queue.put(arg)

window=AppWindow()
window.show()
sys.exit(app.exec_())

Answer 1

A QThread , unlike a threading.Thread , gets terminated as soon as there are no more references to it. Sticking the QThread instance in a global list or a list that's an instance variable of a long-lived class will preserve a reference to the thread, and prevent it from being destroyed.