Python Cookbook

Terminating a Thread

Credit: Doug Fort

Problem

You must terminate a thread from the outside, but Python doesn’t let one thread brutally kill another, so you need a controlled-termination idiom.

Solution

A frequently asked question is: How do I kill a thread? The answer is: You don’t. Instead, you kindly ask it to go away. The thread must periodically check if it’s been asked to go away and then comply (typically after some kind of clean-up):

import threading

class TestThread(threading.Thread):

    def _ _init_ _(self, name='TestThread'):
        """ constructor, setting initial variables """
        self._stopevent = threading.Event(  )
        self._sleepperiod = 1.0

        threading.Thread._ _init_ _(self, name=name)

    def run(self):
        """ main control loop """
        print "%s starts" % (self.getName(  ),)

        count = 0
        while not self._stopevent.isSet(  ):
            count += 1
            print "loop %d" % (count,)
            self._stopevent.wait(self._sleepperiod)

        print "%s ends" % (self.getName(  ),)

    def join(self, timeout=None):
        """ Stop the thread. """
        self._stopevent.set(  )
        threading.Thread.join(self, timeout)

if _ _name_ _ == "_ _main_ _":
    testthread = TestThread(  )
    testthread.start(  )

    import time
    time.sleep(10.0)

    testthread.join(  )

Discussion

Often, you will want to control a thread from the outside, but the ability to kill it is, well, overkill. Python doesn’t give you this ability, and thus forces you to design your thread systems more carefully. This recipe is based on the idea of a thread whose main function uses a loop. Periodically, the loop checks if a ...