Is it possible to terminate a running thread without setting/checking any flags/semaphores/etc.?

It is generally a bad pattern to kill a thread abruptly, in Python and in any language. Think of the following cases:

• the thread is holding a critical resource that must be closed properly
• the thread has created several other threads that must be killed as well.

The nice way of handling this if you can afford it (if you are managing your own threads) is to have an exit_request flag that each threads checks on regular interval to see if it is time for it to exit.

For example:

import threading

class StoppableThread(threading.Thread):
    """Thread class with a stop() method. The thread itself has to check
    regularly for the stopped() condition."""

    def __init__(self,  *args, **kwargs):
        super(StoppableThread, self).__init__(*args, **kwargs)
        self._stop_event = threading.Event()

    def stop(self):
        self._stop_event.set()

    def stopped(self):
        return self._stop_event.is_set()

In this code, you should call stop() on the thread when you want it to exit, and wait for the thread to exit properly using join(). The thread should check the stop flag at regular intervals.

There are cases however when you really need to kill a thread. An example is when you are wrapping an external library that is busy for long calls and you want to interrupt it.

The following code allows (with some restrictions) to raise an Exception in a Python thread:

def _async_raise(tid, exctype):
    '''Raises an exception in the threads with id tid'''
    if not inspect.isclass(exctype):
        raise TypeError("Only types can be raised (not instances)")
    res = ctypes.pythonapi.PyThreadState_SetAsyncExc(ctypes.c_long(tid),
                                                     ctypes.py_object(exctype))
    if res == 0:
        raise ValueError("invalid thread id")
    elif res != 1:
        # "if it returns a number greater than one, you're in trouble,
        # and you should call it again with exc=NULL to revert the effect"
        ctypes.pythonapi.PyThreadState_SetAsyncExc(ctypes.c_long(tid), None)
        raise SystemError("PyThreadState_SetAsyncExc failed")

class ThreadWithExc(threading.Thread):
    '''A thread class that supports raising exception in the thread from
       another thread.
    '''
    def _get_my_tid(self):
        """determines this (self's) thread id

        CAREFUL : this function is executed in the context of the caller
        thread, to get the identity of the thread represented by this
        instance.
        """
        if not self.isAlive():
            raise threading.ThreadError("the thread is not active")

        # do we have it cached?
        if hasattr(self, "_thread_id"):
            return self._thread_id

        # no, look for it in the _active dict
        for tid, tobj in threading._active.items():
            if tobj is self:
                self._thread_id = tid
                return tid

        # TODO: in python 2.6, there's a simpler way to do : self.ident

        raise AssertionError("could not determine the thread's id")

    def raiseExc(self, exctype):
        """Raises the given exception type in the context of this thread.

        If the thread is busy in a system call (time.sleep(),
        socket.accept(), ...), the exception is simply ignored.

        If you are sure that your exception should terminate the thread,
        one way to ensure that it works is:

            t = ThreadWithExc( ... )
            ...
            t.raiseExc( SomeException )
            while t.isAlive():
                time.sleep( 0.1 )
                t.raiseExc( SomeException )

        If the exception is to be caught by the thread, you need a way to
        check that your thread has caught it.

        CAREFUL : this function is executed in the context of the
        caller thread, to raise an excpetion in the context of the
        thread represented by this instance.
        """
        _async_raise( self._get_my_tid(), exctype )

(Based on Killable Threads by Tomer Filiba. The quote about the return value of PyThreadState_SetAsyncExc appears to be from an old version of Python.)

As noted in the documentation, this is not a magic bullet because if the thread is busy outside the Python interpreter, it will not catch the interruption.

A good usage pattern of this code is to have the thread catch a specific exception and perform the cleanup. That way, you can interrupt a task and still have proper cleanup.

There is no official API to do that, no.

You need to use platform API to kill the thread, e.g. pthread_kill, or TerminateThread. You can access such API e.g. through pythonwin, or through ctypes.

Notice that this is inherently unsafe. It will likely lead to uncollectable garbage (from local variables of the stack frames that become garbage), and may lead to deadlocks, if the thread being killed has the GIL at the point when it is killed.

A multiprocessing.Process can p.terminate()

In the cases where I want to kill a thread, but do not want to use flags/locks/signals/semaphores/events/whatever, I promote the threads to full blown processes. For code that makes use of just a few threads the overhead is not that bad.

E.g. this comes in handy to easily terminate helper “threads” which execute blocking I/O

The conversion is trivial: In related code replace all threading.Thread with multiprocessing.Process and all queue.Queue with multiprocessing.Queue and add the required calls of p.terminate() to your parent process which wants to kill its child p

See the Python documentation for multiprocessing.

If you are trying to terminate the whole program you can set the thread as a “daemon”. see Thread.daemon

As others have mentioned, the norm is to set a stop flag. For something lightweight (no subclassing of Thread, no global variable), a lambda callback is an option. (Note the parentheses in if stop().)

import threading
import time

def do_work(id, stop):
    print("I am thread", id)
    while True:
        print("I am thread {} doing something".format(id))
        if stop():
            print("  Exiting loop.")
            break
    print("Thread {}, signing off".format(id))


def main():
    stop_threads = False
    workers = []
    for id in range(0,3):
        tmp = threading.Thread(target=do_work, args=(id, lambda: stop_threads))
        workers.append(tmp)
        tmp.start()
    time.sleep(3)
    print('main: done sleeping; time to stop the threads.')
    stop_threads = True
    for worker in workers:
        worker.join()
    print('Finis.')

if __name__ == '__main__':
    main()

Replacing print() with a pr() function that always flushes (sys.stdout.flush()) may improve the precision of the shell output.

(Only tested on Windows/Eclipse/Python3.3)

This is based on thread2 — killable threads (Python recipe)

You need to call PyThreadState_SetasyncExc(), which is only available through ctypes.

This has only been tested on Python 2.7.3, but it is likely to work with other recent 2.x releases.

import ctypes

def terminate_thread(thread):
    """Terminates a python thread from another thread.

    :param thread: a threading.Thread instance
    """
    if not thread.isAlive():
        return

    exc = ctypes.py_object(SystemExit)
    res = ctypes.pythonapi.PyThreadState_SetAsyncExc(
        ctypes.c_long(thread.ident), exc)
    if res == 0:
        raise ValueError("nonexistent thread id")
    elif res > 1:
        # """if it returns a number greater than one, you're in trouble,
        # and you should call it again with exc=NULL to revert the effect"""
        ctypes.pythonapi.PyThreadState_SetAsyncExc(thread.ident, None)
        raise SystemError("PyThreadState_SetAsyncExc failed")

You should never forcibly kill a thread without cooperating with it.

Killing a thread removes any guarantees that try/finally blocks set up so you might leave locks locked, files open, etc.

The only time you can argue that forcibly killing threads is a good idea is to kill a program fast, but never single threads.

In Python, you simply cannot kill a Thread directly.

If you do NOT really need to have a Thread (!), what you can do, instead of using the threading package , is to use the multiprocessing package . Here, to kill a process, you can simply call the method:

yourProcess.terminate()  # kill the process!

Python will kill your process (on Unix through the SIGTERM signal, while on Windows through the TerminateProcess() call). Pay attention to use it while using a Queue or a Pipe! (it may corrupt the data in the Queue/Pipe)

Note that the multiprocessing.Event and the multiprocessing.Semaphore work exactly in the same way of the threading.Event and the threading.Semaphore respectively. In fact, the first ones are clones of the latters.

If you REALLY need to use a Thread, there is no way to kill it directly. What you can do, however, is to use a “daemon thread”. In fact, in Python, a Thread can be flagged as daemon:

yourThread.daemon = True  # set the Thread as a "daemon thread"

The main program will exit when no alive non-daemon threads are left. In other words, when your main thread (which is, of course, a non-daemon thread) will finish its operations, the program will exit even if there are still some daemon threads working.

Note that it is necessary to set a Thread as daemon before the start() method is called!

Of course you can, and should, use daemon even with multiprocessing. Here, when the main process exits, it attempts to terminate all of its daemonic child processes.

Finally, please, note that sys.exit() and os.kill() are not choices.

You can kill a thread by installing trace into the thread that will exit the thread. See attached link for one possible implementation.

Kill a thread in Python

If you are explicitly calling time.sleep() as part of your thread (say polling some external service), an improvement upon Phillipe’s method is to use the timeout in the event‘s wait() method wherever you sleep()

For example:

import threading

class KillableThread(threading.Thread):
    def __init__(self, sleep_interval=1):
        super().__init__()
        self._kill = threading.Event()
        self._interval = sleep_interval

    def run(self):
        while True:
            print("Do Something")

            # If no kill signal is set, sleep for the interval,
            # If kill signal comes in while sleeping, immediately
            #  wake up and handle
            is_killed = self._kill.wait(self._interval)
            if is_killed:
                break

        print("Killing Thread")

    def kill(self):
        self._kill.set()

Then to run it

t = KillableThread(sleep_interval=5)
t.start()
# Every 5 seconds it prints:
#: Do Something
t.kill()
#: Killing Thread

The advantage of using wait() instead of sleep()ing and regularly checking the event is that you can program in longer intervals of sleep, the thread is stopped almost immediately (when you would otherwise be sleep()ing) and in my opinion, the code for handling exit is significantly simpler.

It is better if you don’t kill a thread. A way could be to introduce a “try” block into the thread’s cycle and to throw an exception when you want to stop the thread (for example a break/return/… that stops your for/while/…). I’ve used this on my app and it works…

It is definitely possible to implement a Thread.stop method as shown in the following example code:

import sys
import threading
import time


class StopThread(StopIteration):
    pass

threading.SystemExit = SystemExit, StopThread


class Thread2(threading.Thread):

    def stop(self):
        self.__stop = True

    def _bootstrap(self):
        if threading._trace_hook is not None:
            raise ValueError('Cannot run thread with tracing!')
        self.__stop = False
        sys.settrace(self.__trace)
        super()._bootstrap()

    def __trace(self, frame, event, arg):
        if self.__stop:
            raise StopThread()
        return self.__trace


class Thread3(threading.Thread):

    def _bootstrap(self, stop_thread=False):
        def stop():
            nonlocal stop_thread
            stop_thread = True
        self.stop = stop

        def tracer(*_):
            if stop_thread:
                raise StopThread()
            return tracer
        sys.settrace(tracer)
        super()._bootstrap()

###############################################################################


def main():
    test1 = Thread2(target=printer)
    test1.start()
    time.sleep(1)
    test1.stop()
    test1.join()
    test2 = Thread2(target=speed_test)
    test2.start()
    time.sleep(1)
    test2.stop()
    test2.join()
    test3 = Thread3(target=speed_test)
    test3.start()
    time.sleep(1)
    test3.stop()
    test3.join()


def printer():
    while True:
        print(time.time() % 1)
        time.sleep(0.1)


def speed_test(count=0):
    try:
        while True:
            count += 1
    except StopThread:
        print('Count =', count)

if __name__ == '__main__':
    main()

The Thread3 class appears to run code approximately 33% faster than the Thread2 class.

from ctypes import *
pthread = cdll.LoadLibrary("libpthread-2.15.so")
pthread.pthread_cancel(c_ulong(t.ident))

t is your Thread object.

Read the python source (Modules/threadmodule.c and Python/thread_pthread.h) you can see the Thread.ident is an pthread_t type, so you can do anything pthread can do in python use libpthread.

Following workaround can be used to kill a thread:

kill_threads = False

def doSomething():
    global kill_threads
    while True:
        if kill_threads:
            thread.exit()
        ......
        ......

thread.start_new_thread(doSomething, ())

This can be used even for terminating threads, whose code is written in another module, from main thread. We can declare a global variable in that module and use it to terminate thread/s spawned in that module.

I usually use this to terminate all the threads at the program exit. This might not be the perfect way to terminate thread/s but could help.

I’m way late to this game, but I’ve been wrestling with a similar question and the following appears to both resolve the issue perfectly for me AND lets me do some basic thread state checking and cleanup when the daemonized sub-thread exits:

import threading
import time
import atexit

def do_work():

  i = 0
  @atexit.register
  def goodbye():
    print ("'CLEANLY' kill sub-thread with value: %s [THREAD: %s]" %
           (i, threading.currentThread().ident))

  while True:
    print i
    i += 1
    time.sleep(1)

t = threading.Thread(target=do_work)
t.daemon = True
t.start()

def after_timeout():
  print "KILL MAIN THREAD: %s" % threading.currentThread().ident
  raise SystemExit

threading.Timer(2, after_timeout).start()

Yields:

0
1
KILL MAIN THREAD: 140013208254208
'CLEANLY' kill sub-thread with value: 2 [THREAD: 140013674317568]

One thing I want to add is that if you read official documentation in threading lib Python, it’s recommended to avoid use of “demonic” threads, when you don’t want threads end abruptly, with the flag that Paolo Rovelli mentioned.

From official documentation:

Daemon threads are abruptly stopped at shutdown. Their resources (such as open files, database transactions, etc.) may not be released properly. If you want your threads to stop gracefully, make them non-daemonic and use a suitable signaling mechanism such as an Event.

I think that creating daemonic threads depends of your application, but in general (and in my opinion) it’s better to avoid killing them or making them daemonic. In multiprocessing you can use is_alive() to check process status and “terminate” for finish them (Also you avoid GIL problems). But you can find more problems, sometimes, when you execute your code in Windows.

And always remember that if you have “live threads”, the Python interpreter will be running for wait them. (Because of this daemonic can help you if don’t matter abruptly ends).

There is a library built for this purpose, stopit. Although some of the same cautions listed herein still apply, at least this library presents a regular, repeatable technique for achieving the stated goal.

While it’s rather old, this might be a handy solution for some:

A little module that extends the threading’s module functionality — allows one thread to raise exceptions in the context of another thread. By raising SystemExit, you can finally kill python threads.

import threading
import ctypes     

def _async_raise(tid, excobj):
    res = ctypes.pythonapi.PyThreadState_SetAsyncExc(tid, ctypes.py_object(excobj))
    if res == 0:
        raise ValueError("nonexistent thread id")
    elif res > 1:
        # """if it returns a number greater than one, you're in trouble, 
        # and you should call it again with exc=NULL to revert the effect"""
        ctypes.pythonapi.PyThreadState_SetAsyncExc(tid, 0)
        raise SystemError("PyThreadState_SetAsyncExc failed")

class Thread(threading.Thread):
    def raise_exc(self, excobj):
        assert self.isAlive(), "thread must be started"
        for tid, tobj in threading._active.items():
            if tobj is self:
                _async_raise(tid, excobj)
                return

        # the thread was alive when we entered the loop, but was not found 
        # in the dict, hence it must have been already terminated. should we raise
        # an exception here? silently ignore?

    def terminate(self):
        # must raise the SystemExit type, instead of a SystemExit() instance
        # due to a bug in PyThreadState_SetAsyncExc
        self.raise_exc(SystemExit)

So, it allows a “thread to raise exceptions in the context of another thread” and in this way, the terminated thread can handle the termination without regularly checking an abort flag.

However, according to its original source, there are some issues with this code.

• The exception will be raised only when executing python bytecode. If your thread calls a native/built-in blocking function, the exception will be raised only when execution returns to the python code.
  • There is also an issue if the built-in function internally calls PyErr_Clear(), which would effectively cancel your pending exception. You can try to raise it again.
• Only exception types can be raised safely. Exception instances are likely to cause unexpected behavior, and are thus restricted.
  • For example: t1.raise_exc(TypeError) and not t1.raise_exc(TypeError(“blah”)).
  • IMHO it’s a bug, and I reported it as one. For more info, http://mail.python.org/pipermail/python-dev/2006-August/068158.html
• I asked to expose this function in the built-in thread module, but since ctypes has become a standard library (as of 2.5), and this
  feature is not likely to be implementation-agnostic, it may be kept
  unexposed.

This seems to work with pywin32 on windows 7

my_thread = threading.Thread()
my_thread.start()
my_thread._Thread__stop()

Pieter Hintjens — one of the founders of the ØMQ-project — says, using ØMQ and avoiding synchronization primitives like locks, mutexes, events etc., is the sanest and securest way to write multi-threaded programs:

http://zguide.zeromq.org/py:all#Multithreading-with-ZeroMQ

This includes telling a child thread, that it should cancel its work. This would be done by equipping the thread with a ØMQ-socket and polling on that socket for a message saying that it should cancel.

The link also provides an example on multi-threaded python code with ØMQ.

Asuming, that you want to have multiple threads of the same function, this is IMHO the easiest implementation to stop one by id:

import time
from threading import Thread

def doit(id=0):
    doit.stop=0
    print("start id:%d"%id)
    while 1:
        time.sleep(1)
        print(".")
        if doit.stop==id:
            doit.stop=0
            break
    print("end thread %d"%id)

t5=Thread(target=doit, args=(5,))
t6=Thread(target=doit, args=(6,))

t5.start() ; t6.start()
time.sleep(2)
doit.stop =5  #kill t5
time.sleep(2)
doit.stop =6  #kill t6

The nice thing is here, you can have multiple of same and different functions, and stop them all by functionname.stop

If you want to have only one thread of the function then you don’t need to remember the id. Just stop, if doit.stop > 0.

Just to build up on @SCB’s idea (which was exactly what I needed) to create a KillableThread subclass with a customized function:

from threading import Thread, Event

class KillableThread(Thread):
    def __init__(self, sleep_interval=1, target=None, name=None, args=(), kwargs={}):
        super().__init__(None, target, name, args, kwargs)
        self._kill = Event()
        self._interval = sleep_interval
        print(self._target)

    def run(self):
        while True:
            # Call custom function with arguments
            self._target(*self._args)

        # If no kill signal is set, sleep for the interval,
        # If kill signal comes in while sleeping, immediately
        #  wake up and handle
        is_killed = self._kill.wait(self._interval)
        if is_killed:
            break

    print("Killing Thread")

def kill(self):
    self._kill.set()

if __name__ == '__main__':

    def print_msg(msg):
        print(msg)

    t = KillableThread(10, print_msg, args=("hello world"))
    t.start()
    time.sleep(6)
    print("About to kill thread")
    t.kill()

Naturally, like with @SBC, the thread doesn’t wait to run a new loop to stop. In this example, you would see the “Killing Thread” message printed right after the “About to kill thread” instead of waiting for 4 more seconds for the thread to complete (since we have slept for 6 seconds already).

Second argument in KillableThread constructor is your custom function (print_msg here). Args argument are the arguments that will be used when calling the function ((“hello world”)) here.

As mentioned in @Kozyarchuk’s answer, installing trace works. Since this answer contained no code, here is a working ready-to-use example:

import sys, threading, time 

class TraceThread(threading.Thread): 
    def __init__(self, *args, **keywords): 
        threading.Thread.__init__(self, *args, **keywords) 
        self.killed = False
    def start(self): 
        self._run = self.run 
        self.run = self.settrace_and_run
        threading.Thread.start(self) 
    def settrace_and_run(self): 
        sys.settrace(self.globaltrace) 
        self._run()
    def globaltrace(self, frame, event, arg): 
        return self.localtrace if event == 'call' else None
    def localtrace(self, frame, event, arg): 
        if self.killed and event == 'line': 
            raise SystemExit() 
        return self.localtrace 

def f(): 
    while True: 
        print('1') 
        time.sleep(2)
        print('2') 
        time.sleep(2)
        print('3') 
        time.sleep(2)

t = TraceThread(target=f) 
t.start() 
time.sleep(2.5) 
t.killed = True

It stops after having printed 1 and 2. 3 is not printed.

You can execute your command in a process and then kill it using the process id. I needed to sync between two thread one of which doesn’t return by itself.

processIds = []

def executeRecord(command):
    print(command)

    process = subprocess.Popen(command, stdout=subprocess.PIPE)
    processIds.append(process.pid)
    print(processIds[0])

    #Command that doesn't return by itself
    process.stdout.read().decode("utf-8")
    return;


def recordThread(command, timeOut):

    thread = Thread(target=executeRecord, args=(command,))
    thread.start()
    thread.join(timeOut)

    os.kill(processIds.pop(), signal.SIGINT)

    return;

Start the sub thread with setDaemon(True).

def bootstrap(_filename):
    mb = ModelBootstrap(filename=_filename) # Has many Daemon threads. All get stopped automatically when main thread is stopped.

t = threading.Thread(target=bootstrap,args=('models.conf',))
t.setDaemon(False)

while True:
    t.start()
    time.sleep(10) # I am just allowing the sub-thread to run for 10 sec. You can listen on an event to stop execution.
    print('Thread stopped')
    break

This is a bad answer, see the comments

Here’s how to do it:

from threading import *

...

for thread in enumerate():
    if thread.isAlive():
        try:
            thread._Thread__stop()
        except:
            print(str(thread.getName()) + ' could not be terminated'))

Give it a few seconds then your thread should be stopped. Check also the thread._Thread__delete() method.

I’d recommend a thread.quit() method for convenience. For example if you have a socket in your thread, I’d recommend creating a quit() method in your socket-handle class, terminate the socket, then run a thread._Thread__stop() inside of your quit().

If you really need the ability to kill a sub-task, use an alternate implementation. multiprocessing and gevent both support indiscriminately killing a “thread”.

Python’s threading does not support cancellation. Do not even try. Your code is very likely to deadlock, corrupt or leak memory, or have other unintended “interesting” hard-to-debug effects which happen rarely and nondeterministically.