是否有可能终止一个正在运行的线程而不设置/检查任何标志/信号/等等?
当前回答
在Python和任何语言中,突然终止线程通常都是一种糟糕的模式。考虑以下情况:
线程持有一个必须正确关闭的关键资源 线程创建了其他几个线程,这些线程也必须被杀死。
如果你能负担得起(如果你在管理自己的线程),处理这个问题的好方法是有一个exit_request标志,每个线程都会定期检查它,看看它是否到了退出的时候。
例如:
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()
在这段代码中,当您希望线程退出时,应该在线程上调用stop(),并使用join()等待线程正确退出。线程应该定期检查停止标志。
然而,在某些情况下,您确实需要终止一个线程。例如,当您正在包装一个外部库时,该库正忙于长时间的调用,并且您希望中断它。
下面的代码允许(有一些限制)在Python线程中引发异常:
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 an 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 exception in the context of the
thread represented by this instance.
"""
_async_raise( self._get_my_tid(), exctype )
(根据Tomer Filiba的《Killable Threads》改编。关于PyThreadState_SetAsyncExc返回值的引用似乎来自旧版本的Python。)
正如文档中所指出的,这不是一个神奇的子弹,因为如果线程在Python解释器之外忙,它将不会捕获中断。
这段代码的一个良好使用模式是让线程捕获特定的异常并执行清理。这样,您可以中断任务,但仍然可以进行适当的清理。
其他回答
可以通过在将退出线程的线程中安装trace来终止线程。请参阅所附的链接,了解一种可能的实现。
在Python中杀死一个线程
正如其他人所提到的,规范是设置一个停止标志。对于一些轻量级的东西(没有Thread的子类化,没有全局变量),lambda回调是一个选项。(注意if stop()中的括号。)
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()
将print()替换为始终刷新的pr()函数(sys.stdout.flush())可以提高shell输出的精度。
(仅在Windows/Eclipse/Python3.3上测试)
这似乎与windows 7上的pywin32一起工作
my_thread = threading.Thread()
my_thread.start()
my_thread._Thread__stop()
如果您试图终止整个程序,您可以将线程设置为“守护进程”。看到 Thread.daemon
实现一个线程是绝对可能的。方法,如下例代码所示:
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()
Thread3类运行代码的速度似乎比Thread2类快大约33%。
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