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Alex Leblois
Alex Leblois

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Posted on    October-09-2014 11:45 PM

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


Kamlakar Singh
Kamlakar Singh

Total Post:194

Points:1396
Posted on    October-09-2014 11:48 PM

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 him 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):

            super(StoppableThread, self).__init__()

            self._stop = threading.Event()

 

        def stop(self):

            self._stop.set()

 

        def stopped(self):

            return self._stop.isSet()

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(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(tid, 0)

        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 )

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.


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