How to Gracefully and Forcibly Kill a Thread in Python?

Killing a Thread in Python: A Guide

In Python, thread termination is generally discouraged due to potential resource issues. However, there are instances when forceful termination is necessary.

Standard Approach: Exit Flags and Event Objects

The best practice involves setting an exit flag that threads periodically check, enabling a graceful exit. For instance, using a StoppableThread class with an exit_request flag allows threads to check for termination and exit accordingly.

class StoppableThread(threading.Thread):
    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()

Forced Termination: Breaking the Execution Flow

In extreme cases, direct thread termination is necessary. However, this approach carries risks if the thread holds critical resources.

One method involves raising an exception in the thread using the _async_raise function. This raises an exception asynchronously, allowing the thread to catch and handle it gracefully.

def _async_raise(tid, exctype):
    res = ctypes.pythonapi.PyThreadState_SetAsyncExc(ctypes.c_long(tid),
                                                     ctypes.py_object(exctype))
    ...

class ThreadWithExc(threading.Thread):
    def raise_exc(self, exctype):
        _async_raise(self._get_my_tid(), exctype )

Cautions and Considerations

Killing threads abruptly can result in resource leaks or incomplete task execution. Thus, it's essential to employ the forced termination approach judiciously and always favor graceful exits. Additionally, the _async_raise method may not always be effective if the thread is performing intensive operations.

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