Python - threading.Timer stays alive after calling cancel() method
multithreading, python
Solution
A `Timer` is a subclass of a `Thread` and its implementation is really simple. It waits the provided time by subscribing to the event `finished`.
So when you set the event by `Timer.cancel` it is guaranteed that the function does not get called. But it is not guaranteed that the Timer thread will directly continue (and exit).
So the point is that the thread of the timer can still be alive after the execution of `cancel`, but the function will not get executed. So checking for `finished` is safe, while testing for `Thread.is_alive` (newer API, use this!) is a race condition in this case.
Hint: You can verify this by placing a `time.sleep` after calling `cancel`. Then it will just print:
<_MainThread(MainThread, started 10872)>
Timer is finished
Problem
I noticed the following behavior in the following code (using threading.Timer class): ``` import threading def ontimer(): print threading.current_thread() def main(): timer = threading.Timer(2, ontimer) timer.start() print threading.current_thread() timer.cancel() if timer.isAlive(): print "Timer is still alive" if timer.finished: print "Timer is finished" if __name__ == "__main__": main() ``` The output of the code is: ``` <_MainThread(MainThread, started 5836)> Timer is still alive Timer is finished ``` As We notice from the output, that the timer object is still alive and finished in the same time. In fact, I would like to call a similar function hundreds of times, and I wonder whether those "living" timers may affect the performance. I would like to stop or cancel the timer object in a proper way. Am I doing it right? Thank you