解決python ThreadPoolExecutor 線程池中的異常捕獲問題
問題
最近寫了涉及線程池及線程的 python 腳本，運(yùn)行過程中發(fā)現(xiàn)一個(gè)有趣的現(xiàn)象，線程池中的工作線程出現(xiàn)問題，引發(fā)了異常，但是主線程沒有捕獲異常，還在發(fā)現(xiàn) BUG 之前一度以為線程池代碼正常返回。
先說重點(diǎn)
這里主要想介紹 python concurrent.futuresthread.ThreadPoolExecutor 線程池中的 worker 引發(fā)異常的時(shí)候，并不會(huì)直接向上拋起異常，而是需要主線程通過調(diào)用concurrent.futures.Future.exception(timeout=None) 方法主動(dòng)獲取 worker 的異常。
問題重現(xiàn)及解決
引子
問題主要由這樣一段代碼引起的：
def thread_executor():
logger.info("I am slave. I am working. I am going to sleep 3s")
sleep(3)
logger.info("Exit thread executor")
def main():
thread_obj = threading.Thread(target=thread_executor)
while True:
logger.info("Master starts thread worker")
try:
# 工作線程由于某種異常而結(jié)束并退出了，想重啟工作線程的工作，但又不想重復(fù)創(chuàng)建線程
thread_obj.start() # 這一行會(huì)報(bào)錯(cuò)，同一線程不能重復(fù)啟動(dòng)
except Exception as e:
logger.error("Master start thread error", exc_info=True)
raise e
logger.info("Master is going to sleep 5s")
sleep(5)
上面這段代碼的功能如注釋中解釋的，主要要實(shí)現(xiàn)類似生產(chǎn)者消費(fèi)者的功能，工作線程一直去生產(chǎn)資源，主線程去消費(fèi)工作線程生產(chǎn)的資源。但是工作線程由于異常推出了，想重新啟動(dòng)生產(chǎn)工作。顯然，這個(gè)代碼會(huì)報(bào)錯(cuò)。
運(yùn)行結(jié)果:
thread: MainThread [INFO] Master starts thread worker
thread: Thread-1 [INFO] I am slave. I am working. I am going to sleep 3s
thread: MainThread [INFO] Master is going to sleep 5s
thread: Thread-1 [INFO] Exit thread executor because of some exception
thread: MainThread [INFO] Master starts thread worker
thread: MainThread [ERROR] Master start thread error
Traceback (most recent call last):
File "xxx.py", line 47, in main
thread_obj.start()
File "E:\anaconda\lib\threading.py", line 843, in start
raise RuntimeError("threads can only be started once")
RuntimeError: threads can only be started once
Traceback (most recent call last):
File "xxx.py", line 56, in <module>
main()
File "xxx.py", line 50, in main
raise e
File "xxx.py", line 47, in main
thread_obj.start()
File "E:\anaconda\lib\threading.py", line 843, in start
raise RuntimeError("threads can only be started once")
RuntimeError: threads can only be started once
切入正題
然而腳本還有其他業(yè)務(wù)代碼要運(yùn)行，所以需要把上面的資源生產(chǎn)和消費(fèi)的代碼放到一個(gè)線程里完成，所以引入線程池來執(zhí)行這段代碼：
def thread_executor():
while True:
logger.info("I am slave. I am working. I am going to sleep 3s")
sleep(3)
logger.info("Exit thread executor because of some exception")
break
def main():
thread_obj = threading.Thread(target=thread_executor)
while True:
logger.info("Master starts thread worker")
# 工作線程由于某種異常而結(jié)束并退出了，想重啟工作線程的工作，但又不想重復(fù)創(chuàng)建線程
# 沒有想到這里會(huì)有異常
thread_obj.start() # 這一行會(huì)報(bào)錯(cuò)，同一線程不能重復(fù)啟動(dòng)
logger.info("Master is going to sleep 5s")
sleep(5)
def thread_pool_main():
thread_obj = ThreadPoolExecutor(max_workers=1, thread_name_prefix="WorkExecutor")
logger.info("Master ThreadPool Executor starts thread worker")
thread_obj.submit(main)
while True:
logger.info("Master ThreadPool Executor is going to sleep 5s")
sleep(5)
if __name__ == '__main__':
thread_pool_main()
代碼運(yùn)行結(jié)果如下:
INFO [thread: MainThread] Master ThreadPool Executor starts thread worker INFO [thread: WorkExecutor_0] Master starts thread worker INFO [thread: MainThread] Master ThreadPool Executor is going to sleep 5s INFO [thread: Thread-1] I am slave. I am working. I am going to sleep 3s INFO [thread: WorkExecutor_0] Master is going to sleep 5s INFO [thread: Thread-1] Exit thread executor because of some exception INFO [thread: MainThread] Master ThreadPool Executor is going to sleep 5s INFO [thread: WorkExecutor_0] Master starts thread worker INFO [thread: MainThread] Master ThreadPool Executor is going to sleep 5s INFO [thread: MainThread] Master ThreadPool Executor is going to sleep 5s INFO [thread: MainThread] Master ThreadPool Executor is going to sleep 5s INFO [thread: MainThread] Master ThreadPool Executor is going to sleep 5s INFO [thread: MainThread] Master ThreadPool Executor is going to sleep 5s INFO [thread: MainThread] Master ThreadPool Executor is going to sleep 5s INFO [thread: MainThread] Master ThreadPool Executor is going to sleep 5s ... ...
顯然，由上面的結(jié)果，在線程池 worker 執(zhí)行到 INFO [thread: WorkExecutor_0] Master starts thread worker 的時(shí)候，是會(huì)有異常產(chǎn)生的，但是整個(gè)代碼并沒有拋棄任何異常。
解決方法
發(fā)現(xiàn)上面的 bug 后，想在線程池 worker 出錯(cuò)的時(shí)候，把異常記錄到日志。查閱資料，要獲取線程池的異常信息，需要調(diào)用 concurrent.futures.Future.exception(timeout=None) 方法，為了記錄日志，這里加了線程池執(zhí)行結(jié)束的回調(diào)函數(shù)。同時(shí)，日志中記錄異常信息，用了 logging.exception() 方法。
def thread_executor():
while True:
logger.info("I am slave. I am working. I am going to sleep 3s")
sleep(3)
logger.info("Exit thread executor because of some exception")
break
def main():
thread_obj = threading.Thread(target=thread_executor)
while True:
logger.info("Master starts thread worker")
# 工作線程由于某種異常而結(jié)束并退出了，想重啟工作線程的工作，但又不想重復(fù)創(chuàng)建線程
# 沒有想到這里會(huì)有異常
thread_obj.start() # 這一行會(huì)報(bào)錯(cuò)，同一線程不能重復(fù)啟動(dòng)
logger.info("Master is going to sleep 5s")
sleep(5)
def thread_pool_callback(worker):
logger.info("called thread pool executor callback function")
worker_exception = worker.exception()
if worker_exception:
logger.exception("Worker return exception: {}".format(worker_exception))
def thread_pool_main():
thread_obj = ThreadPoolExecutor(max_workers=1, thread_name_prefix="WorkExecutor")
logger.info("Master ThreadPool Executor starts thread worker")
thread_pool_exc = thread_obj.submit(main)
thread_pool_exc.add_done_callback(thread_pool_callback)
# logger.info("thread pool exception: {}".format(thread_pool_exc.exception()))
while True:
logger.info("Master ThreadPool Executor is going to sleep 5s")
sleep(5)
if __name__ == '__main__':
thread_pool_main()
代碼運(yùn)行結(jié)果:
INFO [thread: MainThread] Master ThreadPool Executor starts thread worker
INFO [thread: WorkExecutor_0] Master starts thread worker
INFO [thread: MainThread] Master ThreadPool Executor is going to sleep 5s
INFO [thread: Thread-1] I am slave. I am working. I am going to sleep 3s
INFO [thread: WorkExecutor_0] Master is going to sleep 5s
INFO [thread: Thread-1] Exit thread executor because of some exception
INFO [thread: MainThread] Master ThreadPool Executor is going to sleep 5s
INFO [thread: WorkExecutor_0] Master starts thread worker
INFO [thread: WorkExecutor_0] called thread pool executor callback function
ERROR [thread: WorkExecutor_0] Worker return exception: threads can only be started once
Traceback (most recent call last):
File "E:\anaconda\lib\concurrent\futures\thread.py", line 57, in run
result = self.fn(*self.args, **self.kwargs)
File "xxxx.py", line 46, in main
thread_obj.start() # 這一行會(huì)報(bào)錯(cuò)，同一線程不能重復(fù)啟動(dòng)
File "E:\anaconda\lib\threading.py", line 843, in start
raise RuntimeError("threads can only be started once")
RuntimeError: threads can only be started once
INFO [thread: MainThread] Master ThreadPool Executor is going to sleep 5s
INFO [thread: MainThread] Master ThreadPool Executor is going to sleep 5s
INFO [thread: MainThread] Master ThreadPool Executor is going to sleep 5s
... ...
最終的寫法
其實(shí)，上面寫法中，想重復(fù)利用一個(gè)線程去實(shí)現(xiàn)生產(chǎn)者線程的實(shí)現(xiàn)方法是有問題的，在此處，一般情況下，線程執(zhí)行結(jié)束后，線程資源會(huì)被會(huì)被操作系統(tǒng)，所以線程不能被重復(fù)調(diào)用 start() 。
一種可行的實(shí)現(xiàn)方式就是，用線程池替代。當(dāng)然，這樣做得注意上面提到的線程池執(zhí)行體的異常捕獲問題。
def thread_executor():
while True:
logger.info("I am slave. I am working. I am going to sleep 3s")
sleep(3)
logger.info("Exit thread executor because of some exception")
break
def executor_callback(worker):
logger.info("called worker callback function")
worker_exception = worker.exception()
if worker_exception:
logger.exception("Worker return exception: {}".format(worker_exception))
# raise worker_exception
def main():
slave_thread_pool = ThreadPoolExecutor(max_workers=1, thread_name_prefix="SlaveExecutor")
restart_flag = False
while True:
logger.info("Master starts thread worker")
if not restart_flag:
restart_flag = not restart_flag
logger.info("Restart Slave work")
slave_thread_pool.submit(thread_executor).add_done_callback(executor_callback)
logger.info("Master is going to sleep 5s")
sleep(5)
總結(jié)
這個(gè)問題主要還是因?yàn)閷?duì) Python 的 concurrent.futuresthread.ThreadPoolExecutor 不夠了解導(dǎo)致的，接觸這個(gè)包是在書本上，但是書本沒完全介紹包的全部 API 及用法，所以代碼產(chǎn)生異常情況后，DEBUG 了許久在真正找到問題所在。查閱 python docs 后才對(duì)其完整用法有所認(rèn)識(shí)，所以，以后學(xué)習(xí)新的 python 包的時(shí)候還是可以查一查官方文檔的。
參考資料
英文版: docs of python concurrent.futures
中文版: python docs concurrent.futures — 啟動(dòng)并行任務(wù)
exception(timeout=None)
返回由調(diào)用引發(fā)的異常。如果調(diào)用還沒完成那么這個(gè)方法將等待 timeout 秒。如果在 timeout 秒內(nèi)沒有執(zhí)行完成，concurrent.futures.TimeoutError 將會(huì)被觸發(fā)。timeout 可以是整數(shù)或浮點(diǎn)數(shù)。如果 timeout 沒有指定或?yàn)?None，那么等待時(shí)間就沒有限制。
如果 futrue 在完成前被取消則 CancelledError 將被觸發(fā)。
如果調(diào)用正常完成那么返回 None。
add_done_callback(fn)
附加可調(diào)用 fn 到期程。當(dāng)期程被取消或完成運(yùn)行時(shí)，將會(huì)調(diào)用 fn，而這個(gè)期程將作為它唯一的參數(shù)。
加入的可調(diào)用對(duì)象總被屬于添加它們的進(jìn)程中的線程按加入的順序調(diào)用。如果可調(diào)用對(duì)象引發(fā)一個(gè) Exception 子類，它會(huì)被記錄下來并被忽略掉。如果可調(diào)用對(duì)象引發(fā)一個(gè) BaseException 子類，這個(gè)行為沒有定義。
如果期程已經(jīng)完成或已取消，fn 會(huì)被立即調(diào)用。
以上這篇解決python ThreadPoolExecutor 線程池中的異常捕獲問題就是小編分享給大家的全部?jī)?nèi)容了，希望能給大家一個(gè)參考，也希望大家多多支持腳本之家。
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