【Python@Thread】鎖樣本

標籤：同步原語   __str__   append   exit   read   start   style   rand   name   

　　當出現競態條件時候，即在同一個時刻只有一個線程可以進入臨界區，需要使用同步。

常見的同步原語有兩種：鎖/互斥，訊號量。

　　鎖是最簡單，最低級的機制。

首先看一個不使用鎖時候的多線程樣本：

from atexit import registerfrom time import sleep, ctimefrom threading import currentThread, Threadfrom random import randrangeclass cleanOutput(list):    def __str__(self):        return ‘,‘.join(self)loops = [randrange(2, 5) for x in range(randrange(3, 7))]remaining = cleanOutput()def loop(nsec):    myname = currentThread().name    remaining.append(myname)    print(‘{0} starting at {1}‘.format(myname, ctime()))    sleep(nsec)    remaining.remove(myname)    print(‘{0} end at {1}‘.format(myname, ctime()))    print(‘remaining {0}‘.format(remaining))def main():    for i in loops:        Thread(target=loop, args=(i,)).start()@registerdef _atexit():    print("end!")if __name__ == ‘__main__‘:    main()

輸出結果1:

Thread-1 starting at Tue Dec 20 23:12:03 2016
Thread-2 starting at Tue Dec 20 23:12:03 2016
Thread-3 starting at Tue Dec 20 23:12:03 2016
Thread-4 starting at Tue Dec 20 23:12:03 2016
Thread-5 starting at Tue Dec 20 23:12:03 2016
Thread-6 starting at Tue Dec 20 23:12:03 2016
Thread-2 end at Tue Dec 20 23:12:05 2016
Thread-3 end at Tue Dec 20 23:12:05 2016
Thread-5 end at Tue Dec 20 23:12:05 2016
remaining Thread-1,Thread-4,Thread-6
remaining Thread-1,Thread-4,Thread-6
remaining Thread-1,Thread-4,Thread-6
Thread-1 end at Tue Dec 20 23:12:06 2016
remaining Thread-4,Thread-6
Thread-6 end at Tue Dec 20 23:12:06 2016
remaining Thread-4
Thread-4 end at Tue Dec 20 23:12:06 2016
remaining
end!

輸出結果2：

Thread-1 starting at Tue Dec 20 23:18:45 2016
Thread-2 starting at Tue Dec 20 23:18:45 2016
Thread-3 starting at Tue Dec 20 23:18:45 2016
Thread-4 starting at Tue Dec 20 23:18:45 2016
Thread-1 end at Tue Dec 20 23:18:47 2016
remaining Thread-2,Thread-3,Thread-4
Thread-3 end at Tue Dec 20 23:18:47 2016
Thread-2 end at Tue Dec 20 23:18:47 2016
remaining Thread-4
remaining Thread-4
Thread-4 end at Tue Dec 20 23:18:48 2016
remaining
end!

可以看到輸出的結果非常奇怪，當多個線程同時使用remaining列表時候，結果會出現意外。

當使用鎖時，即

 1 from atexit import register 2 from time import sleep, ctime 3 from threading import currentThread, Thread, Lock 4 from random import randrange 5  6  7 class cleanOutput(list): 8     def __str__(self): 9         return ‘,‘.join(self)10 11 loops = [randrange(2, 5) for x in range(randrange(3, 7))]12 remaining = cleanOutput()13 lock = Lock()14 15 16 def loop(nsec):17     myname = currentThread().name18     with lock:　　　　　　　　　　#也可以使用lock.acquire()和lock.release()19         remaining.append(myname)　　　　　　　　　　20         print(‘{0} starting at {1}‘.format(myname, ctime()))21     sleep(nsec)22     with lock:23         remaining.remove(myname)24         print(‘{0} end at {1}‘.format(myname, ctime()))25         print(‘remaining {0}‘.format(remaining))26 27 28 def main():29     for i in loops:30         Thread(target=loop, args=(i,)).start()31 32 33 @register34 def _atexit():35     print("end!")36 37 38 if __name__ == ‘__main__‘:39     main()

輸出結果為：

Thread-1 starting at Tue Dec 20 23:22:53 2016
Thread-2 starting at Tue Dec 20 23:22:53 2016
Thread-3 starting at Tue Dec 20 23:22:53 2016
Thread-4 starting at Tue Dec 20 23:22:53 2016
Thread-5 starting at Tue Dec 20 23:22:53 2016
Thread-6 starting at Tue Dec 20 23:22:53 2016
Thread-1 end at Tue Dec 20 23:22:55 2016
remaining Thread-2,Thread-3,Thread-4,Thread-5,Thread-6
Thread-4 end at Tue Dec 20 23:22:55 2016
remaining Thread-2,Thread-3,Thread-5,Thread-6
Thread-2 end at Tue Dec 20 23:22:55 2016
remaining Thread-3,Thread-5,Thread-6
Thread-5 end at Tue Dec 20 23:22:56 2016
remaining Thread-3,Thread-6
Thread-6 end at Tue Dec 20 23:22:57 2016
remaining Thread-3
Thread-3 end at Tue Dec 20 23:22:57 2016
remaining
end!

結果正常了

 

參考資料：Python核心編程.第四章.Wesley Chun著

【[email protected]】鎖樣本