同步锁
import time, threadingdef addNum(): global num num -= 1num = 100thread_list = []for i in range(100): t = threading.Thread(target=addNum) t.start() thread_list.append(t)for t in thread_list: t.join()print('final num:', num)运行结果:final num: 0 import time, threadingdef addNum(): global num #num -= 1 tmp = num time.sleep(0.00001) num = tmp - 1num = 100thread_list = []for i in range(100): t = threading.Thread(target=addNum) t.start() thread_list.append(t)for t in thread_list: t.join()print('final num:', num)运行结果:final num: 93 或final num: 91或final num: 94 原因：
第一个程序中，num -= 1 这种写法，程序执行动作太快(完成这个动作在 cup 切换的时间内) 第二个程序中，把 num -= 1 , 加入了 sleep 时间，100个线程存在没有执行完就进行了切换，导致全局的 num 没有正常返回。引用下大神的图发现总结得非常好：
在上面的例子中 使用 join 方法会把整个线程停住，造成了串行，失去了多线程的意义，我们只需要在涉及到计算公共数据的时候串行执行即可。 使用同步锁处理计算公共的数据
import time, threadingdef addNum(): global num lock.acquire() tmp = num time.sleep(0.00001) num = tmp - 1 lock.release()num = 100lock = threading.Lock()thread_list = []for i in range(100): t = threading.Thread(target=addNum) t.start() thread_list.append(t)for t in thread_list: t.join()print('final num:', num)运算结果:final num: 0 线程死锁和递归锁
import threading, timeclass myThread(threading.Thread): def doA(self): lockA.acquire() print(self.name, "gotlockA", time.ctime()) time.sleep(3) lockB.acquire() print(self.name, "gotlockB", time.ctime()) lockB.release() lockA.release() def doB(self): lockB.acquire() print(self.name, "gotlockB", time.ctime()) time.sleep(2) lockA.acquire() print(self.name, "gotlockA", time.ctime()) lockA.release() lockB.release() def run(self): self.doA() self.doB()if __name__ == '__main__': lockA = threading.Lock() lockB = threading.Lock() threads = [] for i in range(5): threads.append(myThread()) for t in threads: t.start() for t in threads: t.join()#运行结果:Thread-1 gotlockA Sat Jul 28 15:09:31 2018Thread-1 gotlockB Sat Jul 28 15:09:34 2018Thread-1 gotlockB Sat Jul 28 15:09:34 2018Thread-2 gotlockA Sat Jul 28 15:09:34 2018
使用递归锁
import threading, timeclass myThread(threading.Thread): def doA(self): lock.acquire() print(self.name, "gotlockA", time.ctime()) time.sleep(3) lock.acquire() print(self.name, "gotlockB", time.ctime()) lock.release() lock.release() def doB(self): lock.acquire() print(self.name, "gotlockB", time.ctime()) time.sleep(2) lock.acquire() print(self.name, "gotlockA", time.ctime()) lock.release() lock.release() def run(self): self.doA() self.doB()if __name__ == '__main__': lock = threading.RLock() threads = [] for i in range(5): threads.append(myThread()) for t in threads: t.start() for t in threads: t.join()运行结果:Thread-1 gotlockA Sat Jul 28 15:19:35 2018Thread-1 gotlockB Sat Jul 28 15:19:38 2018Thread-1 gotlockB Sat Jul 28 15:19:38 2018Thread-1 gotlockA Sat Jul 28 15:19:40 2018Thread-3 gotlockA Sat Jul 28 15:19:40 2018Thread-3 gotlockB Sat Jul 28 15:19:43 2018Thread-3 gotlockB Sat Jul 28 15:19:43 2018Thread-3 gotlockA Sat Jul 28 15:19:45 2018Thread-5 gotlockA Sat Jul 28 15:19:45 2018Thread-5 gotlockB Sat Jul 28 15:19:48 2018Thread-5 gotlockB Sat Jul 28 15:19:48 2018Thread-5 gotlockA Sat Jul 28 15:19:50 2018Thread-4 gotlockA Sat Jul 28 15:19:50 2018Thread-4 gotlockB Sat Jul 28 15:19:53 2018Thread-4 gotlockB Sat Jul 28 15:19:53 2018Thread-4 gotlockA Sat Jul 28 15:19:55 2018Thread-2 gotlockA Sat Jul 28 15:19:55 2018Thread-2 gotlockB Sat Jul 28 15:19:58 2018Thread-2 gotlockB Sat Jul 28 15:19:58 2018Thread-2 gotlockA Sat Jul 28 15:20:00 2018
信号量
信号量用来控制线程并发数的，BoundedSemaphore或Semaphore管理一个内置的计数 器，每当调用acquire()时-1，调用release()时+1,计数器不能小于0，当计数器为 0时，acquire()将阻塞线程至同步锁定状态，直到其他线程调用release()。(类似于停车位的概念)。
BoundedSemaphore与Semaphore的唯一区别在于前者将在调用release()时检查计数 器的值是否超过了计数器的初始值，如果超过了将抛出一个异常。import threading, timeclass myThread(threading.Thread): def run(self): if semaphore.acquire(): print(self.name) time.sleep(5) semaphore.release()if __name__ == "__main__": semaphore = threading.Semaphore(5) thrs = [] for i in range(20): thrs.append(myThread()) for t in thrs: t.start()#运行结果:Thread-1Thread-2Thread-3Thread-4Thread-5Thread-6Thread-7Thread-9Thread-10Thread-8Thread-11Thread-13Thread-14Thread-12Thread-15Thread-18Thread-16Thread-17Thread-19Thread-20
import threading, timeclass myThread(threading.Thread): def run(self): if semaphore.acquire(): print(self.name) time.sleep(5) semaphore.release()if __name__ == "__main__": semaphore = threading.BoundedSemaphore(5) thrs = [] for i in range(20): thrs.append(myThread()) for t in thrs: t.start()#运行结果:Thread-1Thread-2Thread-3Thread-4Thread-5Thread-6Thread-8Thread-10Thread-9Thread-7Thread-12Thread-14Thread-15Thread-13Thread-11Thread-16Thread-17Thread-20Thread-19Thread-18