Python 线程同步锁, 信号量

同步锁

import  time, threading

def addNum():
    global num
    num -= 1

num = 100

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)

运行结果:
final num: 0

import  time, threading

def addNum():
    global num
    #num -= 1
    tmp = num
    time.sleep(0.00001)
    num = tmp - 1

num = 100

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, threading

def addNum():
    global num

    lock.acquire()
    tmp = num
    time.sleep(0.00001)
    num = tmp - 1
    lock.release()


num = 100
lock = 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, time

class 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 2018
Thread-1 gotlockB Sat Jul 28 15:09:34 2018
Thread-1 gotlockB Sat Jul 28 15:09:34 2018
Thread-2 gotlockA Sat Jul 28 15:09:34 2018

使用递归锁

import  threading, time

class 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 2018
Thread-1 gotlockB Sat Jul 28 15:19:38 2018
Thread-1 gotlockB Sat Jul 28 15:19:38 2018
Thread-1 gotlockA Sat Jul 28 15:19:40 2018
Thread-3 gotlockA Sat Jul 28 15:19:40 2018
Thread-3 gotlockB Sat Jul 28 15:19:43 2018
Thread-3 gotlockB Sat Jul 28 15:19:43 2018
Thread-3 gotlockA Sat Jul 28 15:19:45 2018
Thread-5 gotlockA Sat Jul 28 15:19:45 2018
Thread-5 gotlockB Sat Jul 28 15:19:48 2018
Thread-5 gotlockB Sat Jul 28 15:19:48 2018
Thread-5 gotlockA Sat Jul 28 15:19:50 2018
Thread-4 gotlockA Sat Jul 28 15:19:50 2018
Thread-4 gotlockB Sat Jul 28 15:19:53 2018
Thread-4 gotlockB Sat Jul 28 15:19:53 2018
Thread-4 gotlockA Sat Jul 28 15:19:55 2018
Thread-2 gotlockA Sat Jul 28 15:19:55 2018
Thread-2 gotlockB Sat Jul 28 15:19:58 2018
Thread-2 gotlockB Sat Jul 28 15:19:58 2018
Thread-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, time

class 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-1
Thread-2
Thread-3
Thread-4
Thread-5
Thread-6
Thread-7
Thread-9
Thread-10
Thread-8
Thread-11
Thread-13
Thread-14
Thread-12
Thread-15
Thread-18
Thread-16
Thread-17
Thread-19
Thread-20

import threading, time

class 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-1
Thread-2
Thread-3
Thread-4
Thread-5
Thread-6
Thread-8
Thread-10
Thread-9
Thread-7
Thread-12
Thread-14
Thread-15
Thread-13
Thread-11
Thread-16
Thread-17
Thread-20
Thread-19
Thread-18