037多线程同步

##多线程资源共享使用问题

import threading
import time

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(num)

上面程序运行，num不会减到0。cpu执行了一个线程的tmp = num后，就切换到其他线程，有很多执行了tmp = num，所以所有线程的tmp大都是等于100的，执行num = tmp - 1的时候，num就可能等于99或者98之类的。

解决办法，加锁
步骤：1、新建锁
2、调用acquire()方法锁上，中间是执行代码
3、调用release()方法释放锁

 1 import threading
 2 import time
 3 
 4 def addNum():
 5     global num
 6     # num -= 1
 7     r.acquire()          # 锁上
 8     tmp = num
 9     time.sleep(0.00001)
10     num = tmp-1
11     r.release()       # 释放锁
12 
13 r = threading.Lock()         # 拿到锁对象
14 num = 100
15 thread_list = []
16 for i in range(100):
17     t = threading.Thread(target=addNum)
18     t.start()
19     thread_list.append(t)
20 
21 for t in thread_list:
22     t.join()
23 
24 print(num)

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两个锁以上可能导致死锁，这里就不写例子了。
重用锁：threading.Rlock()，在对象里面的方法需要，比如下面这个取钱的例子

#01同步例子

 1 import threading
 2 import time
 3 class Tickets:
 4     def __init__(self):
 5     self.ticket = 100
 6 
 7     def sell_ticket(self):
 8         while self.ticket > 0:
 9             loc.acquire()
10             print('卖出',self.ticket,'票',threading.current_thread())
11             self.ticket -= 1
12             loc.release()
13             time.sleep(0.0001)
14 
15 if __name__ == '__main__':
16     ticket = Tickets()
17     t1 = threading.Thread(target=ticket.sell_ticket)
18     t2 = threading.Thread(target=ticket.sell_ticket)
19     t3 = threading.Thread(target=ticket.sell_ticket)
20     loc = threading.Lock()
21     t1.start()
22     t2.start()
23     t3.start()

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