python多线程threading及锁机制

1，多线程无锁情况下

import threading
import time

balance = 0

def change_it(n):
    global balance
    balance = balance + n
    balance = balance - n

def run_thread(n):
    for i in range(10000):       
        change_it(n)

t1 = threading.Thread(target = run_thread, args = (5, ))  //创建一个线程t1
t2 = threading.Thread(target = run_thread, args = (8, ))  //创建一个线程t2
t1.start()  //开启线程
t2.start()
t1.join()    //等待线程结束
t2.join()
print(balance)
打印结果理论上是0，实际上不确定，因为线程t1和线程t2存在竞争资源的情况，导致结果的不确定性，可以多运行几次来观察结果

2，给资源加锁

import threading
import time

balance = 0
lock = threading.Lock()  //创建锁

def change_it(n):
    global balance
    balance = balance + n
    balance = balance - n

def run_thread(n):
    for i in range(10000):
        lock.acquire()   //使用锁
        change_it(n)
        lock.release()   //释放锁

t1 = threading.Thread(target = run_thread, args = (5, ))
t2 = threading.Thread(target = run_thread, args = (8, ))
t1.start()
t2.start()
t1.join()
t2.join()
print(balance)

由于多线程是不安全的，有必要给资源加锁，打印结果必定是0