python基础===多线程

https://www.cnblogs.com/wj-1314/p/8263328.html

threading 模块

先上代码：

 import time, threading

 def loop():
     print("thread %s is running..." %threading.current_thread().name)
     n = 0
     while n<5:
         n += 1
         print('thread %s >>> %s'%(threading.current_thread().name, n))
         time.sleep(1)
     print("thread %s ended." %threading.current_thread().name)

 def loop1():
     x = 1
     while x <3:
         print(x,"loop1的线程")
         x += 1
         time.sleep(2)

 if __name__ == "__main__":
     print("thread %s is running ..." %threading.current_thread().name)
     t = threading.Thread(target = loop, name = "LoopThread")
     t1 = threading.Thread(target = loop1)
     t.start()
     t1.start()
     t.join()
     t1.join()
     print("thread %s ended." % threading.current_thread().name)

 """
 thread MainThread is running ...
 thread LoopThread is running...1
 thread LoopThread >>> 1
 loop1的线程
 thread LoopThread >>> 2
 2thread LoopThread >>> 3
 loop1的线程
 thread LoopThread >>> 4
 thread LoopThread >>> 5
 thread LoopThread ended.
 thread MainThread ended.

 """

由于任何进程默认就会启动一个线程，我们把该线程称为主线程，主线程又可以启动新的线程，Python的threading模块有个threading.current_thread() 获取当前线程的实例。

但是有些情况下，多线程会产生场景上的错误，比如

import time, threading

# 假定这是你的银行存款:
balance = 0

def change_it(n):
    # 先存后取，结果应该为0:
    global balance
    balance = balance + n
    balance = balance - n

def run_thread(n):
    for i in range(100000):
        change_it(n)

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)

结果有时会报错

如果我们要确保balance计算正确，就要给change_it()上一把锁，当某个线程开始执行change_it()时，我们说，该线程因为获得了锁，因此其他线程不能同时执行change_it()，只能等待，直到锁被释放后，获得该锁以后才能改。由于锁只有一个，无论多少线程，同一时刻最多只有一个线程持有该锁，所以，不会造成修改的冲突。创建一个锁就是通过threading.Lock()来实现：

import time, threading
lock = threading.Lock()

# 假定这是你的银行存款:
balance = 0

def change_it(n):
    # 先存后取，结果应该为0:
    global balance
    balance = balance + n
    balance = balance - n

def run_thread(n):
    for i in range(100000):

        lock.acquire()
        try:
            change_it(n)
        finally:
            lock.release()

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)