Python 多线程编程(二):threading 模块中 Lock 类的用法详解
在前面一篇博文《Python多线程编程(一）：threading 模块 Thread 类的用法详解 》 我有简单介绍怎么利用 threading 模块进行多线程的编码。
但那只是多线程编码最简单的部分，真正难的其实是多个线程之间的通信和数据同步。
大概可以这样讲，多线程最难的是如何正确协调各个线程修改同一份数据。
以卖票的例子作为说明。
买票有多个窗口，假设有 3 个好了，窗口之间共享一个票池，每个窗口都可以买票直到票池里面没有票可以卖。
不难写出下面的代码。
import threading
import random
class WindowThread(threading.Thread):
def __init__(self,name):
threading.Thread.__init__(self,name=name)
self.name = name
self.tickts = 0
def run(self):
global tickt_count
while tickt_count > 0:
print('%s notice:There has %d tickts remain ' %(self.name,tickt_count))
if tickt_count > 2:
number = random.randint(1,2)
else:
number = 1
tickt_count -= number
self.tickts += number
print('%s have buy %d tickt,the remain tickt\'t count is %d .Already buy %d \n'
% (self.name, number, tickt_count, self.tickts))
print('%s notice:There is no tickt can sold! Already sold %d'%(self.name,self.tickts))
tickt_count = 10
window1 = WindowThread('window1')
window2 = WindowThread('window2')
window3 = WindowThread('window3')
window1.start()
window2.start()
window3.start()
window1.join()
window2.join()
window3.join()
print('tickt count ',tickt_count)
假设有 3 个窗口，总共有 10 张票。
在每个窗口排队购票时，窗口会告诉你当前还剩多少票。
if tickt_count > 2:
number = random.randint(1,2)
else:
number = 1
tickt_count -= number
self.tickts += number
每个人最多购买 2 张票，如果票池当中只剩下 1 张票时，那么也只能购买一张票了。
tickt_count 是票池里面剩余的票的数量。
tickts 是每个窗口已经卖出的数量。
购买结束后，要更新票池里面剩余票的数量，和更新本窗口卖出去的票数。
最后，所有的窗口无票可卖的时候，结束。然后打印 票池里面的票数，理论上应该是 0，因为票已经卖光了。
我们看结果运行如何：
window1 notice:There has 10 tickts remain
window2 notice:There has 10 tickts remain
window1 have buy 2 tickt,the remain tickt't count is 8 .Already buy 2
window2 have buy 1 tickt,the remain tickt't count is 7 .Already buy 1
window1 notice:There has 7 tickts remain
window1 have buy 2 tickt,the remain tickt't count is 5 .Already buy 4
window1 notice:There has 5 tickts remain
window1 have buy 1 tickt,the remain tickt't count is 4 .Already buy 5
window1 notice:There has 4 tickts remain
window1 have buy 1 tickt,the remain tickt't count is 3 .Already buy 6
window1 notice:There has 3 tickts remain
window1 have buy 2 tickt,the remain tickt't count is 1 .Already buy 8
window3 notice:There has 1 tickts remain
window1 notice:There has 1 tickts remain
window1 have buy 1 tickt,the remain tickt't count is 0 .Already buy 9
window1 notice:There is no tickt can sold! Already sold 9
window3 have buy 1 tickt,the remain tickt't count is -1 .Already buy 1
window3 notice:There is no tickt can sold! Already sold 1
window2 notice:There is no tickt can sold! Already sold 1
tickt count -1
多运行几次，可以发现一个现象，那就是结果不正确。
3 个窗口的卖出的总数对不上。
最后显示 tickt count 为 -1，而不是 0，这显然不符合我们的预期。
Q:为什么会这样呢？
A:因为我们没有保护好多个线程之间，共享的数据。
Q:怎么能解决呢？
A:Lock 机制可以解决
什么是 Lock?
Lock 中文称为锁，是一种初级的多线程同步的手段。
Lock 有 locked 和 unlocked 两种状态,而这两中状态之间是可以转换的.
- 当 Lock 是 unlocked 状态时候，某个线程调用 acquire() 可以获取这个 Lock,并且将 Lock将状态转换成 locked 状态，并且线程不会阻塞。
- 但当 Lock 是 locked 状态时，某个线程调用 acquire() 会阻塞自己，直到其他的线程将 Lock 的状态变成 unlocked。
- 当 Lock 是 locked 状态时，调用 release() 方法，可以释放一个 Lock，这样其它线程就可以获取这个 Lock 了。
- 但当 Lock 是 unlocked 状态时，某个线程调用 release(),程序会抛出 RuntimeError 异常。
所以，acquire() 和 release() 方法在单个线程当中都是成对使用的。
有效利用 Lock 的状态转换机制，就可以避免多个线程同时修改同一份数据。
于是，我们可以进行代码的改写。
import threading
import random
class WindowThread(threading.Thread):
def __init__(self,name,lock):
threading.Thread.__init__(self,name=name)
self.name = name
self.tickts = 0
self.lock = lock
def run(self):
global tickt_count
while tickt_count > 0:
print('%s notice:There has %d tickts remain ' %(self.name,tickt_count))
self.lock.acquire()
if tickt_count > 0:
if tickt_count > 2:
number = random.randint(1,2)
else:
number = 1
tickt_count -= number
self.tickts += number
print('%s have buy %d tickt,the remain tickt\'t count is %d .Already buy %d \n'
% (self.name, number, tickt_count, self.tickts))
self.lock.release()
print('%s notice:There is no tickt can sold! Already sold %d'%(self.name,self.tickts))
tickt_count = 10
lock = threading.Lock()
window1 = WindowThread('window1',lock)
window2 = WindowThread('window2',lock)
window3 = WindowThread('window3',lock)
window1.start()
window2.start()
window3.start()
window1.join()
window2.join()
window3.join()
print('tickt count ',tickt_count)
还是 3 个线程，但代码少许不一样。
lock = threading.Lock()
window1 = WindowThread('window1',lock)
window2 = WindowThread('window2',lock)
window3 = WindowThread('window3',lock)
3 个线程共用 1 个 Lock 对象。
self.lock.acquire()
if tickt_count > 0:
if tickt_count > 2:
number = random.randint(1,2)
else:
number = 1
tickt_count -= number
self.tickts += number
print('%s have buy %d tickt,the remain tickt\'t count is %d .Already buy %d \n'
% (self.name, number, tickt_count, self.tickts))
self.lock.release()
进行关键数据操作的时候，用 Lock 锁起来，这样每次就只能一个线程对 tickt_count 数量进行修改。
最终程序结果运行如下：
window1 notice:There has 10 tickts remain
window1 have buy 1 tickt,the remain tickt't count is 9 .Already buy 1
window2 notice:There has 9 tickts remain
window1 notice:There has 9 tickts remain
window2 have buy 1 tickt,the remain tickt't count is 8 .Already buy 1
window3 notice:There has 8 tickts remain
window2 notice:There has 8 tickts remain
window2 have buy 2 tickt,the remain tickt't count is 6 .Already buy 3
window2 notice:There has 6 tickts remain
window3 have buy 2 tickt,the remain tickt't count is 4 .Already buy 2
window3 notice:There has 4 tickts remain
window2 have buy 2 tickt,the remain tickt't count is 2 .Already buy 5
window2 notice:There has 2 tickts remain
window1 have buy 1 tickt,the remain tickt't count is 1 .Already buy 2
window1 notice:There has 1 tickts remain
window3 have buy 1 tickt,the remain tickt't count is 0 .Already buy 3
window3 notice:There is no tickt can sold! Already sold 3
window2 notice:There is no tickt can sold! Already sold 5
window1 notice:There is no tickt can sold! Already sold 2
tickt count 0
可以多试几次，窗口卖出的票数都是可以对的上号的，并且最终票池里面的数量是 0，不会发生之前出现为 -1 的情况。
所以，自此我们就通过引入 Lock 同步机制，进行了一个很简单化的多线程编码示例。
默认情况，当一个 Lock 是 locked 状态时调用 acquire()，会阻塞线程本身。
但我们可以设置不阻塞，或者是阻塞指定时间。
#不阻塞
lock.acquire(False)
#阻塞指定时间，如 3 秒钟,当然 python3 的版本才有这个功能
lock.acquire(timeout=3)
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