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Python 学习笔记 - 线程(2)
前面学习了线程基本的概念和创建线程的两种方法，现在看看多线程如何处理竞争条件（racing condition）的问题。
比如说，下面的例子中 我使用了第二种创建的方式，自定义一个类，继承Thread类，然后自定义run（）来执行我的方法。在这个run方法里面，每次都对全局变量加1
在主线程里面，他调用一个自己定义的函数，在这个函数里面创建了5000个线程；每个线程都加入一个列表，然后对每个对象都使用join，这是确保主线程等着直到所有子线程完成。最后输出结果
import time
import threading
some_var = 0
class IncrementThread(threading.Thread):
def run(self):
#we want to read a global variable
#and then increment it
global some_var
read_value = some_var
print ("some_var in %s is %d" % (self.name, read_value))
some_var = read_value + 1
print ("some_var in %s after increment is %d" % (self.name, some_var))
def use_increment_thread():
threads = []
start=time.time()
for i in range(5000):
t = IncrementThread()
threads.append(t)
t.start()
for t in threads:
t.join()
print("Total time %s"%(time.time()-start))
print ("After 5000 modifications, some_var should have become 5000")
print ("After 5000 modifications, some_var is %d" % (some_var,))
use_increment_thread()
------------------
Total time 1.7780036926269531
After 5000 modifications, some_var should have become 5000
After 5000 modifications, some_var is 4987可以看见结果并不是5000，这是为啥呢？ 如果查看过程，会发现有些线程刚刚获取了一个值，还未来得及处理，执行的权力就转交给了另外一个线程，这样就导致计数错误。为了确保每一个线程都成功的执行了他应该执行的代码，我们可以加一把锁。
some_var in Thread-1524 is 1523 some_var in Thread-1524 after increment is 1524 some_var in Thread-1525 is 1524 some_var in Thread-1526 is 1524 some_var in Thread-1526 after increment is 1525 some_var in Thread-1527 is 1525
下面是修订过的代码，通过使用Lock()函数，我们在执行代码前acquire(),之后release（），在当前线程完成这段代码之前，其他的线程不可以执行相同的操作。
some_var = 0
lock=threading.Lock()
class IncrementThread(threading.Thread):
def run(self):
#we want to read a global variable
#and then increment it
global some_var
lock.acquire()
read_value = some_var
print ("some_var in %s is %d" % (self.name, read_value))
some_var = read_value + 1
print ("some_var in %s after increment is %d" % (self.name, some_var))
lock.release()
def use_increment_thread():
threads = []
start=time.time()
for i in range(5000):
t = IncrementThread()
threads.append(t)
t.start()
for t in threads:
t.join()
print("Total time %s"%(time.time()-start))
print ("After 5000 modifications, some_var should have become 5000")
print ("After 5000 modifications, some_var is %d" % (some_var,))
use_increment_thread()
---------------
Total time 1.6369926929473877
After 5000 modifications, some_var should have become 5000
After 5000 modifications, some_var is 5000本文出自 “麻婆豆腐” 博客，请务必保留此出处http://beanxyz.blog.51cto.com/5570417/1866269
Python 学习笔记 - 线程(2)
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