浅析Python多线程

　　今天看了几篇博客，主要讲解线程的实例以及如何避免线程间的竞争，觉得感觉对自己很有用，所以在此先写先来以备以后自己查阅.

　　实例一：我们将要请求三个不同的url

1.单线程：

1 import time 2 from urllib.request import urlopen 3 4 5 def get_responses(): 6 urls = [ 7 'http://www.baidu.com', 8 'http://www.taobao.com', 9 'http://www.alibaba.com', 10 ] 11 start = time.time() 12 for url in urls: 13 print(url) 14 resp = urlopen(url) 15 print(resp.getcode()) #得到状态码 16 print("spent time:%s" % (time.time()-start)) 17 18 get_responses()

解释：url顺序的被请求 除非cpu从一个url获得了回应，否则不会去请求下一个url 网络请求会花费较长的时间，所以cpu在等待网络请求的返回时间内一直处于闲置状态。

输出为：http://www.baidu.com200http://www.taobao.com200http://www.alibaba.com200spent time:1.1927924156188965

2.多线程：

from urllib.request import urlopen import time from threading import Thread class GetUrlThread(Thread): def __init__(self, url): self.url = url super(GetUrlThread, self).__init__() def run(self): resp = urlopen(self.url) print(self.url, resp.getcode()) def get_responses(): urls = [ 'http://www.baidu.com', 'http://www.taobao.com', 'http://www.alibaba.com', ] start = time.time() threads = [] for url in urls: t = GetUrlThread(url) threads.append(t) t.start() for t in threads: t.join() print("spent time:%s" % (time.time()-start)) get_responses()

解释： 意识到了程序在执行时间上的提升 我们写了一个多线程程序来减少cpu的等待时间，当我们在等待一个线程内的网络请求返回时，这时cpu可以切换到其他线程去进行其他线程内的网络请求。 我们期望一个线程处理一个url，所以实例化线程类的时候我们传了一个url。 线程运行意味着执行类里的run()方法。 无论如何我们想每个线程必须执行run()。 为每个url创建一个线程并且调用start()方法，这告诉了cpu可以执行线程中的run()方法了。 我们希望所有的线程执行完毕的时候再计算花费的时间，所以调用了join()方法。 join()可以通知主线程等待这个线程结束后，才可以执行下一条指令。 每个线程我们都调用了join()方法，所以我们是在所有线程执行完毕后计算的运行时间。 关于线程： cpu可能不会在调用start()后马上执行run()方法。 你不能确定run()在不同线程建间的执行顺序。 对于单独的一个线程，可以保证run()方法里的语句是按照顺序执行的。 这就是因为线程内的url会首先被请求，然后打印出返回的结果。

输出为：http://www.baidu.com 200http://www.alibaba.com 200http://www.taobao.com 200spent time:0.6294200420379639

实例二：全局变量的线程安全问题（race condition）

1.BUG版：

from threading import Thread import time #define a global variable some_var = 0 class IncrementThread(Thread): def run(self): # we want to read a global variable # and then increment it global some_var read_var = some_var print("some_var in %s is %d" % (self.name, read_var)) time.sleep(0.1) some_var = read_var + 1 print("some_var in %s is %d" % (self.name, some_var)) def use_increment_thread(): threads = [] for i in range(50): t = IncrementThread() threads.append(t) t.start() for t in threads: t.join() print("After 50 modifications, some_var should have become 50") print("After 50 modifications, some_var is %d" % some_var) use_increment_thread()

解释： 

有一个全局变量，所有的线程都想修改它。 所有的线程应该在这个全局变量上加 1 。 有50个线程，最后这个数值应该变成50，但是它却没有。 为什么没有达到50？ 在some_var是15的时候，线程t1读取了some_var，这个时刻cpu将控制权给了另一个线程t2。 t2线程读到的some_var也是15 t1和t2都把some_var加到16 当时我们期望的是t1 t2两个线程使some_var + 2变成17 在这里就有了资源竞争。 相同的情况也可能发生在其它的线程间，所以出现了最后的结果小于50的情况。 输出为：some_var in Thread-1 is 0some_var in Thread-2 is 0some_var in Thread-3 is 0some_var in Thread-4 is 0some_var in Thread-5 is 0some_var in Thread-6 is 0some_var in Thread-7 is 0some_var in Thread-8 is 0some_var in Thread-9 is 0some_var in Thread-10 is 0some_var in Thread-11 is 0some_var in Thread-12 is 0some_var in Thread-13 is 0some_var in Thread-14 is 0some_var in Thread-15 is 0some_var in Thread-16 is 0some_var in Thread-17 is 0some_var in Thread-18 is 0some_var in Thread-19 is 0some_var in Thread-20 is 0some_var in Thread-21 is 0some_var in Thread-22 is 0some_var in Thread-23 is 0some_var in Thread-24 is 0some_var in Thread-25 is 0some_var in Thread-26 is 0some_var in Thread-27 is 0some_var in Thread-28 is 0some_var in Thread-29 is 0some_var in Thread-30 is 0some_var in Thread-31 is 0some_var in Thread-32 is 0some_var in Thread-33 is 0some_var in Thread-34 is 0some_var in Thread-35 is 0some_var in Thread-36 is 0some_var in Thread-37 is 0some_var in Thread-38 is 0some_var in Thread-39 is 0some_var in Thread-40 is 0some_var in Thread-41 is 0some_var in Thread-42 is 0some_var in Thread-43 is 0some_var in Thread-44 is 0some_var in Thread-45 is 0some_var in Thread-46 is 0some_var in Thread-47 is 0some_var in Thread-48 is 0some_var in Thread-49 is 0some_var in Thread-50 is 0some_var in Thread-6 is 1some_var in Thread-5 is 1some_var in Thread-2 is 1some_var in Thread-4 is 1some_var in Thread-1 is 1some_var in Thread-3 is 1some_var in Thread-12 is 1some_var in Thread-13 is 1some_var in Thread-11 is 1some_var in Thread-10 is 1some_var in Thread-9 is 1some_var in Thread-7 is 1some_var in Thread-8 is 1some_var in Thread-21 is 1some_var in Thread-20 is 1some_var in Thread-19 is 1some_var in Thread-18 is 1some_var in Thread-17 is 1some_var in Thread-15 is 1some_var in Thread-14 is 1some_var in Thread-16 is 1some_var in Thread-26 is 1some_var in Thread-25 is 1some_var in Thread-24 is 1some_var in Thread-22 is 1some_var in Thread-23 is 1some_var in Thread-31 is 1some_var in Thread-29 is 1some_var in Thread-28 is 1some_var in Thread-27 is 1some_var in Thread-30 is 1some_var in Thread-38 is 1some_var in Thread-37 is 1some_var in Thread-36 is 1some_var in Thread-35 is 1some_var in Thread-32 is 1some_var in Thread-33 is 1some_var in Thread-34 is 1some_var in Thread-44 is 1some_var in Thread-43 is 1some_var in Thread-42 is 1some_var in Thread-41 is 1some_var in Thread-40 is 1some_var in Thread-39 is 1some_var in Thread-50 is 1some_var in Thread-49 is 1some_var in Thread-48 is 1some_var in Thread-47 is 1some_var in Thread-45 is 1some_var in Thread-46 is 1After 50 modifications, some_var should have become 50After 50 modifications, some_var is 1

解决竞争带锁版：

1 from threading import Lock, Thread 2 import time 3 lock = Lock() 4 some_var = 0 5 6 class IncrementThread(Thread): 7 def run(self): 8 #we want to read a global variable 9 #and then increment it 10 global some_var 11 lock.acquire() 12 read_value = some_var 13 print("some_var in %s is %d" % (self.name, read_value)) 14 time.sleep(0.1) 15 some_var = read_value + 1 16 print("some_var in %s after increment is %d" % (self.name, some_var)) 17 lock.release() 18 19 def use_increment_thread(): 20 threads = [] 21 for i in range(50): 22 t = IncrementThread() 23 threads.append(t) 24 t.start() 25 for t in threads: 26 t.join() 27 print("After 50 modifications, some_var should have become 50") 28 print("After 50 modifications, some_var is %d" % (some_var,)) 29 30 use_increment_thread()

解释： 

Lock 用来防止竞争条件 如果在执行一些操作之前，线程t1获得了锁。其他的线程在t1释放Lock之前，不会执行相同的操作 我们想要确定的是一旦线程t1已经读取了some_var，直到t1完成了修改some_var，其他的线程才可以读取some_var 这样读取和修改some_var成了逻辑上的原子操作。 输出为：some_var in Thread-1 is 0some_var in Thread-1 after increment is 1some_var in Thread-2 is 1some_var in Thread-2 after increment is 2some_var in Thread-3 is 2some_var in Thread-3 after increment is 3some_var in Thread-4 is 3some_var in Thread-4 after increment is 4some_var in Thread-5 is 4some_var in Thread-5 after increment is 5some_var in Thread-6 is 5some_var in Thread-6 after increment is 6some_var in Thread-7 is 6some_var in Thread-7 after increment is 7some_var in Thread-8 is 7some_var in Thread-8 after increment is 8some_var in Thread-9 is 8some_var in Thread-9 after increment is 9some_var in Thread-10 is 9some_var in Thread-10 after increment is 10some_var in Thread-11 is 10some_var in Thread-11 after increment is 11some_var in Thread-12 is 11some_var in Thread-12 after increment is 12some_var in Thread-13 is 12some_var in Thread-13 after increment is 13some_var in Thread-14 is 13some_var in Thread-14 after increment is 14some_var in Thread-15 is 14some_var in Thread-15 after increment is 15some_var in Thread-16 is 15some_var in Thread-16 after increment is 16some_var in Thread-17 is 16some_var in Thread-17 after increment is 17some_var in Thread-18 is 17some_var in Thread-18 after increment is 18some_var in Thread-19 is 18some_var in Thread-19 after increment is 19some_var in Thread-20 is 19some_var in Thread-20 after increment is 20some_var in Thread-21 is 20some_var in Thread-21 after increment is 21some_var in Thread-22 is 21some_var in Thread-22 after increment is 22some_var in Thread-23 is 22some_var in Thread-23 after increment is 23some_var in Thread-24 is 23some_var in Thread-24 after increment is 24some_var in Thread-25 is 24some_var in Thread-25 after increment is 25some_var in Thread-26 is 25some_var in Thread-26 after increment is 26some_var in Thread-27 is 26some_var in Thread-27 after increment is 27some_var in Thread-28 is 27some_var in Thread-28 after increment is 28some_var in Thread-29 is 28some_var in Thread-29 after increment is 29some_var in Thread-30 is 29some_var in Thread-30 after increment is 30some_var in Thread-31 is 30some_var in Thread-31 after increment is 31some_var in Thread-32 is 31some_var in Thread-32 after increment is 32some_var in Thread-33 is 32some_var in Thread-33 after increment is 33some_var in Thread-34 is 33some_var in Thread-34 after increment is 34some_var in Thread-35 is 34some_var in Thread-35 after increment is 35some_var in Thread-36 is 35some_var in Thread-36 after increment is 36some_var in Thread-37 is 36some_var in Thread-37 after increment is 37some_var in Thread-38 is 37some_var in Thread-38 after increment is 38some_var in Thread-39 is 38some_var in Thread-39 after increment is 39some_var in Thread-40 is 39some_var in Thread-40 after increment is 40some_var in Thread-41 is 40some_var in Thread-41 after increment is 41some_var in Thread-42 is 41some_var in Thread-42 after increment is 42some_var in Thread-43 is 42some_var in Thread-43 after increment is 43some_var in Thread-44 is 43some_var in Thread-44 after increment is 44some_var in Thread-45 is 44some_var in Thread-45 after increment is 45some_var in Thread-46 is 45some_var in Thread-46 after increment is 46some_var in Thread-47 is 46some_var in Thread-47 after increment is 47some_var in Thread-48 is 47some_var in Thread-48 after increment is 48some_var in Thread-49 is 48some_var in Thread-49 after increment is 49some_var in Thread-50 is 49some_var in Thread-50 after increment is 50After 50 modifications, some_var should have become 50After 50 modifications, some_var is 50

实例三：多线程环境下的原子操作

BUG版本：

1 from threading import Thread 2 import time 3 4 class CreateListThread(Thread): 5 def run(self): 6 self.entries = [] 7 for i in range(10): 8 # time.sleep(0.1) 9 self.entries.append(i) 10 for each in self.entries: 11 print(each, end = " ") 12 time.sleep(0.1) 13 14 def use_create_list_thread(): 15 for i in range(3): 16 t = CreateListThread() 17 t.start() 18 19 use_create_list_thread()

 

 

解释：当一个线程正在打印的时候，cpu切换到了另一个线程，所以产生了不正确的结果。我们需要确保print self.entries是个逻辑上的原子操作，以防打印时被其他线程打断。因为打印的速度太快，我在此有意放大了这个时间，加了一个time.sleep(0.1) 输出为：0 0 0 1 1 1 2 2 2 3 3 3 4 4 4 5 5 5 6 6 6 7 7 7 8 8 8 9 9 9

2.加锁保证操作的原子性

1 from threading import Thread, Lock 2 import time 3 4 lock = Lock() 5 6 7 class CreateListThread(Thread): 8 def run(self): 9 self.entries = [] 10 for i in range(10): 11 time.sleep(0.1) 12 self.entries.append(i) 13 lock.acquire() 14 for each in self.entries: 15 print(each, end = " ") 16 time.sleep(0.1) 17 lock.release() 18 19 20 def use_create_list_thread(): 21 for i in range(3): 22 t = CreateListThread() 23 t.start() 24 25 use_create_list_thread()

输出为：0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9

转载于:https://www.cnblogs.com/Hwan/p/5956118.html

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