深入理解 Python 中的多线程 新手必看
示例1 import time import urllib2 defget_responses(): urls=[ ‘http://www.baidu.com',‘http://www.amazon.com',‘http://www.ebay.com',‘http://www.alibaba.com',‘http://www.aspzz.cn' ] start=time.time() forurlinurls: printurl resp=urllib2.urlopen(url) printresp.getcode() print”Elapsed time: %s”%(time.time()-start) get_responses() 输出是: 解释: import urllib2 import time from threading import Thread classGetUrlThread(Thread): def__init__(self,url): self.url=url super(GetUrlThread,self).__init__() defrun(self): resp=urllib2.urlopen(self.url) printself.url,resp.getcode() defget_responses(): urls=[ ‘http://www.baidu.com',‘http://www.aspzz.cn' ] start=time.time() threads=[] forurlinurls: t=GetUrlThread(url) threads.append(t) t.start() fortinthreads: t.join() print”Elapsed time: %s”%(time.time()-start) get_responses() 输出: 解释: 意识到了程序在执行时间上的提升 关于线程: cpu可能不会在调用start()后马上执行run()方法。 实例2 我们将会用一个程序演示一下多线程间的资源竞争,并修复这个问题。 from threading import Thread #define a global variable some_var=0 classIncrementThread(Thread): defrun(self): #we want to read a global variable #and then increment it globalsome_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) defuse_increment_thread(): threads=[] foriinrange(50): t=IncrementThread() threads.append(t) t.start() fortinthreads: 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() 多次运行这个程序,你会看到多种不同的结果。 from threading import Lock,Thread lock=Lock() some_var=0 classIncrementThread(Thread): defrun(self): #we want to read a global variable #and then increment it globalsome_var lock.acquire() read_value=some_var print”some_var in %s is %d”%(self.name,some_var) lock.release() defuse_increment_thread(): threads=[] foriinrange(50): t=IncrementThread() threads.append(t) t.start() fortinthreads: t.join() print”After 50 modifications,) use_increment_thread() 再次运行这个程序,达到了我们预期的结果。 from threading import Thread import time classCreateListThread(Thread): defrun(self): self.entries=[] foriinrange(10): time.sleep(1) self.entries.append(i) printself.entries defuse_create_list_thread(): foriinrange(3): t=CreateListThread() t.start() use_create_list_thread() 运行几次后发现并没有打印出争取的结果。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。 from threading import Thread,Lock import time lock=Lock() classCreateListThread(Thread): defrun(self): self.entries=[] foriinrange(10): time.sleep(1) self.entries.append(i) lock.acquire() printself.entries lock.release() defuse_create_list_thread(): foriinrange(3): t=CreateListThread() t.start() use_create_list_thread() 这次我们看到了正确的结果。证明了一个线程不可以修改其他线程内部的变量(非全局变量)。 (编辑:李大同) 【声明】本站内容均来自网络,其相关言论仅代表作者个人观点,不代表本站立场。若无意侵犯到您的权利,请及时与联系站长删除相关内容! |