Python 多线程编程及同步处理
综述?? 多线程是程序设计中的一个重要方面，尤其是在服务器Deamon程序方面。无论何种系统，线程调度的开销都比传统的进程要快得多。? Python可以方便地支持多线程。可以快速创建线程、互斥锁、信号量等等元素，支持线程读写同步互斥。美中不足的是，Python的运行在Python虚拟机上，创建的多线程可能是虚拟的线程，需要由Python虚拟机来轮询调度，这大大降低了Python多线程的可用性。希望高版本的Python可以解决这个问题，发挥多CPU的最大效率。? 网上有些朋友说要获得真正多CPU的好处，有两种方法：? 1.可以创建多个进程而不是线程，进程数和cpu一样多。
? 2.使用Jython 或 IronPython，可以得到真正的多线程。?? 闲话少说，下面看看Python如何建立线程?? Python线程创建???使用threading模块的 Thread类? 类接口如下?
class?Thread( group=None, target=None, name=None, args=(), kwargs={})?? 需要关注的参数是target和args. target?是需要子线程运行的目标函数，args是函数的参数，以tuple的形式传递。??以下代码创建一个指向函数worker?的子线程def worker(a_tid,a_account):?????...??th = threading.Thread(target=worker,args=(i,acc) ) ;?? 启动这个线程th.start()?? 等待线程返回threading.Thread.join(th)?或者th.join()?? 如果你可以对要处理的数据进行很好的划分，而且线程之间无须通信，那么你可以使用：创建=》运行=》回收的方式编写你的多线程程序。但是如果线程之间需要访问共同的对象，则需要引入互斥锁或者信号量对资源进行互斥访问。??下面讲讲如何创建互斥锁创建锁?g_mutex = threading.Lock()??....?使用锁??????for?... :?????????#锁定，从下一句代码到释放前互斥访问?????????g_mutex.acquire()?????????a_account.deposite(1)?????????#释放?????????g_mutex.release()?? 最后，模拟一个公交地铁IC卡缴车费的多线程程序? 有10个读卡器，每个读卡器收费器每次扣除用户一块钱进入总账中，每读卡器每天一共被刷10000000次。账户原有100块。所以最后的总账应该为10000100。先不使用互斥锁来进行锁定（注释掉了锁定代码），看看后果如何。?import time,datetime?
import threading?
def worker(a_tid,a_account):?
????global g_mutex?
????print "Str " , a_tid, datetime.datetime.now()?
????for i in range(1000000):?
????????#g_mutex.acquire()?
????????a_account.deposite(1)?
????????#g_mutex.release()?
????print "End " , a_tid , datetime.datetime.now()?
?????
class Account:?
????def __init__ (self, a_base ):?
????????self.m_amount=a_base?
????def deposite(self,a_amount):?
????????self.m_amount+=a_amount?
????def withdraw(self,a_amount):?
????????self.m_amount-=a_amount?????
?????????
if __name__ == "__main__":?
????global g_mutex?
????count = 0?
????dstart = datetime.datetime.now()?
????print "Main Thread Start At: " , dstart?
????#init thread_pool?
????thread_pool = []?
????#init mutex?
????g_mutex = threading.Lock()?
????# init thread items?
????acc = Account(100)?
????for i in range(10):?
????????th = threading.Thread(target=worker,args=(i,acc) ) ;?
????????thread_pool.append(th)?
?????????
????# start threads one by one?????????
????for i in range(10):?
????????thread_pool[i].start()?
?????
????#collect all threads?
????for i in range(10):?
????????threading.Thread.join(thread_pool[i])?
????dend = datetime.datetime.now()?
????print "count=",acc.m_amount?
????print "Main Thread End at: " ,dend , " time span " , dend-dstart;?? 注意，先不用互斥锁进行临界段访问控制，运行结果如下：?Main Thread Start At:????2009-01-13 00:17:55.296000?
Str????0 2009-01-13 00:17:55.312000?
Str????1 2009-01-13 00:17:55.453000?
Str????2 2009-01-13 00:17:55.484000?
Str????3 2009-01-13 00:17:55.531000?
Str????4 2009-01-13 00:17:55.562000?
Str????5 2009-01-13 00:17:55.609000?
Str????6 2009-01-13 00:17:55.640000?
Str????7 2009-01-13 00:17:55.687000?
Str????8 2009-01-13 00:17:55.718000?
Str????9 2009-01-13 00:17:55.781000?
End????0 2009-01-13 00:18:06.250000?
End????1 2009-01-13 00:18:07.500000?
End????4 2009-01-13 00:18:07.531000?
End????2 2009-01-13 00:18:07.562000?
End????3 2009-01-13 00:18:07.593000?
End????9 2009-01-13 00:18:07.609000?
End????7 2009-01-13 00:18:07.640000?
End????8 2009-01-13 00:18:07.671000?
End????5 2009-01-13 00:18:07.687000?
End????6 2009-01-13 00:18:07.718000?
count= 3434612?
Main Thread End at:????2009-01-13 00:18:07.718000????time span????0:00:12.422000?? 从结果看到，程序确实是多线程运行的。但是由于没有对对象Account进行互斥访问，所以结果是错误的，只有3434612，比原预计少了很多。???把上面阴影部分代码的注释打开，运行结果如下Main Thread Start At:????2009-01-13 00:26:12.156000?
Str????0 2009-01-13 00:26:12.156000?
Str????1 2009-01-13 00:26:12.390000?
Str????2 2009-01-13 00:26:12.437000?
Str????3 2009-01-13 00:26:12.468000?
Str????4 2009-01-13 00:26:12.515000?
Str????5 2009-01-13 00:26:12.562000?
Str????6 2009-01-13 00:26:12.593000?
Str????7 2009-01-13 00:26:12.640000?
Str????8 2009-01-13 00:26:12.671000?
Str????9 2009-01-13 00:26:12.718000?
End????0 2009-01-13 00:27:01.781000?
End????1 2009-01-13 00:27:05.890000?
End????5 2009-01-13 00:27:06.046000?
End????7 2009-01-13 00:27:06.078000?
End????4 2009-01-13 00:27:06.109000?
End????2 2009-01-13 00:27:06.140000?
End????6 2009-01-13 00:27:06.156000?
End????8 2009-01-13 00:27:06.187000?
End????3 2009-01-13 00:27:06.203000?
End????9 2009-01-13 00:27:06.234000?
count= 10000100?
Main Thread End at:????2009-01-13 00:27:06.234000????time span????0:00:54.078000?? 这次可以看到，结果正确了。运行时间比不进行互斥多了很多，需要花54秒才能运行（我机器烂，没钱更新，呵呵），不过这也是同步的代价，没办法。?
