python中的多线程学习

#! /usr/bin/env pythonimport osimport timeimport threading# 进程函数def printthread(n):    print (n,"-->进程创建")    for a in range(4):        print (a)        time.sleep(1)    print (n,"-->进程结束")    sem.release()if __name__ =='__main__':    maxThread=5    #    sem=threading.BoundedSemaphore(maxThread)    for a in range(12):        sem.acquire()        threading.Thread(target=printthread,args=(a,)).start()    print ("All thread has create,Wait for all thread exit.")    for a in range(maxThread):        sem.acquire();    print ("All thread exit")