问如何在Python线程中正确使用join(timeout)？

EN

import threading,time

class Server(threading.Thread):

    def __init__(self, hostname):
        super(Server, self).__init__()
        self.__hostname = hostname

    def run(self):
        print self.__hostname+' left'
        time.sleep(5)
        print self.__hostname+' back'
        sem.release()

#init
sem = threading.BoundedSemaphore(4)
threads = []

for x in xrange(1,5):
    sem.acquire()
    t = Server('thread '+str(x))
    threads.append(t)
    t.start()

for t in threads:
    t.join(2)

print 'why is this line executed by main thread 5 seconds after, not 2?'