我的代码：

import threading
import time

def test1():
    while True:
        print '111111111111'
        time.sleep(2)

def test2():
    while True:
        print '22222222222'
        time.sleep(10)
        print '222222222222end'
        break
    return

def test3():
    #获取所有线程
    thread_list_all = threading.enumerate()
    while True:
        thread_list1 = threading.enumerate()
        thread_list_number = len(thread_list1)
        #如果有线程死掉
        if thread_list_number < 4:
            #查看哪个线程死掉
            dead_thread = list(set(thread_list_all).difference(set(thread_list1)))
            for item_thread in dead_thread:
                print item_thread.getName(),'666666666666'
                #重新启动线程
                item_thread.start()
                # item_thread.join()

        time.sleep(1)


t1 = threading.Thread(target=test1,name=test1)
t2 = threading.Thread(target=test2,name=test2)
t3 = threading.Thread(target=test3,name=test3)

t1.start()
t2.start()
t3.start()

t1.join()
t2.join()
t3.join()

运行报错：

111111111111
22222222222
111111111111
111111111111
111111111111
111111111111
222222222222end
111111111111
<function test2 at 0x7f8df4c180c8> 666666666666
Exception in thread <function test3 at 0x7f8df4c18140>:
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 801, in __bootstrap_inner
    self.run()
  File "/usr/lib/python2.7/threading.py", line 754, in run
    self.__target(*self.__args, **self.__kwargs)
  File "/home/freedom/work/app/sem/testvivo.py", line 28, in test3
    item_thread.start()
  File "/usr/lib/python2.7/threading.py", line 730, in start
    raise RuntimeError("threads can only be started once")
RuntimeError: threads can only be started once

请问我如何才能监控我的线程呢？如果某个线程死掉了，就让他再重新启动。我上面的代码，如果线程3死掉了，就没办法监控了，该如何改我的代码呢？