一个关于多线程的问题，请高手指点

dreamerx2004

问题在代码的注释里，谢谢

############################ Begin Python Code ############################
import thread
import dummy_thread
import datetime
import time

NUM = 0

def addNum():
    global NUM
    for i in range(3000000):
        NUM = NUM + 1
   

st = datetime.datetime.now()

for i in range(3):
    #addNum()
    #thread.start_new_thread(addNum, ())
    dummy_thread.start_new_thread(addNum, ())
    #Q1: module "thread" needs a lock to ensure sync of the global variable, module "dummy_thread" doesn't, why?

et = datetime.datetime.now()

#Q2: calling sleep() is important to run this script when using module "thread" but not to module "dummy_thread" (on win7), why?
time.sleep(0.1)

print NUM
print et - st
############################  End Python Code   ############################