python多线程同步机制Lock

#!/usr/bin/env python
# -*- coding: utf-8 -*-

import threading
import time

value = 0
lock = threading.Lock()

def add():
    global value
    with lock:
        new_value = value + 1
        time.sleep(0.001)
        value = new_value

if __name__ == '__main__':
    threads = []
    for i in range(100):
        t = threading.Thread(target=add)
        t.start()
        threads.append(t)

    for t in threads:
        t.join()

    print value
    print 'Main end'

如果将with lock去掉，将得不到最终的值100.