一个python多进程计数器的问题

Question

写了个多进程的脚本，然后其中有个计数器，大致逻辑如下

def worker(co):
    # working
    co[0] += 1
    if co[0] % 10000 == 0:
        logging.info("Parsed {}".format(co[0]))
        
def main():
    co = Manager.list()
    co[0] = 1
    pool = Pool()
    for i in range(10):
        pool.apply_async(worker, (co,))

最终程序运行时发现个有趣的问题

INFO: 2017-04-11 16:42:19,288 - 13582 - Parsed 879999
INFO: 2017-04-11 16:42:19,292 - 13583 - Parsed 880000
INFO: 2017-04-11 16:42:19,295 - 13593 - Parsed 880001
INFO: 2017-04-11 16:42:19,297 - 13597 - Parsed 880001

这里的880001，多一个我能理解，当if co[0] % 10000 == 0判断时，其他进程已经做了加一操作了，但这个879999少一个，我就有点想不通了？

Answer 1

The process that meets the printing conditions is just about to print, and other processes are doing the +1 operation, getting the value, +1, and assigning the value back requires several instructions. In fact, any value may appear, because the instruction to get the value can occur at any time.