多进程写入文件

from multiprocessing import Poolfrom time import sleepimport timedef mycallback(x):    with open('123.txt', 'a+') as f:        f.writelines(str(x) + "\n")        print x        sleep(1)def sayHi(num):    return numif __name__ == '__main__':    e1 = time.time()    pool = Pool(4)    for i in range(10):        pool.apply_async(sayHi, (i,), callback=mycallback)    pool.close()    pool.join()    e2 = time.time()    print float(e2 - e1)