python 读取数据再写入，文件大小总会出现变差？：报错

python 2.7.8 

def file_by_block(file, begin, end):
    with open(file, 'rb') as fp:
        fp.seek(begin)  #寻找文件中某个给定的位置
        lenthofdata = end - begin  #希望读取的大小
        data = fp.read(lenthofdata)
        with open('C:\\Users\\Administrator\\Desktop\\TEMP\\b.text', 'ab+') as fd:
            fd.writelines(data)

无论用writelines还是write，用'a','ab','ab+'，总是不能把data完整的写入到fd中。求高人指点，先谢！

多线程调用上面读取写入的部分：

def process_pid(pid, array, file, rlock):
    '''
    多个动作pid同时开动。
    '''
    FILE_SIZE = getFilesize(file)
    while 1:
        rlock.acquire()
        begin = max(array)
        end = array[pid] = (begin + BLOCKSIZE) if (begin + BLOCKSIZE) < FILE_SIZE else FILE_SIZE
        rlock.release()

        if begin == FILE_SIZE:
            break
        elif end < FILE_SIZE:
            file_by_range(file, begin, end,pid)
        elif end == FILE_SIZE:
            file_by_range(file, begin, end,pid)
        else:
            break

def main():

    file = "C:\\Users\\Administrator\\Desktop\\a.mkv"
    FILE_SIZE = getFilesize(file)

    rlock = RLock()
    array = Array('l', WORKERS, lock=rlock)
    threads = []
    for i in range(WORKERS):
        p = Process(target=process_pid(), args=[i, array, file, rlock])
        threads.append(p)

    for i in range(WORKERS):
        threads[i].start()

    for i in range(WORKERS):
        threads[i].join()

    print 'in main :', datetime.datetime.now().strftime("%Y-%d-%m %H:%M:%S")

if __name__ == '__main__':
    main()