scrapy 爬取小说保存的txt内容章节乱序，Pipeline中如何排序保存

import scrapy

class MyItem(scrapy.Item):

    # define the fields for your item here like:

    # name = scrapy.Field()

    num = scrapy.Field() 

    desc = scrapy.Field()

    dname = scrapy.Field()       

    pass

import codecs

import os

import pdb



class MyPipeline(object):

    def process_item(self, item, spider):

        base_dir = os.getcwd()

        fiename = base_dir + '/小说.txt'

        with open(fiename, "a",encoding='utf-8') as f: 

            f.write(item['dname'] + "\n")

            f.write(item['desc'] +"\n\n") 

       return item