scrapy爬取大规模数据存储于文本形式时，如何保证一个文本达到一定大小后，自动存储于另一个新文本中

import os
from scrapy.exporters import JsonLinesItemExporter
class BkPipeline(object):
def __init__(self):
self.time_stamp=0
self.fp=open("file_{}.json".format(str(self.time_stamp)),'ab')
self.exporter=JsonLinesItemExporter(self.fp,ensure_ascii=False,encoding='utf-8')
def open_spider(self,spider):
print("爬虫开始啦")
def process_item(self, item, spider):
self.exporter.export_item(item)
file_size=os.path.getsize('file_{}.json'.format(str(self.time_stamp)))/(1024**2)
if file_size>5:
self.fp.close()
self.time_stamp+=1
self.fp=open("file_{}.json".format(str(self.time_stamp)),'ab')
self.exporter = JsonLinesItemExporter(self.fp, ensure_ascii=False, encoding='utf-8')
return item
def close_spider(self,spider):
self.fp.close()
print("爬虫结束啦！！！！！！")