如题，本人用Python爬取内容时，出现了ValueError: can only parse strings这个错误，该如何解决？

源代码如下：
#-*-coding:utf8-*-

from lxml import etree
from multiprocessing.dummy import Pool as ThreadPool
import requests
import json
import sys

reload(sys)

sys.setdefaultencoding('utf-8')

def towrite(contentdict):
    f.writelines(u'回帖时间：' + str(contentdict['topic_reply_time'])+'\n')
    f.writelines(u'回帖内容：' + unicode(contentdict['topic_reply_content'])+'\n')
    f.writelines(u'回帖人：' + contentdict['user_name']+'\n\n')

def spider(url):
    html=requests.get(url)
    selector=etree.HTML(html)
    content_field=selector.xpath('//div[@class="l_post j_l_post l_post_bright "]')
    item={}
    for each in content_field:
        reply_info=json.load(each.xpath('@data-field')[0].replace('&quot',''))
        author=reply_info['author']['user_name']
        content=each.xpath('div[@class="d_post_content_main"]/div/cc/div[@class="d_post_content j_d_post_content  clearfix"]/text()')[0]
        reply_time=reply_info['content']['date']
        print author
        print content
        print reply_time
        item['user_name']=author
        item['topic_reply_content']=content
        item['topic_reply_time']=reply_time
        towrite(item)

if __name__=='__main__':
    pool=ThreadPool(4)
    f=open('content.txt','a')
    page=[]
    for i in range(1,21):
        newpage='http://tieba.baidu.com/p/3522395718?pn='+str(i)
        page.append(newpage)

    results=pool.map(spider, page)
    pool.close()
    pool.join()
    f.close()