为什么索引器错误列表索引超出范围

Question

我不太明白，当我不断得到索引误差，说超出范围。是因为列表链接是空的吗？这里写链接的格式是什么

class POSpider(CrawlSpider):
    name = 'po'
    start_urls = ['https://www.poets.org/poetsorg/poems']
    allowed_domains = ['poets.org/poetsorg/poems']

    def parse(self, response):
        items=[]
        l=response.xpath('//*[@class="themes"]//a//@href').extract()
        theme_ids=[]
        for item in l:
            theme_ids.append(item[855:1412])
        theme_urls=[]
        for tid in theme_ids:
            theme_urls.append('https://www.poets.org/poetsorg/poems?   field_occasion_tid=All&field_poem_themes_tid='+ tid)
        for link in theme_urls:
            request=scrapy.Request(link,callback=self.parse_layer2,dont_filter=True)
            yield request



    def parse_layer2(self,response):
        items=[]
        p=response.xpath('//*[@id="block-views-poems-poems-block-all"]/div/div//tbody//td[2]//@href')[-1].extract()
        poem_urls=[]
        for item in p:
            poem_urls.append(item)
        for link in poem_urls:
            request=scrapy.Request(link,callback=self.parse_layer3,dont_filter=True)

            yield request


    def parse_layer3(self,response):
        items=[]
        poems=response.xpath('//*[@id="poem-content"]/div[2]/div/div').extract()
        for poem in poems:
            item=PoetryItem()
            s=poem.xpath('*/p/text()').extract()
            t=strip_list(s)
            t=t.encode('ascii','replace').lower()+'\r\n'
            item['poem']=t
            items.append(item)
        return items

这就是我一直得到的结果。在

^{pr2}$