嵌套 Xpath 的 Scrapy 和 XPath 问题

Question

我正在尝试将亚马逊产品读入 scrapy。 使用此 XPath 从随机类别开始：

products = Selector(response).xpath('//div[@class="s-item-container"]')
for product in products:
    item = AmzItem()
    item['title'] = product.xpath('//a[@class="s-access-detail-page"]/@title').extract()[0]
    item['url'] = product.xpath('//a[@class="s-access-detail-page"]/@href').extract()[0]
    yield item

('//div[@class="s-item-container"]') 在一个类别页面上返回带有产品的所有 div - 这是正确的。

现在，我如何获得产品的链接？

// 代表代码中的任何位置 带有@class 的a 应该选择正确的类 但我得到一个：

item['title'] = product.xpath('//a[@class="s-access-detail-page"]/@title').extract()[0] exceptions.IndexError: list index out of range

所以我的匹配这个 XPath 的列表必须是空的 - 但我不明白为什么？

编辑：
HTML 看起来像这样：

<div class="s-item-container" style="height: 343px;">
<div class="a-row a-spacing-base">
    <div class="a-column a-span12 a-text-left">
        <div class="a-section a-spacing-none a-inline-block s-position-relative">
            <a class="a-link-normal a-text-normal" href="https://rads.stackoverflow.com/amzn/click/com/B0105S434A" rel="nofollow noreferrer"><img alt="Product Details" src="http://ecx.images-amazon.com/images/I/41%2BzrAY74UL._AA160_.jpg" onload="viewCompleteImageLoaded(this, new Date().getTime(), 24, false);" class="s-access-image cfMarker" height="160" width="160"></a>
            <div class="a-section a-spacing-none a-text-center">
                <div class="a-row a-spacing-top-mini">
                    <a class="a-size-mini a-link-normal a-text-normal" href="https://rads.stackoverflow.com/amzn/click/com/B0105S434A" rel="nofollow noreferrer">
                        <div class="a-box">
                            <div class="a-box-inner a-padding-mini"><span class="a-color-secondary">See more choices</span></div>
                        </div>
                    </a>
                </div>
            </div>
        </div>
    </div>
</div>
<div class="a-row a-spacing-mini">
    <div class="a-row a-spacing-none">
        <a class="a-link-normal s-access-detail-page  a-text-normal" title="Harry Potter Gryffindor School Fancy Robe Cloak Costume And Tie (Size S)" href="https://rads.stackoverflow.com/amzn/click/com/B0105S434A" rel="nofollow noreferrer">
            <h2 class="a-size-base a-color-null s-inline s-access-title a-text-normal">Harry Potter Gryffindor School Fancy Robe Cloak Costume And Tie (Size S)</h2>
        </a>
    </div>
    <div class="a-row a-spacing-mini"><span class="a-size-small a-color-secondary">by </span><span class="a-size-small a-color-secondary">Legend</span></div>
</div>
<div class="a-row a-spacing-mini">
    <div class="a-row a-spacing-none"><a class="a-size-small a-link-normal a-text-normal" href="http://www.amazon.com/gp/offer-listing/B0105S434A/ref=sr_1_21_olp?s=pet-supplies&amp;ie=UTF8&amp;qid=1435391788&amp;sr=1-21&amp;keywords=pet+supplies&amp;condition=new"><span class="a-size-base a-color-price a-text-bold">$28.99</span><span class="a-letter-space"></span>new<span class="a-letter-space"></span><span class="a-color-secondary">(1 offer)</span><span class="a-letter-space"></span><span class="a-color-secondary a-text-strike"></span></a></div>
</div>
<div class="a-row a-spacing-none"><span name="B0105S434A">
    <span class="a-declarative" data-action="a-popover" data-a-popover="{&quot;max-width&quot;:&quot;700&quot;,&quot;closeButton&quot;:&quot;false&quot;,&quot;position&quot;:&quot;triggerBottom&quot;,&quot;url&quot;:&quot;/review/widgets/average-customer-review/popover/ref=acr_search__popover?ie=UTF8&amp;asin=B0105S434A&amp;contextId=search&amp;ref=acr_search__popover&quot;}"><a href="javascript:void(0)" class="a-popover-trigger a-declarative"><i class="a-icon a-icon-star a-star-4"><span class="a-icon-alt">3.9 out of 5 stars</span></i><i class="a-icon a-icon-popover"></i></a></span></span>
    <a class="a-size-small a-link-normal a-text-normal" href="https://rads.stackoverflow.com/amzn/click/com/B0105S434A" rel="nofollow noreferrer">48</a>
</div>
</div>

Answer 1

应该是：

# ------------- The dot makes the query relative to product
product.xpath('.//a[@class="s-access-detail-page"]/@title')

Answer 2

//a[@class="s-access-detail-page"] 必须是 class="s-access-detail-page"，因为 xpath 使用字符串而不是含义:) 当你有“多类”时，使用 contains 函数

//a[contains(concat(' ', @class, ' '), " s-access-detail-page ")]/@title