Caveats with inspecting the live browser DOM
Since Firefox add-ons operate on a live browser DOM, what you’ll actually see when inspecting the page source is not the original HTML, but a modified one after applying some browser clean up and executing Javascript code. Firefox, in particular, is known for adding
elements to tables. Scrapy, on the other hand, does not modify the original page HTML, so you won’t be able to extract any data if you use in your XPath expressions.Therefore, you should keep in mind the following things when working with Firefox and XPath:
- Disable Firefox Javascript while inspecting the DOM looking for XPaths to be used in Scrapy
- Never use full XPath paths, use relative and clever ones based on attributes (such as id, class, width, etc) or any identifying features like contains(@href, 'image').
- Never include elements in your XPath expressions unless you really know what you’re doing
在浏览器中检查DOM的注意事项
Firefox插件操作的是活动的浏览器DOM(live browser DOM)，这意味着当您检查网页源码的时候， 其已经不是原始的HTML，而是经过浏览器清理并执行一些Javascript代码后的结果。 Firefox是个典型的例子，其会在table中添加
元素。 而Scrapy相反，其并不修改原始的HTML，因此如果在XPath表达式中使用 ，您将获取不到任何数据。所以，当XPath配合Firefox使用时您需要记住以下几点:
- 当检查DOM来查找Scrapy使用的XPath时，禁用Firefox的Javascrpit。
- 永远不要用完整的XPath路径。使用相对及基于属性(例如 id ， class ， width 等)的路径或者具有区别性的特性例如contains(@href, 'image') 。
- 永远不要在XPath表达式中加入 元素，除非您知道您在做什么
Actually,
- Chrome 也会这样
- 使用
.css()CSS 选择器也会有同样的问题from: https://doc.scrapy.org/en/1.3/topics/firefox.html 中文文档