python - xpath解析使用extract()的时候，几种情况有点分不清楚

Question

xpath解析使用extract()的时候，几种情况有点分不清楚,如下截图所示：

什么时候用哪种？

Answer 1

1. Returns a SelectorList object http://scrapy-chs.readthedocs.io/zh_CN/0.24/topics/selectors.html#selectorlist

   SelectorList 类是内建 list 类的子类，提供了一些额外的方法:
   xpath(query)
   css(query)
   extract()
   re()
   __nonzero__()

2. Returns a list (the one that comes with the system) which contains some of the content you extracted

3. Returns the first element of the list in 2 (throws an exception if the list is empty)

4. Returns the first element in the SelectorList in 1 (throws an exception if the list is empty), which is consistent with the effect achieved in 3

5. 4 returns a str (if Python2 is unicode), so 5 will return the first character of str

Answer 2

It is best to use the first method. After extraction, the selector object will be converted into a list type. It is not recommended to use extract()[0] directly, because sometimes a list index error will be reported