python - lxml etree 在之前找到最接近的元素

Question

xml文件结构如下

<a>
    <b>
        <d>
    </b>

    <c attr1="important"/>
    <b>
        <d>
    </b>
    <c attr1="so important" />
    <b></b>
</a>

我的解析器首先获取所有 <d>元素

from lxml import etree
xmltree = etree.parse(document)
elems = xmltree.xpath('//d')

现在的任务是:

从最近的<c>获取属性在当前 <d> 之前标记 标签，如果有的话。

天真的方法是做类似下面的事情

for el in elems:
    it = el.getparent()
    while it != None and it.tag != 'c':
        prev = it.getprevious()
        if prev == None:
            it = it.getparent()
        else:
            it = prev

    if it != None:
        print el, it.get("attr1")

但对我来说这看起来并不简单——我是不是遗漏了文档中的某些内容？如果不实现我自己的迭代器，我该如何解决这个问题？

Answer 1

使用 preceding axis :

The preceding axis indicates all the nodes that precede the context node in the document except any ancestor, attribute and namespace nodes.

for el in elems:
    try:
        print el.xpath("preceding::c[@attr1]")[-1].get("attr1")
    except IndexError:
        print "No preceding 'c' element."

演示:

>>> from lxml import etree
>>> 
>>> data = """
... <a>
...     <b>
...         <d/>
...     </b>
... 
...     <c attr1="important"/>
...     <b>
...         <d/>
...     </b>
...     <c attr1="so important" />
...     <b></b>
... </a>
... """
>>> xmltree = etree.fromstring(data)
>>> elems = xmltree.xpath('//d')
>>> 
>>> for el in elems:
...     try:
...         print el.xpath("preceding::c[@attr1]")[-1].get("attr1")
...     except IndexError:
...         print "No preceding 'c' element."
... 
No preceding 'c' element.
important