XPath 语法错误：谓词无效

我有一个像这样的 XML 文件：

$ cat sample.xml
<Requests>
        <Request>
                <ID>123</ID>
                <Items>
                        <Item>a item</Item>
                        <Item>b item</Item>
                        <Item>c item</Item>
                </Items>
        </Request>
        <Request>
                <ID>456</ID>
                <Items>
                        <Item>d item</Item>
                        <Item>e item</Item>
                </Items>
        </Request>
</Requests>

我只是想提取 XMLRequest对于其孙元素有一定价值的元素Item。这是代码：

bash-4.2$ cat xsearch.py
import sys
import xml.etree.ElementTree as ET


if __name__ == '__main__':
        tree = ET.parse(sys.argv[1])
        root = tree.getroot()
        for request in root.findall(".//Item[.='c item']/../.."):
        #for request in root.findall(".//Request[Items/Item = 'c item']"):
                print(request)

我收到“无效谓词”错误：

bash-4.2$ python3 xsearch.py sample.xml
Traceback (most recent call last):
  File "/usr/lib64/python3.6/xml/etree/ElementPath.py", line 263, in iterfind
    selector = _cache[cache_key]
KeyError: (".//Item[.='c item']/../..", None)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "xsearch.py", line 8, in <module>
    for request in root.findall(".//Item[.='c item']/../.."):
  File "/usr/lib64/python3.6/xml/etree/ElementPath.py", line 304, in findall
    return list(iterfind(elem, path, namespaces))
  File "/usr/lib64/python3.6/xml/etree/ElementPath.py", line 277, in iterfind
    selector.append(ops[token[0]](next, token))
  File "/usr/lib64/python3.6/xml/etree/ElementPath.py", line 233, in     prepare_predicate
    raise SyntaxError("invalid predicate")
SyntaxError: invalid predicate

 

谁能指出我哪里错了？

一般来说，XPath无效谓词错误意味着 XPath 的谓词之一（即 XPath 之间的代码）在语法上存在错误。[ and ].

具体就您的情况而言，存在两个问题：

1. The SyntaxError("invalid predicate")是因为有一个额外的)在谓词中：

    for request in root.findall(".//Item[.='c item')]/../.."):
                                                   ^
   

   另请注意，您可以提升谓词以避免向下导航然后向上导航（../..):

   代替

    .//Item[.='c item']/../..
   

   consider

    .//Request[Items/Item = 'c item']
   

   选择Request具有目标的元素Item.

2. 您正在使用的 XPath 库，ElementTree，不是 XPath 标准的完整实现 https://docs.python.org/2/library/xml.etree.elementtree.html#xpath-support。您可能会浪费大量时间来尝试确定 ElementTree 所支持的内容（".//Items[Item='c item']/.."碰巧在这里工作）并且不支持，但最好只使用更兼容的库，例如lxml https://lxml.de/.