Python 值错误 : XPath error: Unregistered function

Question

<img alt="MediaMarkt" border="0" e-editable="img" src="http://news-de.mediamarkt.de/custloads/298149669/vce/mediamarkt.png" style="display:block;" width="169"/>

我试图从 HTML 获取 src，我有 alt 值，然后使用它我尝试获取图像

company_name = "mediamarkt"
response.xpath(f'//img[lower-case(@alt)="{company_name.lower()}"]') #Error
response.xpath(f"//img[matches(@alt,'{company_name}','i')]") # Error

我得到的错误:

Traceback (most recent call last):
  File "/home/timmy/.local/lib/python3.8/site-packages/parsel/selector.py", line 254, in xpath
    result = xpathev(query, namespaces=nsp,
  File "src/lxml/etree.pyx", line 1582, in lxml.etree._Element.xpath
  File "src/lxml/xpath.pxi", line 305, in lxml.etree.XPathElementEvaluator.__call__
  File "src/lxml/xpath.pxi", line 225, in lxml.etree._XPathEvaluatorBase._handle_result
lxml.etree.XPathEvalError: Unregistered function

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.8/code.py", line 90, in runcode
    exec(code, self.locals)
  File "<console>", line 1, in <module>
  File "/home/timmy/.local/lib/python3.8/site-packages/scrapy/http/response/text.py", line 117, in xpath
    return self.selector.xpath(query, **kwargs)
  File "/home/timmy/.local/lib/python3.8/site-packages/parsel/selector.py", line 260, in xpath
    six.reraise(ValueError, ValueError(msg), sys.exc_info()[2])
  File "/usr/lib/python3/dist-packages/six.py", line 702, in reraise
    raise value.with_traceback(tb)
  File "/home/timmy/.local/lib/python3.8/site-packages/parsel/selector.py", line 254, in xpath
    result = xpathev(query, namespaces=nsp,
  File "src/lxml/etree.pyx", line 1582, in lxml.etree._Element.xpath
  File "src/lxml/xpath.pxi", line 305, in lxml.etree.XPathElementEvaluator.__call__
  File "src/lxml/xpath.pxi", line 225, in lxml.etree._XPathEvaluatorBase._handle_result
ValueError: XPath error: Unregistered function in //img[matches(@alt,'mediamarkt','i')]

我从 case-insensitive matching in xpath? 得到了那些 XPath

Answer 1

lower-case() 和 matches() 都需要 XPath 2.0，但 lxml 仅实现 XPath 1.0。

XPath 1.0 中用于不区分大小写匹配的习语使用 translate(),

translate(@alt, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ', 'abcdefghijklmnopqrstuvwxyz')

在与需要不区分大小写的比较的字符串的小写版本进行比较之前，将大写字符映射为小写字符。

所以，在你的情况下，

response.xpath(f"//img[translate(@alt, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ','abcdefghijklmnopqrstuvwxyz')='{company_name.lower()}']")

对于您的其他 XPath 也是如此。

另见 case insensitive xpath contains() possible?