为什么用lxml处理XHTML文档(在python中)时xpath无法正常工作? [英] Why doesn't xpath work when processing an XHTML document with lxml (in python)?
问题描述
我正在针对以下测试文档进行测试:
I am testing against the following test document:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>hi there</title>
</head>
<body>
<img class="foo" src="bar.png"/>
</body>
</html>
如果我使用lxml.html解析文档，则可以使用xpath获得IMG:
If I parse the document using lxml.html, I can get the IMG with an xpath just fine:
>>> root = lxml.html.fromstring(doc)
>>> root.xpath("//img")
[<Element img at 1879e30>]
但是，如果我将文档解析为XML并尝试获取IMG标签，则会得到空结果:
However, if I parse the document as XML and try to get the IMG tag, I get an empty result:
>>> tree = etree.parse(StringIO(doc))
>>> tree.getroot().xpath("//img")
[]
我可以直接导航到该元素:
I can navigate to the element directly:
>>> tree.getroot().getchildren()[1].getchildren()[0]
<Element {http://www.w3.org/1999/xhtml}img at f56810>
但是，那当然对我处理任意文档没有帮助.我还希望能够查询etree以获得可以直接标识此元素的xpath表达式，从技术上讲，我可以做到这一点:
But of course that doesn't help me process arbitrary documents. I would also expect to be able to query etree to get an xpath expression that will directly identify this element, which, technically I can do:
>>> tree.getpath(tree.getroot().getchildren()[1].getchildren()[0])
'/*/*[2]/*'
>>> tree.getroot().xpath('/*/*[2]/*')
[<Element {http://www.w3.org/1999/xhtml}img at fa1750>]
但是，同样，xpath显然对解析任意文档没有用.
But that xpath is, again, obviously not useful for parsing arbitrary documents.
很显然，我在这里缺少一些关键问题，但我不知道它是什么.我最好的猜测是，它与名称空间有关，但是唯一定义的名称空间是默认名称，我不知道关于名称空间我还需要考虑什么.
Obviously I am missing some key issue here, but I don't know what it is. My best guess is that it has something to do with namespaces but the only namespace defined is the default and I don't know what else I might need to consider in regards to namespaces.
那么，我想念什么?
推荐答案
问题是名称空间.当解析为XML时，img标签位于 http://www.w3.org/1999/xhtml 名称空间，因为它是元素的默认名称空间.您要在没有名称空间的情况下请求img标签.
The problem is the namespaces. When parsed as XML, the img tag is in the http://www.w3.org/1999/xhtml namespace since that is the default namespace for the element. You are asking for the img tag in no namespace.
尝试一下:
>>> tree.getroot().xpath(
... "//xhtml:img",
... namespaces={'xhtml':'http://www.w3.org/1999/xhtml'}
... )
[<Element {http://www.w3.org/1999/xhtml}img at 11a29e0>]
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