Xpath doesn't match

I'm trying to get some elements from a page. Unfortunatel开发者_开发知识库y it results with an empty list. The pretty-printed tree includes this element:

<html:a title="..." href="..." id="..." class="topic_title">...</html:a>

However when I do this on the same tree:

page.xpath('''.//a[@class="topic_title"]''')

I get an empty list. The tree was created with html5lib / lxml treebuilder.

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It seems as if you are dealing with XHTML, so you could register the namespace html before evaluating the XPath expression:

page.xpath('''.//html:a[@class="topic_title"]''',
           namespaces={'html': 'http://www.w3.org/1999/xhtml'})

See also Namespaces and Prefixes:

If your XPath expression uses namespace prefixes, you must define them in a prefix mapping. To this end, pass a dictionary to the namespaces keyword argument that maps the namespace prefixes used in the XPath expression to namespace URIs.