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cssselect is a parser for CSS Selectors that can translate to XPath 1.0

Project description

cssselect is a parser for CSS Selectors Level 3 that can also translate selectors to XPath 1.0 queries. Such queries can be used in lxml to find the matching elements in an XML or HTML document.

This module used to live inside of lxml as lxml.cssselect before it was extracted as a stand-alone project.

The CSSSelector class

The most important class in the cssselect module is CSSSelector. It provides the same interface as lxml’s XPath class, but accepts a CSS selector expression as input:

>>> from cssselect import CSSSelector
>>> sel = CSSSelector('div.content')
>>> sel  #doctest: +ELLIPSIS
<CSSSelector ... for 'div.content'>
>>> sel.css
'div.content'

The selector actually compiles to XPath, and you can see the expression by inspecting the object:

>>> sel.path
"descendant-or-self::div[contains(concat(' ', normalize-space(@class), ' '), ' content ')]"

To use the selector, simply call it with a document or element object:

>>> from lxml.etree import fromstring
>>> h = fromstring('''<div id="outer">
...   <div id="inner" class="content body">
...       text
...   </div></div>''')
>>> [e.get('id') for e in sel(h)]
['inner']

CSS Selectors

This libraries attempts to implement CSS selectors as described in the w3c specification. Many of the pseudo-classes do not apply in this context, including all dynamic pseudo-classes. In particular these will not be available:

  • link state: :link, :visited, :target
  • actions: :hover, :active, :focus
  • UI states: :enabled, :disabled, :indeterminate (:checked and :unchecked are available)

Also, none of the pseudo-elements apply, because the selector only returns elements and pseudo-elements select portions of text, like ::first-line.

Namespaces

In CSS you can use namespace-prefix|element, similar to namespace-prefix:element in an XPath expression. In fact, it maps one-to-one, and the same rules are used to map namespace prefixes to namespace URIs.

Limitations

These applicable pseudoclasses are not yet implemented:

  • :lang(language)
  • *:first-of-type, *:last-of-type, *:nth-of-type, *:nth-last-of-type, *:only-of-type. All of these work when you specify an element type, but not with *

Unlike XPath you cannot provide parameters in your expressions – all expressions are completely static.

XPath has underspecified string quoting rules (there seems to be no string quoting at all), so if you use expressions that contain characters that requiring quoting you might have problems with the translation from CSS to XPath.

Project details


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