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cssselect

Contents

Quickstart

Use HTMLTranslator for HTML documents, GenericTranslator for "generic" XML documents. (The former has a more useful translation for some selectors, based on HTML-specific element types or attributes.)

pycon

>>> from cssselect import GenericTranslator, SelectorError >>> try: ... expression = GenericTranslator().css_to_xpath('div.content') ... except SelectorError: ... print('Invalid selector.') ... >>> print(expression) descendant-or-self::div[@class and contains(concat(' ', normalize-space(@class), ' '), ' content ')]

The resulting expression can be used with lxml's XPath engine:

pycon

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

User API

In CSS3 Selectors terms, the top-level object is a group of selectors, a sequence of comma-separated selectors. For example, div, h1.title + p is a group of two selectors.

parse

Selector()

FunctionalPseudoElement

GenericTranslator

HTMLTranslator

Exceptions

SelectorError

SelectorSyntaxError

ExpressionError

Supported selectors

This library implements CSS3 selectors as described in the W3C specification. In this context however, there is no interactivity or history of visited links. Therefore, these pseudo-classes are accepted but never match anything:

  • :hover
  • :active
  • :focus
  • :target
  • :visited

Additionally, these depend on document knowledge and only have a useful implementation in HTMLTranslator. In GenericTranslator, they never match:

  • :link
  • :enabled
  • :disabled
  • :checked

These applicable pseudo-classes are not yet implemented:

  • *: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 *

On the other hand, cssselect supports some selectors that are not in the Level 3 specification.

These parts of the Level 4 specification are supported (note that a large part of the Level 4 additions is not applicable to cssselect similarly to :hover or not representable in XPath 1.0 so the complete specification is unlikely to be implemented):

  • The :scope pseudo-class. Limitation: it can only be used at a start of a selector.
  • The :is(), :where() and :has() pseudo-classes. Limitation: :has() cannot contain nested :has() or :not().

These are non-standard extensions:

  • The :contains(text) pseudo-class that existed in an early draft but was then removed.
  • The != attribute operator. [foo!=bar] is the same as :not([foo=bar]).
  • :not() accepts a sequence of simple selectors, not just single simple selector. For example, :not(a.important[rel]) is allowed, even though the negation contains 3 simple selectors.

Customizing the translation

Just like HTMLTranslator is a subclass of GenericTranslator, you can make new sub-classes of either of them and override some methods. This enables you, for example, to customize how some pseudo-class is implemented without forking or monkey-patching cssselect.

The "customization API" is the set of methods in translation classes and their signature. You can look at the source code to see how it works. However, be aware that this API is not very stable yet. It might change and break your sub-class.

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. How prefixes are mapped to namespace URIs depends on the XPath implementation.