.. _topics-selectors:

=========
Selectors
=========

When you're scraping web pages, the most common task you need to perform is
to extract data from the HTML source. There are several libraries available to
achieve this, such as:

-   `BeautifulSoup`_ is a very popular web scraping library among Python
    programmers which constructs a Python object based on the structure of the
    HTML code and also deals with bad markup reasonably well, but it has one
    drawback: it's slow.

-   `lxml`_ is an XML parsing library (which also parses HTML) with a pythonic
    API based on :mod:`~xml.etree.ElementTree`. (lxml is not part of the Python
    standard library.)

Scrapy comes with its own mechanism for extracting data. They're called
selectors because they "select" certain parts of the HTML document specified
either by `XPath`_ or `CSS`_ expressions.

`XPath`_ is a language for selecting nodes in XML documents, which can also be
used with HTML. `CSS`_ is a language for applying styles to HTML documents. It
defines selectors to associate those styles with specific HTML elements.

.. note::
    Scrapy Selectors is a thin wrapper around `parsel`_ library; the purpose of
    this wrapper is to provide better integration with Scrapy Response objects.

    `parsel`_ is a stand-alone web scraping library which can be used without
    Scrapy. It uses `lxml`_ library under the hood, and implements an
    easy API on top of lxml API. It means Scrapy selectors are very similar
    in speed and parsing accuracy to lxml.

.. _BeautifulSoup: https://www.crummy.com/software/BeautifulSoup/
.. _lxml: https://lxml.de/
.. _XPath: https://www.w3.org/TR/xpath/all/
.. _CSS: https://www.w3.org/TR/selectors
.. _parsel: https://parsel.readthedocs.io/en/latest/

Using selectors
===============

Constructing selectors
----------------------

.. highlight:: python

Response objects expose a :class:`~scrapy.Selector` instance
on ``.selector`` attribute:

.. code-block:: pycon

    >>> response.selector.xpath("//span/text()").get()
    'good'

Querying responses using XPath and CSS is so common that responses include two
more shortcuts: ``response.xpath()`` and ``response.css()``:

.. code-block:: pycon

    >>> response.xpath("//span/text()").get()
    'good'
    >>> response.css("span::text").get()
    'good'

Scrapy selectors are instances of :class:`~scrapy.Selector` class
constructed by passing either :class:`~scrapy.http.TextResponse` object or
markup as a string (in ``text`` argument).

Usually there is no need to construct Scrapy selectors manually:
``response`` object is available in Spider callbacks, so in most cases
it is more convenient to use ``response.css()`` and ``response.xpath()``
shortcuts. By using ``response.selector`` or one of these shortcuts
you can also ensure the response body is parsed only once.

But if required, it is possible to use ``Selector`` directly.
Constructing from text:

.. code-block:: pycon

    >>> from scrapy.selector import Selector
    >>> body = "<html><body><span>good</span></body></html>"
    >>> Selector(text=body).xpath("//span/text()").get()
    'good'

Constructing from response - :class:`~scrapy.http.HtmlResponse` is one of
:class:`~scrapy.http.TextResponse` subclasses:

.. code-block:: pycon

    >>> from scrapy.selector import Selector
    >>> from scrapy.http import HtmlResponse
    >>> response = HtmlResponse(url="http://example.com", body=body)
    >>> Selector(response=response).xpath("//span/text()").get()
    'good'

``Selector`` automatically chooses the best parsing rules
(XML vs HTML) based on input type.

Using selectors
---------------

To explain how to use the selectors we'll use the ``Scrapy shell`` (which
provides interactive testing) and an example page located in the Scrapy
documentation server:

    https://docs.scrapy.org/en/latest/_static/selectors-sample1.html

.. _topics-selectors-htmlcode:

For the sake of completeness, here's its full HTML code:

.. literalinclude:: ../_static/selectors-sample1.html
   :language: html

.. highlight:: sh

First, let's open the shell::

    scrapy shell https://docs.scrapy.org/en/latest/_static/selectors-sample1.html

Then, after the shell loads, you'll have the response available as ``response``
shell variable, and its attached selector in ``response.selector`` attribute.

Since we're dealing with HTML, the selector will automatically use an HTML parser.

.. highlight:: python

So, by looking at the :ref:`HTML code <topics-selectors-htmlcode>` of that
page, let's construct an XPath for selecting the text inside the title tag:

.. code-block:: pycon

    >>> response.xpath("//title/text()")
    [<Selector xpath='//title/text()' data='Example website'>]

To actually extract the textual data, you must call the selector ``.get()``
or ``.getall()`` methods, as follows:

.. code-block:: pycon

    >>> response.xpath("//title/text()").getall()
    ['Example website']
    >>> response.xpath("//title/text()").get()
    'Example website'

``.get()`` always returns a single result; if there are several matches,
content of a first match is returned; if there are no matches, None
is returned. ``.getall()`` returns a list with all results.

Notice that CSS selectors can select text or attribute nodes using CSS3
pseudo-elements:

.. code-block:: pycon

    >>> response.css("title::text").get()
    'Example website'

As you can see, ``.xpath()`` and ``.css()`` methods return a
:class:`~scrapy.selector.SelectorList` instance, which is a list of new
selectors. This API can be used for quickly selecting nested data:

.. code-block:: pycon

    >>> response.css("img").xpath("@src").getall()
    ['image1_thumb.jpg',
    'image2_thumb.jpg',
    'image3_thumb.jpg',
    'image4_thumb.jpg',
    'image5_thumb.jpg']

If you want to extract only the first matched element, you can call the
selector ``.get()`` (or its alias ``.extract_first()`` commonly used in
previous Scrapy versions):

.. code-block:: pycon

    >>> response.xpath('//div[@id="images"]/a/text()').get()
    'Name: My image 1 '

It returns ``None`` if no element was found:

.. code-block:: pycon

    >>> response.xpath('//div[@id="not-exists"]/text()').get() is None
    True

A default return value can be provided as an argument, to be used instead
of ``None``:

.. code-block:: pycon

    >>> response.xpath('//div[@id="not-exists"]/text()').get(default="not-found")
    'not-found'

Instead of using e.g. ``'@src'`` XPath it is possible to query for attributes
using ``.attrib`` property of a :class:`~scrapy.Selector`:

.. code-block:: pycon

    >>> [img.attrib["src"] for img in response.css("img")]
    ['image1_thumb.jpg',
    'image2_thumb.jpg',
    'image3_thumb.jpg',
    'image4_thumb.jpg',
    'image5_thumb.jpg']

As a shortcut, ``.attrib`` is also available on SelectorList directly;
it returns attributes for the first matching element:

.. code-block:: pycon

    >>> response.css("img").attrib["src"]
    'image1_thumb.jpg'

This is most useful when only a single result is expected, e.g. when selecting
by id, or selecting unique elements on a web page:

.. code-block:: pycon

    >>> response.css("base").attrib["href"]
    'http://example.com/'

Now we're going to get the base URL and some image links:

.. code-block:: pycon

    >>> response.xpath("//base/@href").get()
    'http://example.com/'

    >>> response.css("base::attr(href)").get()
    'http://example.com/'

    >>> response.css("base").attrib["href"]
    'http://example.com/'

    >>> response.xpath('//a[contains(@href, "image")]/@href').getall()
    ['image1.html',
    'image2.html',
    'image3.html',
    'image4.html',
    'image5.html']

    >>> response.css("a[href*=image]::attr(href)").getall()
    ['image1.html',
    'image2.html',
    'image3.html',
    'image4.html',
    'image5.html']

    >>> response.xpath('//a[contains(@href, "image")]/img/@src').getall()
    ['image1_thumb.jpg',
    'image2_thumb.jpg',
    'image3_thumb.jpg',
    'image4_thumb.jpg',
    'image5_thumb.jpg']

    >>> response.css("a[href*=image] img::attr(src)").getall()
    ['image1_thumb.jpg',
    'image2_thumb.jpg',
    'image3_thumb.jpg',
    'image4_thumb.jpg',
    'image5_thumb.jpg']

.. _topics-selectors-css-extensions:

Extensions to CSS Selectors
---------------------------

Per W3C standards, `CSS selectors`_ do not support selecting text nodes
or attribute values.
But selecting these is so essential in a web scraping context
that Scrapy (parsel) implements a couple of **non-standard pseudo-elements**:

* to select text nodes, use ``::text``
* to select attribute values, use ``::attr(name)`` where *name* is the
  name of the attribute that you want the value of

.. warning::
    These pseudo-elements are Scrapy-/Parsel-specific.
    They will most probably not work with other libraries like
    `lxml`_ or `PyQuery`_.

.. _PyQuery: https://pypi.org/project/pyquery/

Examples:

* ``title::text`` selects children text nodes of a descendant ``<title>`` element:

.. code-block:: pycon

    >>> response.css("title::text").get()
    'Example website'

* ``*::text`` selects all descendant text nodes of the current selector context:

.. code-block:: pycon

    >>> response.css("#images *::text").getall()
    ['\n   ',
    'Name: My image 1 ',
    '\n   ',
    'Name: My image 2 ',
    '\n   ',
    'Name: My image 3 ',
    '\n   ',
    'Name: My image 4 ',
    '\n   ',
    'Name: My image 5 ',
    '\n  ']

* ``foo::text`` returns no results if ``foo`` element exists, but contains
  no text (i.e. text is empty):

.. code-block:: pycon

  >>> response.css("img::text").getall()
  []

  This means ``.css('foo::text').get()`` could return None even if an element
  exists. Use ``default=''`` if you always want a string:

.. code-block:: pycon

    >>> response.css("img::text").get()
    >>> response.css("img::text").get(default="")
    ''

* ``a::attr(href)`` selects the *href* attribute value of descendant links:

.. code-block:: pycon

    >>> response.css("a::attr(href)").getall()
    ['image1.html',
    'image2.html',
    'image3.html',
    'image4.html',
    'image5.html']

.. note::
    See also: :ref:`selecting-attributes`.

.. note::
    You cannot chain these pseudo-elements. But in practice it would not
    make much sense: text nodes do not have attributes, and attribute values
    are string values already and do not have children nodes.

.. _CSS Selectors: https://www.w3.org/TR/selectors-3/#selectors

.. _topics-selectors-nesting-selectors:

Nesting selectors
-----------------

The selection methods (``.xpath()`` or ``.css()``) return a list of selectors
of the same type, so you can call the selection methods for those selectors
too. Here's an example:

.. code-block:: pycon

    >>> links = response.xpath('//a[contains(@href, "image")]')
    >>> links.getall()
    ['<a href="image1.html">Name: My image 1 <br><img src="image1_thumb.jpg"></a>',
    '<a href="image2.html">Name: My image 2 <br><img src="image2_thumb.jpg"></a>',
    '<a href="image3.html">Name: My image 3 <br><img src="image3_thumb.jpg"></a>',
    '<a href="image4.html">Name: My image 4 <br><img src="image4_thumb.jpg"></a>',
    '<a href="image5.html">Name: My image 5 <br><img src="image5_thumb.jpg"></a>']

    >>> for index, link in enumerate(links):
    ...     href_xpath = link.xpath("@href").get()
    ...     img_xpath = link.xpath("img/@src").get()
    ...     print(f"Link number {index} points to url {href_xpath!r} and image {img_xpath!r}")
    ...
    Link number 0 points to url 'image1.html' and image 'image1_thumb.jpg'
    Link number 1 points to url 'image2.html' and image 'image2_thumb.jpg'
    Link number 2 points to url 'image3.html' and image 'image3_thumb.jpg'
    Link number 3 points to url 'image4.html' and image 'image4_thumb.jpg'
    Link number 4 points to url 'image5.html' and image 'image5_thumb.jpg'

.. _selecting-attributes:

Selecting element attributes
----------------------------

There are several ways to get a value of an attribute. First, one can use
XPath syntax:

.. code-block:: pycon

    >>> response.xpath("//a/@href").getall()
    ['image1.html', 'image2.html', 'image3.html', 'image4.html', 'image5.html']

XPath syntax has a few advantages: it is a standard XPath feature, and
``@attributes`` can be used in other parts of an XPath expression - e.g.
it is possible to filter by attribute value.

Scrapy also provides an extension to CSS selectors (``::attr(...)``)
which allows to get attribute values:

.. code-block:: pycon

    >>> response.css("a::attr(href)").getall()
    ['image1.html', 'image2.html', 'image3.html', 'image4.html', 'image5.html']

In addition to that, there is a ``.attrib`` property of Selector.
You can use it if you prefer to lookup attributes in Python
code, without using XPaths or CSS extensions:

.. code-block:: pycon

    >>> [a.attrib["href"] for a in response.css("a")]
    ['image1.html', 'image2.html', 'image3.html', 'image4.html', 'image5.html']

This property is also available on SelectorList; it returns a dictionary
with attributes of a first matching element. It is convenient to use when
a selector is expected to give a single result (e.g. when selecting by element
ID, or when selecting an unique element on a page):

.. code-block:: pycon

    >>> response.css("base").attrib
    {'href': 'http://example.com/'}
    >>> response.css("base").attrib["href"]
    'http://example.com/'

``.attrib`` property of an empty SelectorList is empty:

.. code-block:: pycon

    >>> response.css("foo").attrib
    {}

Using selectors with regular expressions
----------------------------------------

:class:`~scrapy.Selector` also has a ``.re()`` method for extracting
data using regular expressions. However, unlike using ``.xpath()`` or
``.css()`` methods, ``.re()`` returns a list of strings. So you
can't construct nested ``.re()`` calls.

Here's an example used to extract image names from the :ref:`HTML code
<topics-selectors-htmlcode>` above:

.. code-block:: pycon

    >>> response.xpath('//a[contains(@href, "image")]/text()').re(r"Name:\s*(.*)")
    ['My image 1',
    'My image 2',
    'My image 3',
    'My image 4',
    'My image 5']

There's an additional helper reciprocating ``.get()`` (and its
alias ``.extract_first()``) for ``.re()``, named ``.re_first()``.
Use it to extract just the first matching string:

.. code-block:: pycon

    >>> response.xpath('//a[contains(@href, "image")]/text()').re_first(r"Name:\s*(.*)")
    'My image 1'

.. _old-extraction-api:

extract() and extract_first()
-----------------------------

If you're a long-time Scrapy user, you're probably familiar
with ``.extract()`` and ``.extract_first()`` selector methods. Many blog posts
and tutorials are using them as well. These methods are still supported
by Scrapy, there are **no plans** to deprecate them.

However, Scrapy usage docs are now written using ``.get()`` and
``.getall()`` methods. We feel that these new methods result in a more concise
and readable code.

The following examples show how these methods map to each other.

1.  ``SelectorList.get()`` is the same as ``SelectorList.extract_first()``:

.. code-block:: pycon

    >>> response.css("a::attr(href)").get()
    'image1.html'
    >>> response.css("a::attr(href)").extract_first()
    'image1.html'

2.  ``SelectorList.getall()`` is the same as ``SelectorList.extract()``:

.. code-block:: pycon

    >>> response.css("a::attr(href)").getall()
    ['image1.html', 'image2.html', 'image3.html', 'image4.html', 'image5.html']
    >>> response.css("a::attr(href)").extract()
    ['image1.html', 'image2.html', 'image3.html', 'image4.html', 'image5.html']

3.  ``Selector.get()`` is the same as ``Selector.extract()``:

.. code-block:: pycon

    >>> response.css("a::attr(href)")[0].get()
    'image1.html'
    >>> response.css("a::attr(href)")[0].extract()
    'image1.html'

4.  For consistency, there is also ``Selector.getall()``, which returns a list:

.. code-block:: pycon

    >>> response.css("a::attr(href)")[0].getall()
    ['image1.html']

So, the main difference is that output of ``.get()`` and ``.getall()`` methods
is more predictable: ``.get()`` always returns a single result, ``.getall()``
always returns a list of all extracted results. With ``.extract()`` method
it was not always obvious if a result is a list or not; to get a single
result either ``.extract()`` or ``.extract_first()`` should be called.


.. _topics-selectors-xpaths:

Working with XPaths
===================

Here are some tips which may help you to use XPath with Scrapy selectors
effectively. If you are not much familiar with XPath yet,
you may want to take a look first at this `XPath tutorial`_.

.. note::
    Some of the tips are based on `this post from Zyte's blog`_.

.. _`XPath tutorial`: http://www.zvon.org/comp/r/tut-XPath_1.html
.. _this post from Zyte's blog: https://www.zyte.com/blog/xpath-tips-from-the-web-scraping-trenches/


.. _topics-selectors-relative-xpaths:

Working with relative XPaths
----------------------------

Keep in mind that if you are nesting selectors and use an XPath that starts
with ``/``, that XPath will be absolute to the document and not relative to the
``Selector`` you're calling it from.

For example, suppose you want to extract all ``<p>`` elements inside ``<div>``
elements. First, you would get all ``<div>`` elements:

.. code-block:: pycon
    
    >>> divs = response.xpath("//div")

At first, you may be tempted to use the following approach, which is wrong, as
it actually extracts all ``<p>`` elements from the document, not only those
inside ``<div>`` elements:

.. code-block:: pycon

    >>> for p in divs.xpath("//p"):  # this is wrong - gets all <p> from the whole document
    ...     print(p.get())
    ...

This is the proper way to do it (note the dot prefixing the ``.//p`` XPath):

.. code-block:: pycon

    >>> for p in divs.xpath(".//p"):  # extracts all <p> inside
    ...     print(p.get())
    ...

Another common case would be to extract all direct ``<p>`` children:

.. code-block:: pycon

    >>> for p in divs.xpath("p"):
    ...     print(p.get())
    ...

For more details about relative XPaths see the `Location Paths`_ section in the
XPath specification.

.. _Location Paths: https://www.w3.org/TR/xpath/all/#location-paths

When querying by class, consider using CSS
------------------------------------------

Because an element can contain multiple CSS classes, the XPath way to select elements
by class is the rather verbose::

    *[contains(concat(' ', normalize-space(@class), ' '), ' someclass ')]

If you use ``@class='someclass'`` you may end up missing elements that have
other classes, and if you just use ``contains(@class, 'someclass')`` to make up
for that you may end up with more elements that you want, if they have a different
class name that shares the string ``someclass``.

As it turns out, Scrapy selectors allow you to chain selectors, so most of the time
you can just select by class using CSS and then switch to XPath when needed:

.. code-block:: pycon
    
    >>> from scrapy import Selector
    >>> sel = Selector(
    ...     text='<div class="hero shout"><time datetime="2014-07-23 19:00">Special date</time></div>'
    ... )
    >>> sel.css(".shout").xpath("./time/@datetime").getall()
    ['2014-07-23 19:00']

This is cleaner than using the verbose XPath trick shown above. Just remember
to use the ``.`` in the XPath expressions that will follow.

Beware of the difference between //node[1] and (//node)[1]
----------------------------------------------------------

``//node[1]`` selects all the nodes occurring first under their respective parents.

``(//node)[1]`` selects all the nodes in the document, and then gets only the first of them.

Example:

.. code-block:: pycon

    >>> from scrapy import Selector
    >>> sel = Selector(
    ...     text="""
    ...     <ul class="list">
    ...         <li>1</li>
    ...         <li>2</li>
    ...         <li>3</li>
    ...     </ul>
    ...     <ul class="list">
    ...         <li>4</li>
    ...         <li>5</li>
    ...         <li>6</li>
    ...     </ul>"""
    ... )
    >>> xp = lambda x: sel.xpath(x).getall()

This gets all first ``<li>``  elements under whatever it is its parent:

.. code-block:: pycon

    >>> xp("//li[1]")
    ['<li>1</li>', '<li>4</li>']

And this gets the first ``<li>``  element in the whole document:

.. code-block:: pycon

    >>> xp("(//li)[1]")
    ['<li>1</li>']

This gets all first ``<li>``  elements under an ``<ul>``  parent:

.. code-block:: pycon

    >>> xp("//ul/li[1]")
    ['<li>1</li>', '<li>4</li>']

And this gets the first ``<li>``  element under an ``<ul>``  parent in the whole document:

.. code-block:: pycon

    >>> xp("(//ul/li)[1]")
    ['<li>1</li>']

Using text nodes in a condition
-------------------------------

When you need to use the text content as argument to an `XPath string function`_,
avoid using ``.//text()`` and use just ``.`` instead.

This is because the expression ``.//text()`` yields a collection of text elements -- a *node-set*.
And when a node-set is converted to a string, which happens when it is passed as argument to
a string function like ``contains()`` or ``starts-with()``, it results in the text for the first element only.

Example:

.. code-block:: pycon

    >>> from scrapy import Selector
    >>> sel = Selector(
    ...     text='<a href="#">Click here to go to the <strong>Next Page</strong></a>'
    ... )

Converting a *node-set* to string:

.. code-block:: pycon

    >>> sel.xpath("//a//text()").getall()  # take a peek at the node-set
    ['Click here to go to the ', 'Next Page']
    >>> sel.xpath("string(//a[1]//text())").getall()  # convert it to string
    ['Click here to go to the ']

A *node* converted to a string, however, puts together the text of itself plus of all its descendants:

.. code-block:: pycon

    >>> sel.xpath("//a[1]").getall()  # select the first node
    ['<a href="#">Click here to go to the <strong>Next Page</strong></a>']
    >>> sel.xpath("string(//a[1])").getall()  # convert it to string
    ['Click here to go to the Next Page']

So, using the ``.//text()`` node-set won't select anything in this case:

.. code-block:: pycon

    >>> sel.xpath("//a[contains(.//text(), 'Next Page')]").getall()
    []

But using the ``.`` to mean the node, works:

.. code-block:: pycon

    >>> sel.xpath("//a[contains(., 'Next Page')]").getall()
    ['<a href="#">Click here to go to the <strong>Next Page</strong></a>']

.. _`XPath string function`: https://www.w3.org/TR/xpath/all/#section-String-Functions

.. _topics-selectors-xpath-variables:

Variables in XPath expressions
------------------------------

XPath allows you to reference variables in your XPath expressions, using
the ``$somevariable`` syntax. This is somewhat similar to parameterized
queries or prepared statements in the SQL world where you replace
some arguments in your queries with placeholders like ``?``,
which are then substituted with values passed with the query.

Here's an example to match an element based on its "id" attribute value,
without hard-coding it (that was shown previously):

.. code-block:: pycon

    >>> # `$val` used in the expression, a `val` argument needs to be passed
    >>> response.xpath("//div[@id=$val]/a/text()", val="images").get()
    'Name: My image 1 '

Here's another example, to find the "id" attribute of a ``<div>`` tag containing
five ``<a>`` children (here we pass the value ``5`` as an integer):

.. code-block:: pycon

    >>> response.xpath("//div[count(a)=$cnt]/@id", cnt=5).get()
    'images'

All variable references must have a binding value when calling ``.xpath()``
(otherwise you'll get a ``ValueError: XPath error:`` exception).
This is done by passing as many named arguments as necessary.

`parsel`_, the library powering Scrapy selectors, has more details and examples
on `XPath variables`_.

.. _XPath variables: https://parsel.readthedocs.io/en/latest/usage.html#variables-in-xpath-expressions


.. _removing-namespaces:

Removing namespaces
-------------------

When dealing with scraping projects, it is often quite convenient to get rid of
namespaces altogether and just work with element names, to write more
simple/convenient XPaths. You can use the
:meth:`Selector.remove_namespaces` method for that.

Let's show an example that illustrates this with the Python Insider blog atom feed.

.. highlight:: sh

First, we open the shell with the url we want to scrape::

    $ scrapy shell https://feeds.feedburner.com/PythonInsider

This is how the file starts::

    <?xml version="1.0" encoding="UTF-8"?>
    <?xml-stylesheet ...
    <feed xmlns="http://www.w3.org/2005/Atom"
          xmlns:openSearch="http://a9.com/-/spec/opensearchrss/1.0/"
          xmlns:blogger="http://schemas.google.com/blogger/2008"
          xmlns:georss="http://www.georss.org/georss"
          xmlns:gd="http://schemas.google.com/g/2005"
          xmlns:thr="http://purl.org/syndication/thread/1.0"
          xmlns:feedburner="http://rssnamespace.org/feedburner/ext/1.0">
      ...

You can see several namespace declarations including a default
"http://www.w3.org/2005/Atom" and another one using the "gd:" prefix for
"http://schemas.google.com/g/2005".

.. highlight:: python

Once in the shell we can try selecting all ``<link>`` objects and see that it
doesn't work (because the Atom XML namespace is obfuscating those nodes):

.. code-block:: pycon

    >>> response.xpath("//link")
    []

But once we call the :meth:`Selector.remove_namespaces` method, all
nodes can be accessed directly by their names:

.. code-block:: pycon

    >>> response.selector.remove_namespaces()
    >>> response.xpath("//link")
    [<Selector xpath='//link' data='<link rel="alternate" type="text/html" h'>,
        <Selector xpath='//link' data='<link rel="next" type="application/atom+'>,
        ...

If you wonder why the namespace removal procedure isn't always called by default
instead of having to call it manually, this is because of two reasons, which, in order
of relevance, are:

1. Removing namespaces requires to iterate and modify all nodes in the
   document, which is a reasonably expensive operation to perform by default
   for all documents crawled by Scrapy

2. There could be some cases where using namespaces is actually required, in
   case some element names clash between namespaces. These cases are very rare
   though.


Using EXSLT extensions
----------------------

Being built atop `lxml`_, Scrapy selectors support some `EXSLT`_ extensions
and come with these pre-registered namespaces to use in XPath expressions:


======  =====================================    =======================
prefix  namespace                                usage
======  =====================================    =======================
re      \http://exslt.org/regular-expressions    `regular expressions`_
set     \http://exslt.org/sets                   `set manipulation`_
======  =====================================    =======================

Regular expressions
~~~~~~~~~~~~~~~~~~~

The ``test()`` function, for example, can prove quite useful when XPath's
``starts-with()`` or ``contains()`` are not sufficient.

Example selecting links in list item with a "class" attribute ending with a digit:

.. code-block:: pycon

    >>> from scrapy import Selector
    >>> doc = """
    ... <div>
    ...     <ul>
    ...         <li class="item-0"><a href="link1.html">first item</a></li>
    ...         <li class="item-1"><a href="link2.html">second item</a></li>
    ...         <li class="item-inactive"><a href="link3.html">third item</a></li>
    ...         <li class="item-1"><a href="link4.html">fourth item</a></li>
    ...         <li class="item-0"><a href="link5.html">fifth item</a></li>
    ...     </ul>
    ... </div>
    ... """
    >>> sel = Selector(text=doc, type="html")
    >>> sel.xpath("//li//@href").getall()
    ['link1.html', 'link2.html', 'link3.html', 'link4.html', 'link5.html']
    >>> sel.xpath('//li[re:test(@class, "item-\d$")]//@href').getall()
    ['link1.html', 'link2.html', 'link4.html', 'link5.html']

.. warning:: C library ``libxslt`` doesn't natively support EXSLT regular
    expressions so `lxml`_'s implementation uses hooks to Python's ``re`` module.
    Thus, using regexp functions in your XPath expressions may add a small
    performance penalty.

Set operations
~~~~~~~~~~~~~~

These can be handy for excluding parts of a document tree before
extracting text elements for example.

Example extracting microdata (sample content taken from https://schema.org/Product)
with groups of itemscopes and corresponding itemprops:

.. code-block:: pycon

    >>> doc = """
    ... <div itemscope itemtype="http://schema.org/Product">
    ...   <span itemprop="name">Kenmore White 17" Microwave</span>
    ...   <img src="kenmore-microwave-17in.jpg" alt='Kenmore 17" Microwave' />
    ...   <div itemprop="aggregateRating"
    ...     itemscope itemtype="http://schema.org/AggregateRating">
    ...    Rated <span itemprop="ratingValue">3.5</span>/5
    ...    based on <span itemprop="reviewCount">11</span> customer reviews
    ...   </div>
    ...   <div itemprop="offers" itemscope itemtype="http://schema.org/Offer">
    ...     <span itemprop="price">$55.00</span>
    ...     <link itemprop="availability" href="http://schema.org/InStock" />In stock
    ...   </div>
    ...   Product description:
    ...   <span itemprop="description">0.7 cubic feet countertop microwave.
    ...   Has six preset cooking categories and convenience features like
    ...   Add-A-Minute and Child Lock.</span>
    ...   Customer reviews:
    ...   <div itemprop="review" itemscope itemtype="http://schema.org/Review">
    ...     <span itemprop="name">Not a happy camper</span> -
    ...     by <span itemprop="author">Ellie</span>,
    ...     <meta itemprop="datePublished" content="2011-04-01">April 1, 2011
    ...     <div itemprop="reviewRating" itemscope itemtype="http://schema.org/Rating">
    ...       <meta itemprop="worstRating" content = "1">
    ...       <span itemprop="ratingValue">1</span>/
    ...       <span itemprop="bestRating">5</span>stars
    ...     </div>
    ...     <span itemprop="description">The lamp burned out and now I have to replace
    ...     it. </span>
    ...   </div>
    ...   <div itemprop="review" itemscope itemtype="http://schema.org/Review">
    ...     <span itemprop="name">Value purchase</span> -
    ...     by <span itemprop="author">Lucas</span>,
    ...     <meta itemprop="datePublished" content="2011-03-25">March 25, 2011
    ...     <div itemprop="reviewRating" itemscope itemtype="http://schema.org/Rating">
    ...       <meta itemprop="worstRating" content = "1"/>
    ...       <span itemprop="ratingValue">4</span>/
    ...       <span itemprop="bestRating">5</span>stars
    ...     </div>
    ...     <span itemprop="description">Great microwave for the price. It is small and
    ...     fits in my apartment.</span>
    ...   </div>
    ...   ...
    ... </div>
    ... """
    >>> sel = Selector(text=doc, type="html")
    >>> for scope in sel.xpath("//div[@itemscope]"):
    ...     print("current scope:", scope.xpath("@itemtype").getall())
    ...     props = scope.xpath(
    ...         """
    ...                 set:difference(./descendant::*/@itemprop,
    ...                                .//*[@itemscope]/*/@itemprop)"""
    ...     )
    ...     print(f"    properties: {props.getall()}")
    ...     print("")
    ...

    current scope: ['http://schema.org/Product']
        properties: ['name', 'aggregateRating', 'offers', 'description', 'review', 'review']

    current scope: ['http://schema.org/AggregateRating']
        properties: ['ratingValue', 'reviewCount']

    current scope: ['http://schema.org/Offer']
        properties: ['price', 'availability']

    current scope: ['http://schema.org/Review']
        properties: ['name', 'author', 'datePublished', 'reviewRating', 'description']

    current scope: ['http://schema.org/Rating']
        properties: ['worstRating', 'ratingValue', 'bestRating']

    current scope: ['http://schema.org/Review']
        properties: ['name', 'author', 'datePublished', 'reviewRating', 'description']

    current scope: ['http://schema.org/Rating']
        properties: ['worstRating', 'ratingValue', 'bestRating']


Here we first iterate over ``itemscope`` elements, and for each one,
we look for all ``itemprops`` elements and exclude those that are themselves
inside another ``itemscope``.

.. _EXSLT: http://exslt.org/
.. _regular expressions: http://exslt.org/regexp/index.html
.. _set manipulation: http://exslt.org/set/index.html

Other XPath extensions
----------------------

Scrapy selectors also provide a sorely missed XPath extension function
``has-class`` that returns ``True`` for nodes that have all of the specified
HTML classes.

.. highlight:: html

For the following HTML::

    <p class="foo bar-baz">First</p>
    <p class="foo">Second</p>
    <p class="bar">Third</p>
    <p>Fourth</p>

.. highlight:: python

You can use it like this:

.. code-block:: pycon

    >>> response.xpath('//p[has-class("foo")]')
    [<Selector xpath='//p[has-class("foo")]' data='<p class="foo bar-baz">First</p>'>,
    <Selector xpath='//p[has-class("foo")]' data='<p class="foo">Second</p>'>]
    >>> response.xpath('//p[has-class("foo", "bar-baz")]')
    [<Selector xpath='//p[has-class("foo", "bar-baz")]' data='<p class="foo bar-baz">First</p>'>]
    >>> response.xpath('//p[has-class("foo", "bar")]')
    []

So XPath ``//p[has-class("foo", "bar-baz")]`` is roughly equivalent to CSS
``p.foo.bar-baz``.  Please note, that it is slower in most of the cases,
because it's a pure-Python function that's invoked for every node in question
whereas the CSS lookup is translated into XPath and thus runs more efficiently,
so performance-wise its uses are limited to situations that are not easily
described with CSS selectors.

Parsel also simplifies adding your own XPath extensions.

.. autofunction:: parsel.xpathfuncs.set_xpathfunc


.. _topics-selectors-ref:

Built-in Selectors reference
============================

.. module:: scrapy.selector
   :synopsis: Selector class

Selector objects
----------------

.. autoclass:: Selector

  .. automethod:: xpath

      .. note::

          For convenience, this method can be called as ``response.xpath()``

  .. automethod:: css

      .. note::

          For convenience, this method can be called as ``response.css()``

  .. automethod:: get

     See also: :ref:`old-extraction-api`

  .. autoattribute:: attrib

     See also: :ref:`selecting-attributes`.

  .. automethod:: re

  .. automethod:: re_first

  .. automethod:: register_namespace

  .. automethod:: remove_namespaces

  .. automethod:: __bool__

  .. automethod:: getall

     This method is added to Selector for consistency; it is more useful
     with SelectorList. See also: :ref:`old-extraction-api`

SelectorList objects
--------------------

.. autoclass:: SelectorList

   .. automethod:: xpath

   .. automethod:: css

   .. automethod:: getall

      See also: :ref:`old-extraction-api`

   .. automethod:: get

      See also: :ref:`old-extraction-api`

   .. automethod:: re

   .. automethod:: re_first

   .. autoattribute:: attrib

      See also: :ref:`selecting-attributes`.

.. _selector-examples:

Examples
========

.. _selector-examples-html:

Selector examples on HTML response
----------------------------------

Here are some :class:`Selector` examples to illustrate several concepts.
In all cases, we assume there is already a :class:`Selector` instantiated with
a :class:`~scrapy.http.HtmlResponse` object like this:

.. code-block:: python

      sel = Selector(html_response)

1. Select all ``<h1>`` elements from an HTML response body, returning a list of
   :class:`Selector` objects (i.e. a :class:`SelectorList` object):

   .. code-block:: python

      sel.xpath("//h1")

2. Extract the text of all ``<h1>`` elements from an HTML response body,
   returning a list of strings:

   .. code-block:: python

      sel.xpath("//h1").getall()  # this includes the h1 tag
      sel.xpath("//h1/text()").getall()  # this excludes the h1 tag

3. Iterate over all ``<p>`` tags and print their class attribute:


   .. code-block:: python

      for node in sel.xpath("//p"):
          print(node.attrib["class"])


.. _selector-examples-xml:

Selector examples on XML response
---------------------------------

Here are some examples to illustrate concepts for :class:`Selector` objects
instantiated with an :class:`~scrapy.http.XmlResponse` object:

.. code-block:: python

      sel = Selector(xml_response)

1. Select all ``<product>`` elements from an XML response body, returning a list
   of :class:`Selector` objects (i.e. a :class:`SelectorList` object):

   .. code-block:: python

      sel.xpath("//product")

2. Extract all prices from a `Google Base XML feed`_ which requires registering
   a namespace:

   .. code-block:: python

      sel.register_namespace("g", "http://base.google.com/ns/1.0")
      sel.xpath("//g:price").getall()

.. _Google Base XML feed: https://support.google.com/merchants/answer/160589?hl=en&ref_topic=2473799
