scrapy/docs/topics/selectors.rst

.. _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:

 * `BeautifulSoup`_ is a very popular screen 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 a XML parsing library (which also parses HTML) with a pythonic
   API based on `ElementTree`_ (which 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.

Scrapy selectors are built over the `lxml`_ library, which means they're very
similar in speed and parsing accuracy.

This page explains how selectors work and describes their API which is very
small and simple, unlike the `lxml`_ API which is much bigger because the
`lxml`_ library can be used for many other tasks, besides selecting markup
documents.

For a complete reference of the selectors API see
:ref:`Selector reference <topics-selectors-ref>`

.. _BeautifulSoup: http://www.crummy.com/software/BeautifulSoup/
.. _lxml: http://codespeak.net/lxml/
.. _ElementTree: http://docs.python.org/library/xml.etree.elementtree.html
.. _cssselect: https://pypi.python.org/pypi/cssselect/
.. _XPath: http://www.w3.org/TR/xpath
.. _CSS: http://www.w3.org/TR/selectors


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

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

.. highlight:: python

Scrapy selectors are instances of :class:`~scrapy.selector.Selector` class
constructed by passing a `Response` object as first argument, the response's
body is what they're going to be "selecting"::

    from scrapy.spider import Spider
    from scrapy.selector import Selector

    class MySpider(Spider):
        # ...
        def parse(self, response):
            sel = Selector(response)
            # Using XPath query
            print sel.xpath('//p')
            # Using CSS query
            print sel.css('p')
            # Nesting queries
            print sel.xpath('//div[@foo="bar"]').css('span#bold')


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:

    http://doc.scrapy.org/en/latest/_static/selectors-sample1.html

.. _topics-selectors-htmlcode:

Here's its HTML code:

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

.. highlight:: sh

First, let's open the shell::

    scrapy shell http://doc.scrapy.org/en/latest/_static/selectors-sample1.html

Then, after the shell loads, you'll have a selector already instantiated and
ready to use in ``sel`` shell variable.

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 (using an HTML selector) for selecting the text
inside the title tag::

    >>> sel.xpath('//title/text()')
    [<Selector (text) xpath=//title/text()>]

As you can see, the ``.xpath()`` method returns an
:class:`~scrapy.selector.SelectorList` instance, which is a list of new
selectors. This API can be used quickly for extracting nested data.

To actually extract the textual data, you must call the selector ``.extract()``
method, as follows::

    >>> sel.xpath('//title/text()').extract()
    [u'Example website']

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

    >>> sel.css('title::text').extract()
    [u'Example website']

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

    >>> sel.xpath('//base/@href').extract()
    [u'http://example.com/']

    >>> sel.css('base::attr(href)').extract()
    [u'http://example.com/']

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

    >>> sel.css('a[href*=image]::attr(href)').extract()
    [u'image1.html',
     u'image2.html',
     u'image3.html',
     u'image4.html',
     u'image5.html']

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

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

.. _topics-selectors-nesting-selectors:

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

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

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

    >>> for index, link in enumerate(links):
            args = (index, link.xpath('@href').extract(), link.xpath('img/@src').extract())
            print 'Link number %d points to url %s and image %s' % args

    Link number 0 points to url [u'image1.html'] and image [u'image1_thumb.jpg']
    Link number 1 points to url [u'image2.html'] and image [u'image2_thumb.jpg']
    Link number 2 points to url [u'image3.html'] and image [u'image3_thumb.jpg']
    Link number 3 points to url [u'image4.html'] and image [u'image4_thumb.jpg']
    Link number 4 points to url [u'image5.html'] and image [u'image5_thumb.jpg']

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

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

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

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

.. _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::

    >>> divs = sel.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::

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

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

    >>> for p in divs.xpath('.//p')  # extracts all <p> inside
    >>>     print p.extract()

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

    >>> for p in divs.xpath('p')
    >>>     print p.extract()

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

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


.. _topics-selectors-ref:

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

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

.. class:: Selector(response=None, text=None, type=None)

  An instance of :class:`Selector` is a wrapper over response to select
  certain parts of its content.

  ``response`` is a :class:`~scrapy.http.HtmlResponse` or
  :class:`~scrapy.http.XmlResponse` object that will be used for selecting and
  extracting data.

  ``text`` is a unicode string or utf-8 encoded text for cases when a
  ``response`` isn't available. Using ``text`` and ``response`` together is
  undefined behavior.

  ``type`` defines the selector type, it can be ``"html"``, ``"xml"`` or ``None`` (default).

    If ``type`` is ``None``, the selector automatically chooses the best type
    based on ``response`` type (see below), or defaults to ``"html"`` in case it
    is used together with ``text``.

    If ``type`` is ``None`` and a ``response`` is passed, the selector type is
    inferred from the response type as follow:

        * ``"html"`` for :class:`~scrapy.http.HtmlResponse` type
        * ``"xml"`` for :class:`~scrapy.http.XmlResponse` type
        * ``"html"`` for anything else

   Otherwise, if ``type`` is set, the selector type will be forced and no
   detection will occur.

  .. method:: xpath(query)

      Find nodes matching the xpath ``query`` and return the result as a
      :class:`SelectorList` instance with all elements flattened. List
      elements implement :class:`Selector` interface too.

      ``query`` is a string containing the XPATH query to apply.

  .. method:: css(query)

      Apply the given CSS selector and return a :class:`SelectorList` instance.

      ``query`` is a string containing the CSS selector to apply.

      In the background, CSS queries are translated into XPath queries using
      `cssselect`_ library and run ``.xpath()`` method.

  .. method:: extract()

     Serialize and return the matched nodes as a list of unicode strings.
     Percent encoded content is unquoted.

  .. method:: re(regex)

     Apply the given regex and return a list of unicode strings with the
     matches.

     ``regex`` can be either a compiled regular expression or a string which
     will be compiled to a regular expression using ``re.compile(regex)``

  .. method:: register_namespace(prefix, uri)

     Register the given namespace to be used in this :class:`Selector`.
     Without registering namespaces you can't select or extract data from
     non-standard namespaces. See examples below.

  .. method:: remove_namespaces()

     Remove all namespaces, allowing to traverse the document using
     namespace-less xpaths. See example below.

  .. method:: __nonzero__()

     Returns ``True`` if there is any real content selected or ``False``
     otherwise.  In other words, the boolean value of a :class:`Selector` is
     given by the contents it selects.


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

.. class:: SelectorList

   The :class:`SelectorList` class is subclass of the builtin ``list``
   class, which provides a few additional methods.

   .. method:: xpath(query)

       Call the ``.xpath()`` method for each element in this list and return
       their results flattened as another :class:`SelectorList`.

       ``query`` is the same argument as the one in :meth:`Selector.xpath`

   .. method:: css(query)

       Call the ``.css()`` method for each element in this list and return
       their results flattened as another :class:`SelectorList`.

       ``query`` is the same argument as the one in :meth:`Selector.css`

   .. method:: extract()

       Call the ``.extract()`` method for each element is this list and return
       their results flattened, as a list of unicode strings.

   .. method:: re()

       Call the ``.re()`` method for each element is this list and return
       their results flattened, as a list of unicode strings.

   .. method:: __nonzero__()

        returns True if the list is not empty, False otherwise.


Selector examples on HTML response
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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

      sel = Selector(html_response)

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

      sel.xpath("//h1")

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

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

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

      for node in sel.xpath("//p"):
      ...    print node.xpath("@class").extract()

Selector examples on XML response
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here's a couple of examples to illustrate several concepts. In both cases we
assume there is already an :class:`Selector` instantiated with a
:class:`~scrapy.http.XmlResponse` object like this::

      sel = Selector(xml_response)

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

      sel.xpath("//product")

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

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

.. _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 Github blog atom feed.

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

    $ scrapy shell https://github.com/blog.atom

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)::

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

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

    >>> sel.remove_namespaces()
    >>> sel.xpath("//link")
    [<Selector xpath='//link' data=u'<link xmlns="http://www.w3.org/2005/Atom'>,
     <Selector xpath='//link' data=u'<link xmlns="http://www.w3.org/2005/Atom'>,
     ...

If you wonder why the namespace removal procedure is not always called, 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 performs 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.

.. _Google Base XML feed: http://base.google.com/support/bin/answer.py?hl=en&answer=59461