scrapy/docs/topics/firebug.rst

.. _topics-firebug:

==========================
Using Firebug for scraping
==========================

.. note:: Google Directory, the example website used in this guide is no longer
   available as it `has been shut down by Google`_. The concepts in this guide
   are still valid though. If you want to update this guide to use a new
   (working) site, your contribution will be more than welcome!. See :ref:`topics-contributing`
   for information on how to do so.

Introduction
============

This document explains how to use `Firebug`_ (a Firefox add-on) to make the
scraping process easier and more fun. For other useful Firefox add-ons see
:ref:`topics-firefox-addons`. There are some caveats with using Firefox add-ons
to inspect pages, see :ref:`topics-firefox-livedom`.

In this example, we'll show how to use `Firebug`_ to scrape data from the
`Google Directory`_, which contains the same data as the `Open Directory
Project`_ used in the :ref:`tutorial <intro-tutorial>` but with a different
face.

.. _Firebug: http://getfirebug.com
.. _Google Directory: http://directory.google.com/
.. _Open Directory Project: http://www.dmoz.org

Firebug comes with a very useful feature called `Inspect Element`_ which allows
you to inspect the HTML code of the different page elements just by hovering
your mouse over them. Otherwise you would have to search for the tags manually
through the HTML body which can be a very tedious task.

.. _Inspect Element: http://www.youtube.com/watch?v=-pT_pDe54aA

In the following screenshot you can see the `Inspect Element`_ tool in action.

.. image:: _images/firebug1.png
   :width: 913
   :height: 600
   :alt: Inspecting elements with Firebug

At first sight, we can see that the directory is divided in categories, which
are also divided in subcategories.

However, it seems that there are more subcategories than the ones being shown
in this page, so we'll keep looking:

.. image:: _images/firebug2.png
   :width: 819
   :height: 629
   :alt: Inspecting elements with Firebug

As expected, the subcategories contain links to other subcategories, and also
links to actual websites, which is the purpose of the directory.

Getting links to follow
=======================

By looking at the category URLs we can see they share a pattern:

    http://directory.google.com/Category/Subcategory/Another_Subcategory

Once we know that, we are able to construct a regular expression to follow
those links. For example, the following one::

    directory\.google\.com/[A-Z][a-zA-Z_/]+$

So, based on that regular expression we can create the first crawling rule::

    Rule(LinkExtractor(allow='directory.google.com/[A-Z][a-zA-Z_/]+$', ),
        'parse_category',
        follow=True,
    ),

The :class:`~scrapy.contrib.spiders.Rule` object instructs
:class:`~scrapy.contrib.spiders.CrawlSpider` based spiders how to follow the
category links. ``parse_category`` will be a method of the spider which will
process and extract data from those pages.

This is how the spider would look so far::

   from scrapy.contrib.linkextractors import LinkExtractor
   from scrapy.contrib.spiders import CrawlSpider, Rule

   class GoogleDirectorySpider(CrawlSpider):
       name = 'directory.google.com'
       allowed_domains = ['directory.google.com']
       start_urls = ['http://directory.google.com/']

       rules = (
           Rule(LinkExtractor(allow='directory\.google\.com/[A-Z][a-zA-Z_/]+$'),
               'parse_category', follow=True,
           ),
       )

       def parse_category(self, response):
           # write the category page data extraction code here
           pass


Extracting the data
===================

Now we're going to write the code to extract data from those pages.

With the help of Firebug, we'll take a look at some page containing links to
websites (say http://directory.google.com/Top/Arts/Awards/) and find out how we can
extract those links using :ref:`Selectors <topics-selectors>`. We'll also
use the :ref:`Scrapy shell <topics-shell>` to test those XPath's and make sure
they work as we expect.

.. image:: _images/firebug3.png
   :width: 965
   :height: 751
   :alt: Inspecting elements with Firebug

As you can see, the page markup is not very descriptive: the elements don't
contain ``id``, ``class`` or any attribute that clearly identifies them, so
we''ll use the ranking bars as a reference point to select the data to extract
when we construct our XPaths.

After using FireBug, we can see that each link is inside a ``td`` tag, which is
itself inside a ``tr`` tag that also contains the link's ranking bar (in
another ``td``).

So we can select the ranking bar, then find its parent (the ``tr``), and then
finally, the link's ``td`` (which contains the data we want to scrape).

This results in the following XPath::

    //td[descendant::a[contains(@href, "#pagerank")]]/following-sibling::td//a

It's important to use the :ref:`Scrapy shell <topics-shell>` to test these
complex XPath expressions and make sure they work as expected.

Basically, that expression will look for the ranking bar's ``td`` element, and
then select any ``td`` element who has a descendant ``a`` element whose
``href`` attribute contains the string ``#pagerank``"

Of course, this is not the only XPath, and maybe not the simpler one to select
that data. Another approach could be, for example, to find any ``font`` tags
that have that grey colour of the links,

Finally, we can write our ``parse_category()`` method::

    def parse_category(self, response):
        # The path to website links in directory page
        links = response.xpath('//td[descendant::a[contains(@href, "#pagerank")]]/following-sibling::td/font')

        for link in links:
            item = DirectoryItem()
            item['name'] = link.xpath('a/text()').extract()
            item['url'] = link.xpath('a/@href').extract()
            item['description'] = link.xpath('font[2]/text()').extract()
            yield item


Be aware that you may find some elements which appear in Firebug but
not in the original HTML, such as the typical case of ``<tbody>``
elements.

or tags which Therefer   in page HTML
sources may on Firebug inspects the live DOM

.. _has been shut down by Google: http://searchenginewatch.com/article/2096661/Google-Directory-Has-Been-Shut-Down