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213 lines
6.8 KiB
ReStructuredText
.. _intro-overview:
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==================
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Scrapy at a glance
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==================
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Scrapy is an application framework for crawling web sites and extracting
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structured data which can be used for a wide range of useful applications, like
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data mining, information processing or historical archival.
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Even though Scrapy was originally designed for `screen scraping`_ (more
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precisely, `web scraping`_), it can also be used to extract data using APIs
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(such as `Amazon Associates Web Services`_) or as a general purpose web
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crawler.
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.. _screen scraping: http://en.wikipedia.org/wiki/Screen_scraping
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.. _web scraping: http://en.wikipedia.org/wiki/Web_scraping
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.. _Amazon Associates Web Services: http://aws.amazon.com/associates/
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The purpose of this document is to introduce you to the concepts behind Scrapy
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so you can get an idea of how it works and decide if Scrapy is what you need.
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When you're ready to start a project, you can :ref:`start with the tutorial
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<intro-tutorial>`.
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Pick a website
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==============
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So you need to extract some information from a website, but the website doesn't
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provide any API or mechanism to access that info from a computer program.
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Scrapy can help you extract that information. Let's say we want to extract
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information about all torrent files added today in the `mininova`_ torrent
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site.
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.. _mininova: http://www.mininova.org
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The list of all torrents added today can be found in this page:
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http://www.mininova.org/today
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Write a Spider to extract the Items
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===================================
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Now we'll write a Spider which defines the start URL
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(http://www.mininova.org/today), the rules for following links and the rules
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for extracting the data from pages.
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If we take a look at that page content we'll see that all torrent URLs are like
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http://www.mininova.org/tor/NUMBER where ``NUMBER`` is an integer. We'll use
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that to construct the regular expression for the links to follow: ``/tor/\d+``.
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To extracting data, we'll use `XPath`_ to select the part of the document where
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the data is to be extracted from. Let's take one of those torrent pages:
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http://www.mininova.org/tor/2657665
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.. _XPath: http://www.w3.org/TR/xpath
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And look at the page HTML source to construct the XPath to select the data we
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want which is: torrent name, description and size.
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.. highlight:: html
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By looking at the page HTML source we can see that the file name is contained
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inside a ``<h1>`` tag::
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<h1>Home[2009][Eng]XviD-ovd</h1>
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.. highlight:: none
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An XPath expression to extract the name could be::
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//h1/text()
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.. highlight:: html
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And the description is contained inside a ``<div>`` tag with ``id="description"``::
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<h2>Description:</h2>
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<div id="description">
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"HOME" - a documentary film by Yann Arthus-Bertrand
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<br/>
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<br/>
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***
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<br/>
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<br/>
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"We are living in exceptional times. Scientists tell us that we have 10 years to change the way we live, avert the depletion of natural resources and the catastrophic evolution of the Earth's climate.
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...
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.. highlight:: none
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An XPath expression to select the description could be::
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//div[@id='description']
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.. highlight:: html
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Finally, the file size is contained in the second ``<p>`` tag inside the ``<div>``
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tag with ``id=specifications``::
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<div id="specifications">
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<p>
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<strong>Category:</strong>
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<a href="/cat/4">Movies</a> > <a href="/sub/35">Documentary</a>
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</p>
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<p>
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<strong>Total size:</strong>
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699.79 megabyte</p>
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.. highlight:: none
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An XPath expression to select the description could be::
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//div[@id='specifications']/p[2]/text()[2]
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.. highlight:: python
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For more information about XPath see the `XPath reference`_.
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.. _XPath reference: http://www.w3.org/TR/xpath
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Finally, here's the spider code::
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class MininovaSpider(CrawlSpider):
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name = 'mininova.org'
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allowed_domains = ['mininova.org']
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start_urls = ['http://www.mininova.org/today']
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rules = [Rule(SgmlLinkExtractor(allow=['/tor/\d+']), 'parse_torrent')]
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def parse_torrent(self, response):
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x = HtmlXPathSelector(response)
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torrent = TorrentItem()
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torrent['url'] = response.url
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torrent['name'] = x.select("//h1/text()").extract()
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torrent['description'] = x.select("//div[@id='description']").extract()
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torrent['size'] = x.select("//div[@id='info-left']/p[2]/text()[2]").extract()
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return torrent
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For brevity's sake, we intentionally left out the import statements and the
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Torrent class definition (which is included some paragraphs above).
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Write a pipeline to store the items extracted
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=============================================
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Now let's write an :ref:`topics-item-pipeline` that serializes and stores the
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extracted item into a file using `pickle`_::
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import pickle
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class StoreItemPipeline(object):
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def process_item(self, item, spider):
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torrent_id = item['url'].split('/')[-1]
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f = open("torrent-%s.pickle" % torrent_id, "w")
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pickle.dump(item, f)
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f.close()
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.. _pickle: http://docs.python.org/library/pickle.html
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What else?
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==========
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You've seen how to extract and store items from a website using Scrapy, but
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this is just the surface. Scrapy provides a lot of powerful features for making
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scraping easy and efficient, such as:
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* Built-in support for :ref:`selecting and extracting <topics-selectors>` data
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from HTML and XML sources
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* Built-in support for :ref:`generating feed exports <topics-feed-exports>` in
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multiple formats (JSON, CSV, XML) and storing them in multiple backends (FTP,
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S3, filesystem)
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* A media pipeline for :ref:`automatically downloading images <topics-images>`
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(or any other media) associated with the scraped items
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* Support for :ref:`extending Scrapy <extending-scrapy>` by plugging
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your own functionality using middlewares, extensions, and pipelines
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* Wide range of built-in middlewares and extensions for handling of
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compression, cache, cookies, authentication, user-agent spoofing, robots.txt
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handling, statistics, crawl depth restriction, etc
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* An :ref:`Interactive scraping shell console <topics-shell>`, very useful for
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writing and debugging your spiders
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* A builtin :ref:`Web service <topics-webservice>` for monitoring and
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controlling your bot
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* A :ref:`Telnet console <topics-telnetconsole>` for full unrestricted access
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to a Python console inside your Scrapy process, to introspect and debug your
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crawler
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* Built-in facilities for :ref:`logging <topics-logging>`, :ref:`collecting
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stats <topics-stats>`, and :ref:`sending email notifications <topics-email>`
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What's next?
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============
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The next obvious steps are for you to `download Scrapy`_, read :ref:`the
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tutorial <intro-tutorial>` and join `the community`_. Thanks for your
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interest!
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.. _download Scrapy: http://scrapy.org/download/
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.. _the community: http://scrapy.org/community/
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