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ReStructuredText
317 lines
11 KiB
ReStructuredText
.. _faq:
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Frequently Asked Questions
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==========================
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How does Scrapy compare to BeautifulSoup or lxml?
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-------------------------------------------------
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`BeautifulSoup`_ and `lxml`_ are libraries for parsing HTML and XML. Scrapy is
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an application framework for writing web spiders that crawl web sites and
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extract data from them.
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Scrapy provides a built-in mechanism for extracting data (called
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:ref:`selectors <topics-selectors>`) but you can easily use `BeautifulSoup`_
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(or `lxml`_) instead, if you feel more comfortable working with them. After
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all, they're just parsing libraries which can be imported and used from any
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Python code.
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In other words, comparing `BeautifulSoup`_ (or `lxml`_) to Scrapy is like
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comparing `jinja2`_ to `Django`_.
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.. _BeautifulSoup: http://www.crummy.com/software/BeautifulSoup/
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.. _lxml: http://codespeak.net/lxml/
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.. _jinja2: http://jinja.pocoo.org/2/
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.. _Django: http://www.djangoproject.com
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.. _faq-python-versions:
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What Python versions does Scrapy support?
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-----------------------------------------
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Scrapy runs in Python 2.6 and 2.7.
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Does Scrapy work with Python 3.0?
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---------------------------------
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No, and there are no plans to port Scrapy to Python 3.0 yet. At the moment,
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Scrapy works with Python 2.6 and 2.7.
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.. seealso:: :ref:`faq-python-versions`.
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Did Scrapy "steal" X from Django?
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---------------------------------
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Probably, but we don't like that word. We think Django_ is a great open source
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project and an example to follow, so we've used it as an inspiration for
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Scrapy.
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We believe that, if something is already done well, there's no need to reinvent
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it. This concept, besides being one of the foundations for open source and free
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software, not only applies to software but also to documentation, procedures,
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policies, etc. So, instead of going through each problem ourselves, we choose
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to copy ideas from those projects that have already solved them properly, and
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focus on the real problems we need to solve.
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We'd be proud if Scrapy serves as an inspiration for other projects. Feel free
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to steal from us!
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.. _Django: http://www.djangoproject.com
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Does Scrapy work with HTTP proxies?
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-----------------------------------
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Yes. Support for HTTP proxies is provided (since Scrapy 0.8) through the HTTP
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Proxy downloader middleware. See
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:class:`~scrapy.contrib.downloadermiddleware.httpproxy.HttpProxyMiddleware`.
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How can I scrape an item with attributes in different pages?
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------------------------------------------------------------
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See :ref:`topics-request-response-ref-request-callback-arguments`.
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Scrapy crashes with: ImportError: No module named win32api
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----------------------------------------------------------
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You need to install `pywin32`_ because of `this Twisted bug`_.
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.. _pywin32: http://sourceforge.net/projects/pywin32/
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.. _this Twisted bug: http://twistedmatrix.com/trac/ticket/3707
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How can I simulate a user login in my spider?
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---------------------------------------------
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See :ref:`topics-request-response-ref-request-userlogin`.
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Does Scrapy crawl in breath-first or depth-first order?
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-------------------------------------------------------
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By default, Scrapy uses a `LIFO`_ queue for storing pending requests, which
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basically means that it crawls in `DFO order`_. This order is more convenient
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in most cases. If you do want to crawl in true `BFO order`_, you can do it by
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setting the following settings::
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DEPTH_PRIORITY = 1
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SCHEDULER_DISK_QUEUE = 'scrapy.squeue.PickleFifoDiskQueue'
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SCHEDULER_MEMORY_QUEUE = 'scrapy.squeue.FifoMemoryQueue'
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My Scrapy crawler has memory leaks. What can I do?
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--------------------------------------------------
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See :ref:`topics-leaks`.
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Also, Python has a builtin memory leak issue which is described in
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:ref:`topics-leaks-without-leaks`.
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How can I make Scrapy consume less memory?
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------------------------------------------
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See previous question.
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Can I use Basic HTTP Authentication in my spiders?
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--------------------------------------------------
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Yes, see :class:`~scrapy.contrib.downloadermiddleware.httpauth.HttpAuthMiddleware`.
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Why does Scrapy download pages in English instead of my native language?
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------------------------------------------------------------------------
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Try changing the default `Accept-Language`_ request header by overriding the
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:setting:`DEFAULT_REQUEST_HEADERS` setting.
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.. _Accept-Language: http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.4
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Where can I find some example Scrapy projects?
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----------------------------------------------
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See :ref:`intro-examples`.
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Can I run a spider without creating a project?
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----------------------------------------------
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Yes. You can use the :command:`runspider` command. For example, if you have a
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spider written in a ``my_spider.py`` file you can run it with::
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scrapy runspider my_spider.py
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See :command:`runspider` command for more info.
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I get "Filtered offsite request" messages. How can I fix them?
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--------------------------------------------------------------
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Those messages (logged with ``DEBUG`` level) don't necessarily mean there is a
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problem, so you may not need to fix them.
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Those message are thrown by the Offsite Spider Middleware, which is a spider
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middleware (enabled by default) whose purpose is to filter out requests to
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domains outside the ones covered by the spider.
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For more info see:
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:class:`~scrapy.contrib.spidermiddleware.offsite.OffsiteMiddleware`.
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What is the recommended way to deploy a Scrapy crawler in production?
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---------------------------------------------------------------------
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See :ref:`topics-scrapyd`.
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Can I use JSON for large exports?
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---------------------------------
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It'll depend on how large your output is. See :ref:`this warning
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<json-with-large-data>` in :class:`~scrapy.contrib.exporter.JsonItemExporter`
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documentation.
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Can I return (Twisted) deferreds from signal handlers?
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------------------------------------------------------
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Some signals support returning deferreds from their handlers, others don't. See
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the :ref:`topics-signals-ref` to know which ones.
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What does the response status code 999 means?
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---------------------------------------------
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999 is a custom reponse status code used by Yahoo sites to throttle requests.
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Try slowing down the crawling speed by using a download delay of ``2`` (or
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higher) in your spider::
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class MySpider(CrawlSpider):
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name = 'myspider'
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download_delay = 2
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# [ ... rest of the spider code ... ]
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Or by setting a global download delay in your project with the
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:setting:`DOWNLOAD_DELAY` setting.
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Can I call ``pdb.set_trace()`` from my spiders to debug them?
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-------------------------------------------------------------
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Yes, but you can also use the Scrapy shell which allows you too quickly analyze
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(and even modify) the response being processed by your spider, which is, quite
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often, more useful than plain old ``pdb.set_trace()``.
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For more info see :ref:`topics-shell-inspect-response`.
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Simplest way to dump all my scraped items into a JSON/CSV/XML file?
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-------------------------------------------------------------------
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To dump into a JSON file::
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scrapy crawl myspider -o items.json -t json
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To dump into a CSV file::
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scrapy crawl myspider -o items.csv -t csv
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To dump into a XML file::
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scrapy crawl myspider -o items.xml -t xml
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For more information see :ref:`topics-feed-exports`
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What's this huge cryptic ``__VIEWSTATE`` parameter used in some forms?
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----------------------------------------------------------------------
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The ``__VIEWSTATE`` parameter is used in sites built with ASP.NET/VB.NET. For
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more info on how it works see `this page`_. Also, here's an `example spider`_
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which scrapes one of these sites.
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.. _this page: http://search.cpan.org/~ecarroll/HTML-TreeBuilderX-ASP_NET-0.09/lib/HTML/TreeBuilderX/ASP_NET.pm
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.. _example spider: http://github.com/AmbientLighter/rpn-fas/blob/master/fas/spiders/rnp.py
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What's the best way to parse big XML/CSV data feeds?
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----------------------------------------------------
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Parsing big feeds with XPath selectors can be problematic since they need to
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build the DOM of the entire feed in memory, and this can be quite slow and
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consume a lot of memory.
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In order to avoid parsing all the entire feed at once in memory, you can use
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the functions ``xmliter`` and ``csviter`` from ``scrapy.utils.iterators``
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module. In fact, this is what the feed spiders (see :ref:`topics-spiders`) use
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under the cover.
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Does Scrapy manage cookies automatically?
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-----------------------------------------
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Yes, Scrapy receives and keeps track of cookies sent by servers, and sends them
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back on subsequent requests, like any regular web browser does.
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For more info see :ref:`topics-request-response` and :ref:`cookies-mw`.
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How can I see the cookies being sent and received from Scrapy?
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--------------------------------------------------------------
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Enable the :setting:`COOKIES_DEBUG` setting.
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How can I instruct a spider to stop itself?
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-------------------------------------------
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Raise the :exc:`~scrapy.exceptions.CloseSpider` exception from a callback. For
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more info see: :exc:`~scrapy.exceptions.CloseSpider`.
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How can I prevent my Scrapy bot from getting banned?
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----------------------------------------------------
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See :ref:`bans`.
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Should I use spider arguments or settings to configure my spider?
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-----------------------------------------------------------------
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Both :ref:`spider arguments <spiderargs>` and :ref:`settings <topics-settings>`
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can be used to configure your spider. There is no strict rule that mandates to
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use one or the other, but settings are more suited for parameters that, once
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set, don't change much, while spider arguments are meant to change more often,
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even on each spider run and sometimes are required for the spider to run at all
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(for example, to set the start url of a spider).
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To illustrate with an example, assuming you have a spider that needs to log
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into a site to scrape data, and you only want to scrape data from a certain
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section of the site (which varies each time). In that case, the credentials to
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log in would be settings, while the url of the section to scrape would be a
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spider argument.
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I'm scraping a XML document and my XPath selector doesn't return any items
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--------------------------------------------------------------------------
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You may need to remove namespaces. See :ref:`removing-namespaces`.
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I'm getting an error: "cannot import name crawler"
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--------------------------------------------------
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This is caused by Scrapy changes due to the singletons removal. The error is
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most likely raised by a module (extension, middleware, pipeline or spider) in
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your Scrapy project that imports ``crawler`` from ``scrapy.project``. For
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example::
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from scrapy.project import crawler
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class SomeExtension(object):
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def __init__(self):
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self.crawler = crawler
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# ...
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This way to access the crawler object is deprecated, the code should be ported
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to use ``from_crawler`` class method, for example::
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class SomeExtension(object):
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@classmethod
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def from_crawler(cls, crawler):
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o = cls()
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o.crawler = crawler
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return o
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Scrapy command line tool has some backwards compatibility in place to support
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the old import mechanism (with a deprecation warning), but this mechanism may
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not work if you use Scrapy differently (for example, as a library).
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.. _user agents: http://en.wikipedia.org/wiki/User_agent
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.. _LIFO: http://en.wikipedia.org/wiki/LIFO
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.. _DFO order: http://en.wikipedia.org/wiki/Depth-first_search
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.. _BFO order: http://en.wikipedia.org/wiki/Breadth-first_search
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