.. _topics-spider-middleware: ================= Spider Middleware ================= The spider middleware is a framework of hooks into Scrapy's spider processing mechanism where you can plug custom functionality to process the requests that are sent to :ref:`topics-spiders` for processing and to process the responses and item that are generated from spiders. .. _topics-spider-middleware-setting: Activating a spider middleware ============================== To activate a spider middleware component, add it to the :setting:`SPIDER_MIDDLEWARES` setting, which is a dict whose keys are the middleware class path and their values are the middleware orders. Here's an example:: SPIDER_MIDDLEWARES = { 'myproject.middlewares.CustomSpiderMiddleware': 543, } The :setting:`SPIDER_MIDDLEWARES` setting is merged with the :setting:`SPIDER_MIDDLEWARES_BASE` setting defined in Scrapy (and not meant to be overridden) and then sorted by order to get the final sorted list of enabled middlewares: the first middleware is the one closer to the engine and the last is the one closer to the spider. To decide which order to assign to your middleware see the :setting:`SPIDER_MIDDLEWARES_BASE` setting and pick a value according to where you want to insert the middleware. The order does matter because each middleware performs a different action and your middleware could depend on some previous (or subsequent) middleware being applied. If you want to disable a builtin middleware (the ones defined in :setting:`SPIDER_MIDDLEWARES_BASE`, and enabled by default) you must define it in your project :setting:`SPIDER_MIDDLEWARES` setting and assign `None` as its value. For example, if you want to disable the off-site middleware:: SPIDER_MIDDLEWARES = { 'myproject.middlewares.CustomSpiderMiddleware': 543, 'scrapy.contrib.spidermiddleware.offsite.OffsiteMiddleware': None, } Finally, keep in mind that some middlewares may need to be enabled through a particular setting. See each middleware documentation for more info. Writing your own spider middleware ================================== Writing your own spider middleware is easy. Each middleware component is a single Python class that defines one or more of the following methods: .. class:: SpiderMiddleware .. method:: process_spider_input(response, spider) This method is called for each response that goes through the spider middleware and into the spider, for processing. :meth:`process_spider_input` should return either ``None`` or an iterable of :class:`~scrapy.http.Request` or :class:`~scrapy.item.Item` objects. If it returns ``None``, Scrapy will continue processing this response, executing all other middlewares until, finally, the response is handled to the spider for processing. If returns an iterable, Scrapy won't bother calling ANY other spider middleware ``process_spider_input()`` and will return the iterable back in the other direction for the ``process_spider_exception()`` and ``process_spider_output()`` methods to hook it. :param reponse: the response being processed :type response: :class:`~scrapy.http.Response` object :param spider: the spider for which this response is intended :type spider: :class:`~scrapy.spider.BaseSpider` object .. method:: process_spider_output(response, result, spider) This method is called with the results returned from the Spider, after it has processed the response. :meth:`process_spider_output` must return an iterable of :class:`~scrapy.http.Request` or :class:`~scrapy.item.Item` objects. :param response: the response which generated this output from the spider :type response: class:`~scrapy.http.Response` object :param result: the result returned by the spider :type result: an iterable of :class:`~scrapy.http.Request` or :class:`~scrapy.item.Item` objects :param spider: the spider whose result is being processed :type spider: :class:`~scrapy.item.BaseSpider` object .. method:: process_spider_exception(response, exception, spider) This method is called when when a spider or :meth:process_spider_input: method (from other spider middleware) raises an exception. :meth:`process_spider_exception` should return either ``None`` or an iterable of :class:`~scrapy.http.Response` or :class:`~scrapy.item.Item` objects. If it returns ``None``, Scrapy will continue processing this exception, executing any other :meth:`process_spider_exception` in the following middleware components, until no middleware components are left and the exception reaches the engine (where it's logged and discarded). If it returns an iterable the :meth:`process_spider_output` pipeline kicks in, and no other :meth:`process_spider_exception` will be called. :param response: the response being processed when the exception was raised :type response: :class:`~scrapy.http.Response` object :param exception: the exception raised :type exception: `Exception`_ object :param spider: the spider which raised the exception :type spider: :class:`scrapy.spider.BaseSpider` object .. _Exception: http://docs.python.org/library/exceptions.html#exceptions.Exception .. _topics-spider-middleware-ref: Built-in spider middleware reference ==================================== This page describes all spider middleware components that come with Scrapy. For information on how to use them and how to write your own spider middleware, see the :ref:`spider middleware usage guide `. For a list of the components enabled by default (and their orders) see the :setting:`SPIDER_MIDDLEWARES_BASE` setting. DepthMiddleware --------------- .. module:: scrapy.contrib.spidermiddleware.depth :synopsis: Depth Spider Middleware .. class:: DepthMiddleware DepthMiddleware is a scrape middleware used for tracking the depth of each Request inside the site being scraped. It can be used to limit the maximum depth to scrape or things like that. The :class:`DepthMiddleware` can be configured through the following settings (see the settings documentation for more info): * :setting:`DEPTH_LIMIT` - The maximum depth that will be allowed to crawl for any site. If zero, no limit will be imposed. * :setting:`DEPTH_STATS` - Whether to collect depth stats. HttpErrorMiddleware ------------------- .. module:: scrapy.contrib.spidermiddleware.httperror :synopsis: HTTP Error Spider Middleware .. class:: HttpErrorMiddleware Filter out unsuccessful (erroneous) HTTP responses so that spiders don't have to deal with them, which (most of the times) imposes an overhead, consumes more resources, and makes the spider logic more complex. According to the `HTTP standard`_, successful responses are those whose status codes are in the 200-300 range. .. _HTTP standard: http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html If you still want to process response codes outside that range, you can specify which response codes the spider is able to handle using the ``handle_httpstatus_list`` spider attribute. For example, if you want your spider to handle 404 responses you can do this:: class MySpider(CrawlSpider): handle_httpstatus_list = [404] Keep in mind, however, that it's usually a bad idea to handle non-200 responses, unless you really know what you're doing. For more information see: `HTTP Status Code Definitions`_. .. _HTTP Status Code Definitions: http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html OffsiteMiddleware ----------------- .. module:: scrapy.contrib.spidermiddleware.offsite :synopsis: Offiste Spider Middleware .. class:: OffsiteMiddleware Filters out Requests for URLs outside the domains covered by the spider. This middleware filters out every request whose host names doesn't match :attr:`~scrapy.spider.BaseSpider.domain_name`, or the spider :attr:`~scrapy.spider.BaseSpider.domain_name` prefixed by "www.". Spider can add more domains to exclude using :attr:`~scrapy.spider.BaseSpider.extra_domain_names` attribute. RequestLimitMiddleware ---------------------- .. module:: scrapy.contrib.spidermiddleware.requestlimit :synopsis: Request limit Spider Middleware .. class:: RequestLimitMiddleware Limits the maximum number of requests in the scheduler for each spider. When a spider tries to schedule more than the allowed amount of requests, the new requests (returned by the spider) will be dropped. The :class:`RequestLimitMiddleware` can be configured through the following settings (see the settings documentation for more info): * :setting:`REQUESTS_QUEUE_SIZE` - If non zero, it will be used as an upper limit for the amount of requests that can be scheduled per domain. Can be set per spider using ``requests_queue_size`` attribute. RestrictMiddleware ------------------ .. module:: scrapy.contrib.spidermiddleware.restrict :synopsis: Restrict Spider Middleware .. class:: RestrictMiddleware Restricts crawling to fixed set of particular URLs. The :class:`RestrictMiddleware` can be configured through the following settings (see the settings documentation for more info): * :setting:`RESTRICT_TO_URLS` - Set of URLs allowed to crawl. UrlFilterMiddleware ------------------- .. module:: scrapy.contrib.spidermiddleware.urlfilter :synopsis: URL Filter Spider Middleware .. class:: UrlFilterMiddleware Canonicalizes URLs to filter out duplicated ones UrlLengthMiddleware ------------------- .. module:: scrapy.contrib.spidermiddleware.urllength :synopsis: URL Length Spider Middleware .. class:: UrlLengthMiddleware Filters out requests with URLs longer than URLLENGTH_LIMIT The :class:`UrlLengthMiddleware` can be configured through the following settings (see the settings documentation for more info): * :setting:`URLLENGTH_LIMIT` - The maximum URL length to allow for crawled URLs.