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<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> </head>

Downloader Middleware

The downloader middleware is a framework of hooks into Scrapy's request/response processing. It's a light, low-level system for globally altering Scrapy's requests and responses.

Activating a downloader middleware

To activate a downloader middleware component, add it to the :setting:`DOWNLOADER_MIDDLEWARES` setting, which is a dict whose keys are the middleware class paths and their values are the middleware orders.

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Here's an example:

DOWNLOADER_MIDDLEWARES = {
    'myproject.middlewares.CustomDownloaderMiddleware': 543,
}

The :setting:`DOWNLOADER_MIDDLEWARES` setting is merged with the :setting:`DOWNLOADER_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 downloader. In other words, the :meth:`~scrapy.downloadermiddlewares.DownloaderMiddleware.process_request` method of each middleware will be invoked in increasing middleware order (100, 200, 300, ...) and the :meth:`~scrapy.downloadermiddlewares.DownloaderMiddleware.process_response` method of each middleware will be invoked in decreasing order.

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To decide which order to assign to your middleware see the :setting:`DOWNLOADER_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.

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If you want to disable a built-in middleware (the ones defined in :setting:`DOWNLOADER_MIDDLEWARES_BASE` and enabled by default) you must define it in your project's :setting:`DOWNLOADER_MIDDLEWARES` setting and assign None as its value. For example, if you want to disable the user-agent middleware:

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DOWNLOADER_MIDDLEWARES = {
    'myproject.middlewares.CustomDownloaderMiddleware': 543,
    'scrapy.downloadermiddlewares.useragent.UserAgentMiddleware': 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 downloader middleware

Each middleware component is a Python class that defines one or more of the following methods:

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.. module:: scrapy.downloadermiddlewares

Note

Any of the downloader middleware methods may also return a deferred.

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.. method:: process_request(request, spider)

   This method is called for each request that goes through the download
   middleware.

   :meth:`process_request` should either: return ``None``, return a
   :class:`~scrapy.http.Response` object, return a :class:`~scrapy.http.Request`
   object, or raise :exc:`~scrapy.exceptions.IgnoreRequest`.

   If it returns ``None``, Scrapy will continue processing this request, executing all
   other middlewares until, finally, the appropriate downloader handler is called
   the request performed (and its response downloaded).

   If it returns a :class:`~scrapy.http.Response` object, Scrapy won't bother
   calling *any* other :meth:`process_request` or :meth:`process_exception` methods,
   or the appropriate download function; it'll return that response. The :meth:`process_response`
   methods of installed middleware is always called on every response.

   If it returns a :class:`~scrapy.http.Request` object, Scrapy will stop calling
   process_request methods and reschedule the returned request. Once the newly returned
   request is performed, the appropriate middleware chain will be called on
   the downloaded response.

   If it raises an :exc:`~scrapy.exceptions.IgnoreRequest` exception, the
   :meth:`process_exception` methods of installed downloader middleware will be called.
   If none of them handle the exception, the errback function of the request
   (``Request.errback``) is called. If no code handles the raised exception, it is
   ignored and not logged (unlike other exceptions).

   :param request: the request being processed
   :type request: :class:`~scrapy.http.Request` object

   :param spider: the spider for which this request is intended
   :type spider: :class:`~scrapy.spiders.Spider` object

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.. method:: process_response(request, response, spider)

   :meth:`process_response` should either: return a :class:`~scrapy.http.Response`
   object, return a :class:`~scrapy.http.Request` object or
   raise a :exc:`~scrapy.exceptions.IgnoreRequest` exception.

   If it returns a :class:`~scrapy.http.Response` (it could be the same given
   response, or a brand-new one), that response will continue to be processed
   with the :meth:`process_response` of the next middleware in the chain.

   If it returns a :class:`~scrapy.http.Request` object, the middleware chain is
   halted and the returned request is rescheduled to be downloaded in the future.
   This is the same behavior as if a request is returned from :meth:`process_request`.

   If it raises an :exc:`~scrapy.exceptions.IgnoreRequest` exception, the errback
   function of the request (``Request.errback``) is called. If no code handles the raised
   exception, it is ignored and not logged (unlike other exceptions).

   :param request: the request that originated the response
   :type request: is a :class:`~scrapy.http.Request` object

   :param response: 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.spiders.Spider` object

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.. method:: process_exception(request, exception, spider)

   Scrapy calls :meth:`process_exception` when a download handler
   or a :meth:`process_request` (from a downloader middleware) raises an
   exception (including an :exc:`~scrapy.exceptions.IgnoreRequest` exception)

   :meth:`process_exception` should return: either ``None``,
   a :class:`~scrapy.http.Response` object, or a :class:`~scrapy.http.Request` object.

   If it returns ``None``, Scrapy will continue processing this exception,
   executing any other :meth:`process_exception` methods of installed middleware,
   until no middleware is left and the default exception handling kicks in.

   If it returns a :class:`~scrapy.http.Response` object, the :meth:`process_response`
   method chain of installed middleware is started, and Scrapy won't bother calling
   any other :meth:`process_exception` methods of middleware.

   If it returns a :class:`~scrapy.http.Request` object, the returned request is
   rescheduled to be downloaded in the future. This stops the execution of
   :meth:`process_exception` methods of the middleware the same as returning a
   response would.

   :param request: the request that generated the exception
   :type request: is a :class:`~scrapy.http.Request` object

   :param exception: the raised exception
   :type exception: an ``Exception`` object

   :param spider: the spider for which this request is intended
   :type spider: :class:`~scrapy.spiders.Spider` object

Built-in downloader middleware reference

This page describes all downloader middleware components that come with Scrapy. For information on how to use them and how to write your own downloader middleware, see the :ref:`downloader middleware usage guide <topics-downloader-middleware>`.

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For a list of the components enabled by default (and their orders) see the :setting:`DOWNLOADER_MIDDLEWARES_BASE` setting.

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CookiesMiddleware

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.. module:: scrapy.downloadermiddlewares.cookies
   :synopsis: Cookies Downloader Middleware

This middleware enables working with sites that require cookies, such as those that use sessions. It keeps track of cookies sent by web servers, and send them back on subsequent requests (from that spider), just like web browsers do.

The following settings can be used to configure the cookie middleware:

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.. reqmeta:: cookiejar

Multiple cookie sessions per spider

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.. versionadded:: 0.15

There is support for keeping multiple cookie sessions per spider by using the :reqmeta:`cookiejar` Request meta key. By default it uses a single cookie jar (session), but you can pass an identifier to use different ones.

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For example:

for i, url in enumerate(urls):
    yield scrapy.Request(url, meta={'cookiejar': i},
        callback=self.parse_page)

Keep in mind that the :reqmeta:`cookiejar` meta key is not "sticky". You need to keep passing it along on subsequent requests. For example:

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def parse_page(self, response):
    # do some processing
    return scrapy.Request("http://www.example.com/otherpage",
        meta={'cookiejar': response.meta['cookiejar']},
        callback=self.parse_other_page)

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.. setting:: COOKIES_ENABLED

COOKIES_ENABLED

Default: True

Whether to enable the cookies middleware. If disabled, no cookies will be sent to web servers.

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.. setting:: COOKIES_DEBUG

COOKIES_DEBUG

Default: False

If enabled, Scrapy will log all cookies sent in requests (ie. Cookie header) and all cookies received in responses (ie. Set-Cookie header).

Here's an example of a log with :setting:`COOKIES_DEBUG` enabled:

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2011-04-06 14:35:10-0300 [scrapy.core.engine] INFO: Spider opened
2011-04-06 14:35:10-0300 [scrapy.downloadermiddlewares.cookies] DEBUG: Sending cookies to: <GET http://www.diningcity.com/netherlands/index.html>
        Cookie: clientlanguage_nl=en_EN
2011-04-06 14:35:14-0300 [scrapy.downloadermiddlewares.cookies] DEBUG: Received cookies from: <200 http://www.diningcity.com/netherlands/index.html>
        Set-Cookie: JSESSIONID=B~FA4DC0C496C8762AE4F1A620EAB34F38; Path=/
        Set-Cookie: ip_isocode=US
        Set-Cookie: clientlanguage_nl=en_EN; Expires=Thu, 07-Apr-2011 21:21:34 GMT; Path=/
2011-04-06 14:49:50-0300 [scrapy.core.engine] DEBUG: Crawled (200) <GET http://www.diningcity.com/netherlands/index.html> (referer: None)
[...]

DefaultHeadersMiddleware

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.. module:: scrapy.downloadermiddlewares.defaultheaders
   :synopsis: Default Headers Downloader Middleware

This middleware sets all default requests headers specified in the :setting:`DEFAULT_REQUEST_HEADERS` setting.

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DownloadTimeoutMiddleware

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.. module:: scrapy.downloadermiddlewares.downloadtimeout
   :synopsis: Download timeout middleware

This middleware sets the download timeout for requests specified in the :setting:`DOWNLOAD_TIMEOUT` setting or :attr:`download_timeout` spider attribute.

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Note

You can also set download timeout per-request using :reqmeta:`download_timeout` Request.meta key; this is supported even when DownloadTimeoutMiddleware is disabled.

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HttpAuthMiddleware

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.. module:: scrapy.downloadermiddlewares.httpauth
   :synopsis: HTTP Auth downloader middleware

This middleware authenticates all requests generated from certain spiders using Basic access authentication (aka. HTTP auth).

To enable HTTP authentication from certain spiders, set the http_user and http_pass attributes of those spiders.

Example:

from scrapy.spiders import CrawlSpider

class SomeIntranetSiteSpider(CrawlSpider):

    http_user = 'someuser'
    http_pass = 'somepass'
    name = 'intranet.example.com'

    # .. rest of the spider code omitted ...

HttpCacheMiddleware

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.. module:: scrapy.downloadermiddlewares.httpcache
   :synopsis: HTTP Cache downloader middleware

This middleware provides low-level cache to all HTTP requests and responses. It has to be combined with a cache storage backend as well as a cache policy.

Scrapy ships with three HTTP cache storage backends:

You can change the HTTP cache storage backend with the :setting:`HTTPCACHE_STORAGE` setting. Or you can also implement your own storage backend.

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Scrapy ships with two HTTP cache policies:

You can change the HTTP cache policy with the :setting:`HTTPCACHE_POLICY` setting. Or you can also implement your own policy.

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.. reqmeta:: dont_cache

You can also avoid caching a response on every policy using :reqmeta:`dont_cache` meta key equals True.

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Dummy policy (default)

This policy has no awareness of any HTTP Cache-Control directives. Every request and its corresponding response are cached. When the same request is seen again, the response is returned without transferring anything from the Internet.

The Dummy policy is useful for testing spiders faster (without having to wait for downloads every time) and for trying your spider offline, when an Internet connection is not available. The goal is to be able to "replay" a spider run exactly as it ran before.

In order to use this policy, set:

RFC2616 policy

This policy provides a RFC2616 compliant HTTP cache, i.e. with HTTP Cache-Control awareness, aimed at production and used in continuous runs to avoid downloading unmodified data (to save bandwidth and speed up crawls).

what is implemented:

  • Do not attempt to store responses/requests with no-store cache-control directive set

  • Do not serve responses from cache if no-cache cache-control directive is set even for fresh responses

  • Compute freshness lifetime from max-age cache-control directive

  • Compute freshness lifetime from Expires response header

  • Compute freshness lifetime from Last-Modified response header (heuristic used by Firefox)

  • Compute current age from Age response header

  • Compute current age from Date header

  • Revalidate stale responses based on Last-Modified response header

  • Revalidate stale responses based on ETag response header

  • Set Date header for any received response missing it

  • Support max-stale cache-control directive in requests

    This allows spiders to be configured with the full RFC2616 cache policy, but avoid revalidation on a request-by-request basis, while remaining conformant with the HTTP spec.

    Example:

    Add Cache-Control: max-stale=600 to Request headers to accept responses that have exceeded their expiration time by no more than 600 seconds.

    See also: RFC2616, 14.9.3

what is missing:

In order to use this policy, set:

Filesystem storage backend (default)

File system storage backend is available for the HTTP cache middleware.

In order to use this storage backend, set:

Each request/response pair is stored in a different directory containing the following files:

  • request_body - the plain request body
  • request_headers - the request headers (in raw HTTP format)
  • response_body - the plain response body
  • response_headers - the request headers (in raw HTTP format)
  • meta - some metadata of this cache resource in Python repr() format (grep-friendly format)
  • pickled_meta - the same metadata in meta but pickled for more efficient deserialization

The directory name is made from the request fingerprint (see scrapy.utils.request.fingerprint), and one level of subdirectories is used to avoid creating too many files into the same directory (which is inefficient in many file systems). An example directory could be:

/path/to/cache/dir/example.com/72/72811f648e718090f041317756c03adb0ada46c7

DBM storage backend

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A DBM storage backend is also available for the HTTP cache middleware.

By default, it uses the anydbm module, but you can change it with the :setting:`HTTPCACHE_DBM_MODULE` setting.

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In order to use this storage backend, set:

LevelDB storage backend

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.. versionadded:: 0.23

A LevelDB storage backend is also available for the HTTP cache middleware.

This backend is not recommended for development because only one process can access LevelDB databases at the same time, so you can't run a crawl and open the scrapy shell in parallel for the same spider.

In order to use this storage backend:

HTTPCache middleware settings

The :class:`HttpCacheMiddleware` can be configured through the following settings:

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.. setting:: HTTPCACHE_ENABLED

HTTPCACHE_ENABLED

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.. versionadded:: 0.11

Default: False

Whether the HTTP cache will be enabled.

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.. versionchanged:: 0.11
   Before 0.11, :setting:`HTTPCACHE_DIR` was used to enable cache.

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.. setting:: HTTPCACHE_EXPIRATION_SECS

HTTPCACHE_EXPIRATION_SECS

Default: 0

Expiration time for cached requests, in seconds.

Cached requests older than this time will be re-downloaded. If zero, cached requests will never expire.

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.. versionchanged:: 0.11
   Before 0.11, zero meant cached requests always expire.

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.. setting:: HTTPCACHE_DIR

HTTPCACHE_DIR

Default: 'httpcache'

The directory to use for storing the (low-level) HTTP cache. If empty, the HTTP cache will be disabled. If a relative path is given, is taken relative to the project data dir. For more info see: :ref:`topics-project-structure`.

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.. setting:: HTTPCACHE_IGNORE_HTTP_CODES

HTTPCACHE_IGNORE_HTTP_CODES

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.. versionadded:: 0.10

Default: []

Don't cache response with these HTTP codes.

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.. setting:: HTTPCACHE_IGNORE_MISSING

HTTPCACHE_IGNORE_MISSING

Default: False

If enabled, requests not found in the cache will be ignored instead of downloaded.

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.. setting:: HTTPCACHE_IGNORE_SCHEMES

HTTPCACHE_IGNORE_SCHEMES

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.. versionadded:: 0.10

Default: ['file']

Don't cache responses with these URI schemes.

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.. setting:: HTTPCACHE_STORAGE

HTTPCACHE_STORAGE

Default: 'scrapy.extensions.httpcache.FilesystemCacheStorage'

The class which implements the cache storage backend.

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.. setting:: HTTPCACHE_DBM_MODULE

HTTPCACHE_DBM_MODULE

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.. versionadded:: 0.13

Default: 'anydbm'

The database module to use in the :ref:`DBM storage backend <httpcache-storage-dbm>`. This setting is specific to the DBM backend.

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.. setting:: HTTPCACHE_POLICY

HTTPCACHE_POLICY

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.. versionadded:: 0.18

Default: 'scrapy.extensions.httpcache.DummyPolicy'

The class which implements the cache policy.

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.. setting:: HTTPCACHE_GZIP

HTTPCACHE_GZIP

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.. versionadded:: 1.0

Default: False

If enabled, will compress all cached data with gzip. This setting is specific to the Filesystem backend.

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.. setting:: HTTPCACHE_ALWAYS_STORE

HTTPCACHE_ALWAYS_STORE

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.. versionadded:: 1.1

Default: False

If enabled, will cache pages unconditionally.

A spider may wish to have all responses available in the cache, for future use with Cache-Control: max-stale, for instance. The DummyPolicy caches all responses but never revalidates them, and sometimes a more nuanced policy is desirable.

This setting still respects Cache-Control: no-store directives in responses. If you don't want that, filter no-store out of the Cache-Control headers in responses you feedto the cache middleware.

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.. setting:: HTTPCACHE_IGNORE_RESPONSE_CACHE_CONTROLS

HTTPCACHE_IGNORE_RESPONSE_CACHE_CONTROLS

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.. versionadded:: 1.1

Default: []

List of Cache-Control directives in responses to be ignored.

Sites often set "no-store", "no-cache", "must-revalidate", etc., but get upset at the traffic a spider can generate if it respects those directives. This allows to selectively ignore Cache-Control directives that are known to be unimportant for the sites being crawled.

We assume that the spider will not issue Cache-Control directives in requests unless it actually needs them, so directives in requests are not filtered.

HttpCompressionMiddleware

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.. module:: scrapy.downloadermiddlewares.httpcompression
   :synopsis: Http Compression Middleware

This middleware allows compressed (gzip, deflate) traffic to be sent/received from web sites.

This middleware also supports decoding brotli-compressed responses, provided brotlipy is installed.

HttpCompressionMiddleware Settings

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.. setting:: COMPRESSION_ENABLED

COMPRESSION_ENABLED

Default: True

Whether the Compression middleware will be enabled.

HttpProxyMiddleware

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.. module:: scrapy.downloadermiddlewares.httpproxy
   :synopsis: Http Proxy Middleware

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.. versionadded:: 0.8

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.. reqmeta:: proxy

This middleware sets the HTTP proxy to use for requests, by setting the proxy meta value for :class:`~scrapy.http.Request` objects.

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Like the Python standard library modules urllib and urllib2, it obeys the following environment variables:

  • http_proxy
  • https_proxy
  • no_proxy

You can also set the meta key proxy per-request, to a value like http://some_proxy_server:port or http://username:password@some_proxy_server:port. Keep in mind this value will take precedence over http_proxy/https_proxy environment variables, and it will also ignore no_proxy environment variable.

RedirectMiddleware

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.. module:: scrapy.downloadermiddlewares.redirect
   :synopsis: Redirection Middleware

This middleware handles redirection of requests based on response status.

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.. reqmeta:: redirect_urls

The urls which the request goes through (while being redirected) can be found in the redirect_urls :attr:`Request.meta <scrapy.http.Request.meta>` key.

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The :class:`RedirectMiddleware` can be configured through the following settings (see the settings documentation for more info):

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.. reqmeta:: dont_redirect

If :attr:`Request.meta <scrapy.http.Request.meta>` has dont_redirect key set to True, the request will be ignored by this middleware.

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If you want to handle some redirect status codes in your spider, you can specify these in the handle_httpstatus_list spider attribute.

For example, if you want the redirect middleware to ignore 301 and 302 responses (and pass them through to your spider) you can do this:

class MySpider(CrawlSpider):
    handle_httpstatus_list = [301, 302]

The handle_httpstatus_list key of :attr:`Request.meta <scrapy.http.Request.meta>` can also be used to specify which response codes to allow on a per-request basis. You can also set the meta key handle_httpstatus_all to True if you want to allow any response code for a request.

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RedirectMiddleware settings

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.. setting:: REDIRECT_ENABLED

REDIRECT_ENABLED

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.. versionadded:: 0.13

Default: True

Whether the Redirect middleware will be enabled.

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.. setting:: REDIRECT_MAX_TIMES

REDIRECT_MAX_TIMES

Default: 20

The maximum number of redirections that will be followed for a single request.

MetaRefreshMiddleware

This middleware handles redirection of requests based on meta-refresh html tag.

The :class:`MetaRefreshMiddleware` can be configured through the following settings (see the settings documentation for more info):

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This middleware obey :setting:`REDIRECT_MAX_TIMES` setting, :reqmeta:`dont_redirect` and :reqmeta:`redirect_urls` request meta keys as described for :class:`RedirectMiddleware`

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MetaRefreshMiddleware settings

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.. setting:: METAREFRESH_ENABLED

METAREFRESH_ENABLED

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.. versionadded:: 0.17

Default: True

Whether the Meta Refresh middleware will be enabled.

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.. setting:: METAREFRESH_MAXDELAY

METAREFRESH_MAXDELAY

Default: 100

The maximum meta-refresh delay (in seconds) to follow the redirection. Some sites use meta-refresh for redirecting to a session expired page, so we restrict automatic redirection to the maximum delay.

RetryMiddleware

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.. module:: scrapy.downloadermiddlewares.retry
   :synopsis: Retry Middleware

A middleware to retry failed requests that are potentially caused by temporary problems such as a connection timeout or HTTP 500 error.

Failed pages are collected on the scraping process and rescheduled at the end, once the spider has finished crawling all regular (non failed) pages. Once there are no more failed pages to retry, this middleware sends a signal (retry_complete), so other extensions could connect to that signal.

The :class:`RetryMiddleware` can be configured through the following settings (see the settings documentation for more info):

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.. reqmeta:: dont_retry

If :attr:`Request.meta <scrapy.http.Request.meta>` has dont_retry key set to True, the request will be ignored by this middleware.

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RetryMiddleware Settings

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.. setting:: RETRY_ENABLED

RETRY_ENABLED

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.. versionadded:: 0.13

Default: True

Whether the Retry middleware will be enabled.

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.. setting:: RETRY_TIMES

RETRY_TIMES

Default: 2

Maximum number of times to retry, in addition to the first download.

Maximum number of retries can also be specified per-request using :reqmeta:`max_retry_times` attribute of :attr:`Request.meta <scrapy.http.Request.meta>`. When initialized, the :reqmeta:`max_retry_times` meta key takes higher precedence over the :setting:`RETRY_TIMES` setting.

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.. setting:: RETRY_HTTP_CODES

RETRY_HTTP_CODES

Default: [500, 502, 503, 504, 408]

Which HTTP response codes to retry. Other errors (DNS lookup issues, connections lost, etc) are always retried.

In some cases you may want to add 400 to :setting:`RETRY_HTTP_CODES` because it is a common code used to indicate server overload. It is not included by default because HTTP specs say so.

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RobotsTxtMiddleware

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.. module:: scrapy.downloadermiddlewares.robotstxt
   :synopsis: robots.txt middleware

This middleware filters out requests forbidden by the robots.txt exclusion standard.

To make sure Scrapy respects robots.txt make sure the middleware is enabled and the :setting:`ROBOTSTXT_OBEY` setting is enabled.

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.. reqmeta:: dont_obey_robotstxt

If :attr:`Request.meta <scrapy.http.Request.meta>` has dont_obey_robotstxt key set to True the request will be ignored by this middleware even if :setting:`ROBOTSTXT_OBEY` is enabled.

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DownloaderStats

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.. module:: scrapy.downloadermiddlewares.stats
   :synopsis: Downloader Stats Middleware

Middleware that stores stats of all requests, responses and exceptions that pass through it.

To use this middleware you must enable the :setting:`DOWNLOADER_STATS` setting.

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UserAgentMiddleware

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.. module:: scrapy.downloadermiddlewares.useragent
   :synopsis: User Agent Middleware

Middleware that allows spiders to override the default user agent.

In order for a spider to override the default user agent, its user_agent attribute must be set.

AjaxCrawlMiddleware

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.. module:: scrapy.downloadermiddlewares.ajaxcrawl

Middleware that finds 'AJAX crawlable' page variants based on meta-fragment html tag. See https://developers.google.com/webmasters/ajax-crawling/docs/getting-started for more info.

Note

Scrapy finds 'AJAX crawlable' pages for URLs like 'http://example.com/!#foo=bar' even without this middleware. AjaxCrawlMiddleware is necessary when URL doesn't contain '!#'. This is often a case for 'index' or 'main' website pages.

AjaxCrawlMiddleware Settings

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.. setting:: AJAXCRAWL_ENABLED

AJAXCRAWL_ENABLED

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.. versionadded:: 0.21

Default: False

Whether the AjaxCrawlMiddleware will be enabled. You may want to enable it for :ref:`broad crawls <topics-broad-crawls>`.

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HttpProxyMiddleware settings

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.. setting:: HTTPPROXY_ENABLED

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.. setting:: HTTPPROXY_AUTH_ENCODING

HTTPPROXY_ENABLED

Default: True

Whether or not to enable the :class:`HttpProxyMiddleware`.

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HTTPPROXY_AUTH_ENCODING

Default: "latin-1"

The default encoding for proxy authentication on :class:`HttpProxyMiddleware`.

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