.. _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 responses that
are sent to :ref:`topics-spiders` for processing and to process the requests
and items 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:

.. code-block:: python

    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. In other words,
the :meth:`~scrapy.spidermiddlewares.SpiderMiddleware.process_spider_input`
method of each middleware will be invoked in increasing
middleware order (100, 200, 300, ...), and the
:meth:`~scrapy.spidermiddlewares.SpiderMiddleware.process_spider_output` method
of each middleware will be invoked in decreasing order.

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:

.. code-block:: python

    SPIDER_MIDDLEWARES = {
        "scrapy.spidermiddlewares.referer.RefererMiddleware": None,
        "myproject.middlewares.CustomRefererSpiderMiddleware": 700,
    }

Finally, keep in mind that some middlewares may need to be enabled through a
particular setting. See each middleware documentation for more info.

.. _custom-spider-middleware:

Writing your own spider middleware
==================================

Each spider middleware is a :ref:`component <topics-components>` that defines
one or more of these methods:

.. module:: scrapy.spidermiddlewares

.. class:: SpiderMiddleware

    .. method:: process_start(start: AsyncIterator[Any], /) -> AsyncIterator[Any]
        :async:

        Iterate over the output of :meth:`~scrapy.Spider.start` or that
        of the :meth:`process_start` method of an earlier spider middleware,
        overriding it. For example:

        .. code-block:: python

            async def process_start(self, start):
                async for item_or_request in start:
                    yield item_or_request

        You may yield the same type of objects as :meth:`~scrapy.Spider.start`.

        To write spider middlewares that work on Scrapy versions lower than
        2.13, define also a synchronous ``process_start_requests()`` method
        that returns an iterable. For example:

        .. code-block:: python

            def process_start_requests(self, start, spider):
                yield from start

    .. method:: process_spider_input(response)

        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 ``None`` or raise an
        exception.

        If it returns ``None``, Scrapy will continue processing this response,
        executing all other middlewares until, finally, the response is handed
        to the spider for processing.

        If it raises an exception, Scrapy won't bother calling any other spider
        middleware :meth:`process_spider_input` and will call the request
        errback if there is one, otherwise it will start the :meth:`process_spider_exception`
        chain. The output of the errback is chained back in the other
        direction for :meth:`process_spider_output` to process it, or
        :meth:`process_spider_exception` if it raised an exception.

        :param response: the response being processed
        :type response: :class:`~scrapy.http.Response` object

    .. method:: process_spider_output(response, result)

        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.Request` objects and :ref:`item objects
        <topics-items>`.

        Consider defining this method as an :term:`asynchronous generator`,
        which will be a requirement in a future version of Scrapy. However, if
        you plan on sharing your spider middleware with other people, consider
        either :ref:`enforcing Scrapy 2.7 <enforce-component-requirements>`
        as a minimum requirement of your spider middleware, or :ref:`making
        your spider middleware universal <universal-spider-middleware>` so that
        it works with Scrapy versions earlier than Scrapy 2.7.

        :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.Request` objects and
          :ref:`item objects <topics-items>`

    .. method:: process_spider_output_async(response, result)
        :async:

        If defined, this method must be an :term:`asynchronous generator`,
        which will be called instead of :meth:`process_spider_output` if
        ``result`` is an :term:`asynchronous iterable`.

    .. method:: process_spider_exception(response, exception)

        This method is called when a spider or :meth:`process_spider_output`
        method (from a previous spider middleware) raises an exception.

        :meth:`process_spider_exception` should return either ``None`` or an
        iterable of :class:`~scrapy.Request` or :ref:`item <topics-items>`
        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, starting from the next spider middleware, 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: :exc:`Exception` object


Base class for custom spider middlewares
----------------------------------------

Scrapy provides a base class for custom spider middlewares. It's not required
to use it but it can help with simplifying middleware implementations and
reducing the amount of boilerplate code in :ref:`universal middlewares
<universal-spider-middleware>`.

.. module:: scrapy.spidermiddlewares.base

.. autoclass:: BaseSpiderMiddleware
   :members:

.. _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 <topics-spider-middleware>`.

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

DepthMiddleware
---------------

.. module:: scrapy.spidermiddlewares.depth
   :synopsis: Depth Spider Middleware

.. class:: DepthMiddleware

   DepthMiddleware is used for tracking the depth of each Request inside the
   site being scraped. It works by setting ``request.meta['depth'] = 0`` whenever
   there is no value previously set (usually just the first Request) and
   incrementing it by 1 otherwise.

   It can be used to limit the maximum depth to scrape, control Request
   priority based on their depth, and 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_VERBOSE` - Whether to collect the number of
        requests for each depth.
      * :setting:`DEPTH_PRIORITY` - Whether to prioritize the requests based on
        their depth.

HttpErrorMiddleware
-------------------

.. module:: scrapy.spidermiddlewares.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 time) 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: https://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 or
:setting:`HTTPERROR_ALLOWED_CODES` setting.

For example, if you want your spider to handle 404 responses you can do
this:

.. code-block:: python

    from scrapy.spiders import CrawlSpider


    class MySpider(CrawlSpider):
        handle_httpstatus_list = [404]

.. reqmeta:: handle_httpstatus_list

.. reqmeta:: handle_httpstatus_all

The ``handle_httpstatus_list`` key of :attr:`Request.meta
<scrapy.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, and ``False`` to
disable the effects of the ``handle_httpstatus_all`` key.

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: https://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html

HttpErrorMiddleware settings
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. setting:: HTTPERROR_ALLOWED_CODES

HTTPERROR_ALLOWED_CODES
^^^^^^^^^^^^^^^^^^^^^^^

Default: ``[]``

Pass all responses with non-200 status codes contained in this list.

.. setting:: HTTPERROR_ALLOW_ALL

HTTPERROR_ALLOW_ALL
^^^^^^^^^^^^^^^^^^^

Default: ``False``

Pass all responses, regardless of its status code.


RefererMiddleware
-----------------

.. module:: scrapy.spidermiddlewares.referer
   :synopsis: Referer Spider Middleware

.. class:: RefererMiddleware

   Populates Request ``Referer`` header, based on the URL of the Response which
   generated it.

RefererMiddleware settings
~~~~~~~~~~~~~~~~~~~~~~~~~~

.. setting:: REFERER_ENABLED

REFERER_ENABLED
^^^^^^^^^^^^^^^

Default: ``True``

Whether to enable referer middleware.

.. setting:: REFERRER_POLICY

REFERRER_POLICY
^^^^^^^^^^^^^^^

Default: ``'scrapy.spidermiddlewares.referer.DefaultReferrerPolicy'``

.. reqmeta:: referrer_policy

`Referrer Policy`_ to apply when populating Request "Referer" header.

.. note::
    You can also set the Referrer Policy per request,
    using the special ``"referrer_policy"`` :ref:`Request.meta <topics-request-meta>` key,
    with the same acceptable values as for the ``REFERRER_POLICY`` setting.

Acceptable values for REFERRER_POLICY
*************************************

- either a path to a :class:`scrapy.spidermiddlewares.referer.ReferrerPolicy`
  subclass — a custom policy or one of the built-in ones (see classes below),
- or one or more comma-separated standard W3C-defined string values,
- or the special ``"scrapy-default"``.

=======================================  ========================================================================
String value                             Class name (as a string)
=======================================  ========================================================================
``"scrapy-default"`` (default)           :class:`scrapy.spidermiddlewares.referer.DefaultReferrerPolicy`
`"no-referrer"`_                         :class:`scrapy.spidermiddlewares.referer.NoReferrerPolicy`
`"no-referrer-when-downgrade"`_          :class:`scrapy.spidermiddlewares.referer.NoReferrerWhenDowngradePolicy`
`"same-origin"`_                         :class:`scrapy.spidermiddlewares.referer.SameOriginPolicy`
`"origin"`_                              :class:`scrapy.spidermiddlewares.referer.OriginPolicy`
`"strict-origin"`_                       :class:`scrapy.spidermiddlewares.referer.StrictOriginPolicy`
`"origin-when-cross-origin"`_            :class:`scrapy.spidermiddlewares.referer.OriginWhenCrossOriginPolicy`
`"strict-origin-when-cross-origin"`_     :class:`scrapy.spidermiddlewares.referer.StrictOriginWhenCrossOriginPolicy`
`"unsafe-url"`_                          :class:`scrapy.spidermiddlewares.referer.UnsafeUrlPolicy`
=======================================  ========================================================================

.. autoclass:: ReferrerPolicy

.. autoclass:: DefaultReferrerPolicy
.. warning::
    Scrapy's default referrer policy — just like `"no-referrer-when-downgrade"`_,
    the W3C-recommended value for browsers — will send a non-empty
    "Referer" header from any ``http(s)://`` to any ``https://`` URL,
    even if the domain is different.

    `"same-origin"`_ may be a better choice if you want to remove referrer
    information for cross-domain requests.

.. autoclass:: NoReferrerPolicy

.. autoclass:: NoReferrerWhenDowngradePolicy
.. note::
    "no-referrer-when-downgrade" policy is the W3C-recommended default,
    and is used by major web browsers.

    However, it is NOT Scrapy's default referrer policy (see :class:`DefaultReferrerPolicy`).

.. autoclass:: SameOriginPolicy

.. autoclass:: OriginPolicy

.. autoclass:: StrictOriginPolicy

.. autoclass:: OriginWhenCrossOriginPolicy

.. autoclass:: StrictOriginWhenCrossOriginPolicy

.. autoclass:: UnsafeUrlPolicy
.. warning::
    "unsafe-url" policy is NOT recommended.

.. _Referrer Policy: https://www.w3.org/TR/referrer-policy
.. _"no-referrer": https://www.w3.org/TR/referrer-policy/#referrer-policy-no-referrer
.. _"no-referrer-when-downgrade": https://www.w3.org/TR/referrer-policy/#referrer-policy-no-referrer-when-downgrade
.. _"same-origin": https://www.w3.org/TR/referrer-policy/#referrer-policy-same-origin
.. _"origin": https://www.w3.org/TR/referrer-policy/#referrer-policy-origin
.. _"strict-origin": https://www.w3.org/TR/referrer-policy/#referrer-policy-strict-origin
.. _"origin-when-cross-origin": https://www.w3.org/TR/referrer-policy/#referrer-policy-origin-when-cross-origin
.. _"strict-origin-when-cross-origin": https://www.w3.org/TR/referrer-policy/#referrer-policy-strict-origin-when-cross-origin
.. _"unsafe-url": https://www.w3.org/TR/referrer-policy/#referrer-policy-unsafe-url


StartSpiderMiddleware
---------------------

.. module:: scrapy.spidermiddlewares.start

.. autoclass:: StartSpiderMiddleware


UrlLengthMiddleware
-------------------

.. module:: scrapy.spidermiddlewares.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.
