scrapy/docs/topics/addons.rst

.. _topics-addons:

=======
Add-ons
=======

Scrapy's add-on system is a framework which unifies managing and configuring
components that extend Scrapy's core functionality, such as middlewares,
extensions, or pipelines. It provides users with a plug-and-play experience in
Scrapy extension management, and grants extensive configuration control to
developers.


Activating and configuring add-ons
==================================

Add-ons and their configuration live in Scrapy's
:class:`~scrapy.addons.AddonManager`. During a :class:`~scrapy.crawler.Crawler`
initialization the add-on manager will read a list of enabled add-ons from your
``ADDONS`` setting.

The ``ADDONS`` setting is a dict in which every key is an addon class or its
import path and the value is its priority.

This is an example where two add-ons are enabled in a project's
``settings.py``::

    ADDONS = {
        'path.to.someaddon': 0,
        path.to.someaddon2: 1,
    }


Writing your own add-ons
========================

Add-ons are (any) Python objects that include the following method:

.. method:: update_settings(settings)

    This method is called during the initialization of the
    :class:`~scrapy.crawler.Crawler`. Here, you should perform dependency checks
    (e.g. for external Python libraries) and update the
    :class:`~scrapy.settings.Settings` object as wished, e.g. enable components
    for this add-on or set required configuration of other extensions.

    :param settings: The settings object storing Scrapy/component configuration
    :type settings: :class:`~scrapy.settings.Settings`

They can also have the following method:

.. classmethod:: from_crawler(cls, crawler)
   :noindex:

   If present, this class method is called to create an addon instance
   from a :class:`~scrapy.crawler.Crawler`. It must return a new instance
   of the addon. Crawler object provides access to all Scrapy core
   components like settings and signals; it is a way for pipeline to
   access them and hook its functionality into Scrapy.

   :param crawler: The crawler that uses this addon
   :type crawler: :class:`~scrapy.crawler.Crawler`

The settings set by the addon should use the ``addon`` priority (see
:ref:`populating-settings` and :func:`scrapy.settings.BaseSettings.set`)::

    class MyAddon:
        def update_settings(self, settings):
            settings.set("DNSCACHE_ENABLED", True, "addon")

This allows users to override these settings in the project or spider
configuration. This is not possible with settings that are mutable objects,
such as the dict that is a value of :setting:`ITEM_PIPELINES`. In these cases
you can provide an addon-specific setting that governs whether the addon will
modify :setting:`ITEM_PIPELINES`::

    class MyAddon:
        def update_settings(self, settings):
            if settings.getbool("MYADDON_ENABLE_PIPELINE"):
                settings["ITEM_PIPELINES"]["path.to.mypipeline"] = 200

Fallbacks
---------

Some components provided by addons need to fallback to "default"
implementations, e.g. a custom download handler needs to send the request that
it doesn't handle via the default download handler, or a stats collector that
includes some additional processing but otherwise uses the default stats
collector. And it's possible that a project needs to use several custom
components of the same type, e.g. two custom download handlers that support
different kinds of custom requests and still need to use the default download
handler for other requests. To make such use cases easier to configure, we
recommend that such custom components should be written in the following way:

1. The custom component (e.g. ``MyDownloadHandler``) shouldn't inherit from the
   default Scrapy one (e.g.
   ``scrapy.core.downloader.handlers.http.HTTPDownloadHandler``), but instead
   be able to load the class of the fallback component from a special setting
   (e.g. ``MY_FALLBACK_DOWNLOAD_HANDLER``), create an instance of it and use
   it.
2. The addons that include these components should read the current value of
   the default setting (e.g. ``DOWNLOAD_HANDLERS``) in their
   ``update_settings()`` methods, save that value into the fallback setting
   (``MY_FALLBACK_DOWNLOAD_HANDLER`` mentioned earlier) and set the default
   setting to the component provided byt the addon (e.g.
   ``MyDownloadHandler``). If the fallback setting is already set by the user,
   they shouldn't change it.
3. This way, if there are several addons that want to modify the same setting,
   all of them will fallback to the component from the previous one and then to
   the Scrapy default. The order of that depends on the priority order in the
   ``ADDONS`` setting.


Add-on examples
===============

Set some basic configuration::

    class MyAddon:
        def update_settings(self, settings):
            settings["ITEM_PIPELINES"]["path.to.mypipeline"] = 200
            settings.set("DNSCACHE_ENABLED", True, "addon")

Check dependencies::

    class MyAddon:
        def update_settings(self, settings):
            try:
                import boto
            except ImportError:
                raise RuntimeError("MyAddon requires the boto library")
            ...

Access the crawler instance::

    class MyAddon:
        def __init__(self, crawler) -> None:
            super().__init__()
            self.crawler = crawler

        @classmethod
        def from_crawler(cls, crawler: Crawler):
            return cls(crawler)

        def update_settings(self, settings):
            ...

Use a fallback component::

    from scrapy.core.downloader.handlers.http import HTTPDownloadHandler


    fallback_setting = "MY_FALLBACK_DOWNLOAD_HANDLER"


    class MyHandler:
        lazy = False

        def __init__(self, settings, crawler):
            dhcls = load_object(settings.get(fallback_setting))
            self._fallback_handler = create_instance(
                dhcls,
                settings=None,
                crawler=crawler,
            )

        def download_request(self, request, spider):
            if request.meta.get("my_params"):
                # handle the request
                ...
            else:
                return self._fallback_handler.download_request(request, spider)


    class MyAddon:
        def update_settings(self, settings):
            if not settings.get(fallback_setting):
                settings.set(
                    fallback_setting,
                    settings.getwithbase("DOWNLOAD_HANDLERS")["https"],
                    "addon",
                )