.. _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

During :class:~scrapy.crawler.Crawler initialization, the list of enabled add-ons is read from your ADDONS setting.

The ADDONS setting is a dict in which every key is an add-on 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,
    SomeAddonClass: 1,
}

Writing your own add-ons

Add-ons are :ref:components <topics-components> that include one or both of the following methods:

.. 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`

.. classmethod:: update_pre_crawler_settings(cls, settings)

Use this class method instead of the :meth:`update_settings` method to
update :ref:`pre-crawler settings <pre-crawler-settings>` whose value is
used before the :class:`~scrapy.crawler.Crawler` object is created.

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

The settings set by the add-on 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.

When editing the value of a setting instead of overriding it entirely, it is usually best to leave its priority unchanged. For example, when editing a :ref:component priority dictionary <component-priority-dictionaries>.

If the update_settings method raises :exc:scrapy.exceptions.NotConfigured, the add-on will be skipped. This makes it easy to enable an add-on only when some conditions are met.

Fallbacks

Some components provided by add-ons need to fall back 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.http11.HTTP11DownloadHandler), 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 add-ons 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 by the add-on (e.g. MyDownloadHandler). If the fallback setting is already set by the user, it should not be changed.
3. This way, if there are several add-ons that want to modify the same setting, all of them will fall back 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:

.. skip: next .. code-block:: python

from myproject.pipelines import MyPipeline


class MyAddon:
    def update_settings(self, settings):
        settings.set("DNSCACHE_ENABLED", True, "addon")
        settings.remove_from_list("METAREFRESH_IGNORE_TAGS", "noscript")
        settings.setdefault_in_component_priority_dict(
            "ITEM_PIPELINES", MyPipeline, 200
        )

.. _priority-dict-helpers:

.. tip:: When editing a :ref:component priority dictionary <component-priority-dictionaries> setting, like :setting:ITEM_PIPELINES, consider using setting methods like :meth:~scrapy.settings.BaseSettings.replace_in_component_priority_dict, :meth:~scrapy.settings.BaseSettings.set_in_component_priority_dict and :meth:~scrapy.settings.BaseSettings.setdefault_in_component_priority_dict to avoid mistakes.

Check dependencies:

.. code-block:: python

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

Access the crawler instance:

.. code-block:: python

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

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

    def update_settings(self, settings): ...

Use a fallback component:

.. code-block:: python

from scrapy.utils.misc import build_from_crawler


FALLBACK_SETTING = "MY_FALLBACK_DOWNLOAD_HANDLER"


class MyHandler:
    lazy = False

    def __init__(self, crawler):
        dhcls = load_object(crawler.settings.get(FALLBACK_SETTING))
        self._fallback_handler = build_from_crawler(dhcls, crawler)

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

    async def close(self):
        pass


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