# 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 `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 Python classes that include the following method:

update_settings(settings)

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

  Parameters:

    settings (`Settings`) – The settings object storing Scrapy/component configuration

They can also have the following method:

classmethod from_crawler(cls, crawler)

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

  Parameters:

    crawler (`Crawler`) – The crawler that uses this add-on

The settings set by the add-on should use the `addon` priority (see Populating the settings and `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 `ITEM_PIPELINES`. In these cases you can provide an add-on-specific setting that governs whether the add-on will modify `ITEM_PIPELINES`:

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

If the `update_settings` method raises `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.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 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, they shouldn’t change it.

3. This way, if there are several add-ons 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 NotConfigured("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):
            return cls(crawler)

        def update_settings(self, settings): ...

Use a fallback component:

    from scrapy.core.downloader.handlers.http import HTTPDownloadHandler
    from scrapy.utils.misc import build_from_crawler

    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 = build_from_crawler(dhcls, 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",
                )
            settings["DOWNLOAD_HANDLERS"]["https"] = MyHandler