# 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 components that include one or both of the following methods:

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

classmethod update_pre_crawler_settings(cls, settings)

  Use this class method instead of the `update_settings()` method to update pre-crawler settings whose value is used before the `Crawler` object is created.

  Parameters:

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

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.

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 component priority dictionary.

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.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, 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:

    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
            )

Tip

When editing a component priority dictionary setting, like `ITEM_PIPELINES`, consider using setting methods like `replace_in_component_priority_dict()`, `set_in_component_priority_dict()` and `setdefault_in_component_priority_dict()` to avoid mistakes.

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.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