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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.
System Message: ERROR/3 (<stdin>, line 17); backlink
Unknown interpreted text role "class".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 one or both of the following methods:
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Unknown directive type "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`
System Message: ERROR/3 (<stdin>, line 48)
Unknown directive type "classmethod".
.. 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`
They can also have the following method:
System Message: ERROR/3 (<stdin>, line 59)
Unknown directive type "classmethod".
.. classmethod:: from_crawler(cls, crawler) :noindex: If present, this class method is called to create an add-on instance from a :class:`~scrapy.crawler.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. :param crawler: The crawler that uses this add-on :type crawler: :class:`~scrapy.crawler.Crawler`
The settings set by the add-on should use the addon priority (see :ref:`populating-settings` and :func:`scrapy.settings.BaseSettings.set`):
System Message: ERROR/3 (<stdin>, line 71); backlink
Unknown interpreted text role "ref".System Message: ERROR/3 (<stdin>, line 71); backlink
Unknown interpreted text role "func".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 add-on-specific setting that governs whether the add-on will modify :setting:`ITEM_PIPELINES`:
System Message: ERROR/3 (<stdin>, line 78); backlink
Unknown interpreted text role "setting".System Message: ERROR/3 (<stdin>, line 78); backlink
Unknown interpreted text role "setting".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 :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.
System Message: ERROR/3 (<stdin>, line 89); backlink
Unknown interpreted text role "exc".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:
- 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.
- 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.
- 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:
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.. code-block:: python
class MyAddon:
def update_settings(self, settings):
settings["ITEM_PIPELINES"]["path.to.mypipeline"] = 200
settings.set("DNSCACHE_ENABLED", True, "addon")
Check dependencies:
System Message: WARNING/2 (<stdin>, line 139)
Cannot analyze code. Pygments package not found.
.. 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:
System Message: WARNING/2 (<stdin>, line 151)
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.. 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:
System Message: WARNING/2 (<stdin>, line 166)
Cannot analyze code. Pygments package not found.
.. code-block:: python
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