mirror of https://github.com/scrapy/scrapy.git
Document add-ons
This commit is contained in:
Jakob de Maeyer
parent
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2946b67414
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@ -206,6 +206,7 @@ Extending Scrapy
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:hidden:
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topics/architecture
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topics/addons
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topics/downloader-middleware
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topics/spider-middleware
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topics/extensions
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@ -217,6 +218,9 @@ Extending Scrapy
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:doc:`topics/architecture`
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Understand the Scrapy architecture.
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:doc:`topics/addons`
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Enable and configure built-in and third-party extensions.
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:doc:`topics/downloader-middleware`
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Customize how pages get requested and downloaded.
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@ -0,0 +1,387 @@
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.. _topics-addons:
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=======
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Add-ons
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=======
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Scrapy's add-on system is a framework which unifies managing and configuring
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components that extend Scrapy's core functionality, such as middlewares,
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extensions, or pipelines. It provides users with a plug-and-play experience in
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Scrapy extension management, and grants extensive configuration control to
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developers.
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Activating and configuring add-ons
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==================================
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Add-ons and their configuration live in Scrapy's
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:class:`~scrapy.addons.AddonManager`. During Scrapy's start-up process, and
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only then, the add-on manager will read a list of enabled add-ons and their
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configurations from your settings. There are two places where you can provide
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the paths to add-ons you want to enable:
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* the ``INSTALLED_ADDONS`` setting, and
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* the ``scrapy.cfg`` file.
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As Scrapy settings can be modified from many places, e.g. in a project's
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``settings.py``, in a Spider's ``custom_settings`` attribute, or from the
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command line, using the ``INSTALLED_ADDONS`` setting is the preferred way to
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manage add-ons.
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The ``INSTALLED_ADDONS`` setting a tuple in which every item is a path to an
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add-on. The path can be both a Python or a file path. While more precise, it is
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not necessary to specify the full add-on Python path if it is either built into
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Scrapy or lives in your project's ``addons`` submodule.
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The configuration of an add-on, if necessary at all, is stored as a dictionary
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setting whose name is the uppercase add-on name.
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This is an example where an internal add-on and two third-party add-ons (in this
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case with one requiring no configuration) are enabled/configured in a project's
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``settings.py``::
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INSTALLED_ADDONS = (
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'httpcache',
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'path.to.some.addon',
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'path/to/other/addon.py',
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)
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HTTPCACHE = {
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'expiration_secs': 60,
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'ignore_http_codes': [404, 405],
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}
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SOMEADDON = {
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'some_config': True,
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}
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It is also possible to manage add-ons from ``scrapy.cfg``. While the syntax is
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a little friendlier, be aware that this file, and therefore the configuration in
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it, is not bound to a particular Scrapy project. While this should not pose a
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problem when you use the project on your development machine only, a common
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stumbling block is that ``scrapy.cfg`` is not deployed via ``scrapyd-deploy``.
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In ``scrapy.cfg``, section names, prepended with ``addon:``, replace the
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dictionary keys. I.e., the configuration from above would look like this:
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.. code-block:: cfg
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[addon:httpcache]
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expiration_secs = 60
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ignore_http_codes = 404,405
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[addon:path.to.some.addon]
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some_config = true
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[addon:path/to/other/addon.py]
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Enabling and configuring add-ons within Python code
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---------------------------------------------------
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The :class:`~scrapy.addons.AddonManager` will only read from Scrapy's settings
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and from ``scrapy.cfg`` *at the beginning* of Scrapy's start-up process.
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Afterwards, i.e. as soon as the :class:`~scrapy.addons.AddonManager` is
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populated, changing the ``INSTALLED_ADDONS`` setting or any of the add-on
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configuration dictionary settings will have no effect.
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If you want to enable, disable, or configure add-ons in Python code, for example
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when writing your own add-on, you will have to use the
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:class:`~scrapy.addons.AddonManager`. You can access the add-on manager through
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either ``crawler.addons`` or, if you are writing an add-on, through the
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``addons`` argument of the :meth:`update_addons` callback. The add-on manager
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provides many useful methods and attributes to facilitate interacting with the
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add-ons framework, e.g.:
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* an :meth:`~scrapy.addons.AddonManager.add` method to load add-ons,
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* the :attr:`~scrapy.addons.AddonManager.enabled` list of enabled add-ons,
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* :meth:`~scrapy.addons.AddonManager.enable` and
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:meth:`~scrapy.addons.AddonManager.disable` methods,
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* the :attr:`~scrapy.addons.AddonManager.configs` dictionary which holds the
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configuration of all add-ons
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In this example, we ensure that the ``httpcache`` add-on is loaded, and that
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its ``expiration_secs`` configuration is set to ``60``::
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# addons is an instance of AddonManager
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if 'httpcache' not in addons:
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addons.add('httpcache', {'expiration_secs': 60})
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else:
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addons.configs['httpcache']['expiration_secs'] = 60
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Writing your own add-ons
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========================
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Add-ons are (any) Python *objects* that provide Scrapy's *add-on interface*.
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The interface is enforced through ``zope.interface``. This leaves the choice of
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Python object up the developer. Examples:
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* for a small pipeline, the add-on interface could be implemented in the same
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class that also implements the ``open/close_spider`` and ``process_item``
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callbacks
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* for larger add-ons, or for clearer structure, the interface could be provided
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by a stand-alone module
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The absolute minimum interface consists of two attributes:
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.. attribute:: name
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string with add-on name
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.. attribute:: version
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version string (PEP-404, e.g. ``'1.0.1'``)
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Of course, stating just these two attributes will not get you very far. Add-ons
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can provide three callback methods that are called at various stages before the
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crawling process:
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.. method:: update_settings(config, settings)
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This method is called during the initialization of the
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:class:`~scrapy.crawler.Crawler`. Here, you should perform dependency checks
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(e.g. for external Python libraries) and update the
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:class:`~scrapy.settings.Settings` object as wished, e.g. enable components
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for this add-on or set required configuration of other extensions.
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:param config: Configuration of this add-on
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:type config: ``dict``
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:param settings: The settings object storing Scrapy/component configuration
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:type settings: :class:`~scrapy.settings.Settings`
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.. method:: check_configuration(config, crawler)
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This method is called when the :class:`~scrapy.crawler.Crawler` has been
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fully initialized, immediately before it starts crawling. You can perform
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additional dependency and configuration checks here.
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:param config: Configuration of this add-on
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:type config: ``dict``
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:param crawler: Fully initialized Scrapy crawler
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:type crawler: :class:`~scrapy.crawler.Crawler`
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.. method:: update_addons(config, addons)
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This method is called immediately before :meth:`update_settings`, and should
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be used to enable and configure other *add-ons* only.
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When using this callback, be aware that there is no guarantee in which order
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the :meth:`update_addons` callbacks of enabled add-ons will be called.
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Add-ons that are added to the :class:`~scrapy.addons.AddonManager` during
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this callback will also have their :meth:`update_addons` method called.
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:param config: Configuration of this add-on
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:type config: ``dict``
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:param addons: Add-on manager holding all loaded add-ons
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:type addons: :class:`~scrapy.addons.AddonManager`
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Additionally, add-ons may (and should, where appropriate) provide one or more
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attributes that can be used for limited automated detection of possible
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dependency clashes:
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.. attribute:: requires
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list of built-in or custom components needed by this add-on, as strings.
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.. attribute:: modifies
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list of built-in or custom components whose functionality is affected or
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replaced by this add-on (a custom HTTP cache should list ``httpcache`` here)
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.. attribute:: provides
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list of components provided by this add-on (e.g. ``mongodb`` for an
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extension that provides generic read/write access to a MongoDB database)
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The entries in the :attr:`requires` and :attr:`modifies` attributes can be add-on
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names or components from other add-ons' :attr:`provides` attribute. You can
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specify :pep:`440`-style information about required versions. Examples::
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requires = ['httpcache']
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requires = ['otheraddon >= 2.0', 'yetanotheraddon']
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The Python object or module that is pointed to by an add-on path (e.g. given in
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the ``INSTALLED_ADDONS`` setting, or given to
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:meth:`~scrapy.addons.AddonManager.add`) does not necessarily have to be an
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add-on. Instead, it can provide an ``_addon`` attribute. This attribute can be
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either an add-on or another add-on path.
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Add-on base class
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=================
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Scrapy comes with a built-in base class for add-ons which provides some
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convenience functionality:
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* basic settings can be exported via :meth:`~scrapy.addons.Addon.export_basics`,
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configurable via :attr:`~scrapy.addons.Addon.basic_settings`.
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* a single component (e.g. an item pipeline or a downloader middleware) can be
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inserted into Scrapy's settings via
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:meth:`~scrapy.addons.Addon.export_component`, configurable via
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:attr:`~scrapy.addons.Addon.component_type`,
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:attr:`~scrapy.addons.Addon.component_key`,
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:attr:`~scrapy.addons.Addon.component`, and the ``order`` key in
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:attr:`~scrapy.addons.Addon.default_config`.
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* the add-on configuration can be exposed into Scrapy's settings via
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:meth:`~scrapy.addons.Addon.export_config`, configurable via
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:attr:`~scrapy.addons.Addon.default_config`,
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:attr:`~scrapy.addons.Addon.config_mapping`, and
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:attr:`~scrapy.addons.Addon.settings_prefix`.
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By default, the base add-on class will expose the add-on configuration into
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Scrapy's settings namespace, in caps and with the add-on name prepended. It is
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easy to write your own functionality while still being able to use the
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convenience functions by overwriting
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:meth:`~scrapy.addons.Addon.update_settings`.
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.. module:: scrapy.addons
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.. autoclass:: Addon
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:members:
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Add-on examples
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===============
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Set some basic configuration using the :class:`Addon` base class::
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from scrapy.addons import Addon
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class MyAddon(Addon):
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name = 'myaddon'
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version = '1.0'
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component = 'path.to.mypipeline'
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component_type = 'ITEM_PIPELINES'
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component_order = 200
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basic_settings = {
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'DNSCACHE_ENABLED': False,
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}
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Check dependencies::
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from scrapy.addons import Addon
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class MyAddon(Addon):
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name = 'myaddon'
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version = '1.0'
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def update_settings(self, config, settings):
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try:
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import boto
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except ImportError:
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raise RuntimeError("myaddon requires the boto library")
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else:
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self.export_config(config, settings)
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Enable a component that lives relative to the add-on (see
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:ref:`topics-api-settings`)::
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from scrapy.addons import Addon
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class MyAddon(Addon):
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name = 'myaddon'
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version = '1.0'
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component = __name__ + '.downloadermw.coolmw'
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component_type = 'DOWNLOADER_MIDDLEWARES'
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component_order = 900
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Instantiate components ad hoc::
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from path.to.my.pipelines import MySQLPipeline
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class MyAddon(object):
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name = 'myaddon'
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version = '1.0'
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def update_settings(self, config, settings):
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mysqlpl = MySQLPipeline(password=config['password'])
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settings.set(
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'ITEM_PIPELINES',
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{mysqlpl: 200},
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priority='addon',
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)
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Provide add-on interface along component interface::
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class MyPipeline(object):
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name = 'mypipeline'
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version = '1.0'
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def process_item(self, item, spider):
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# Do some processing here
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return item
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def update_settings(self, config, settings):
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settings.set(
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'ITEM_PIPELINES',
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{self: 200},
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priority='addon',
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)
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Enable another addon (see :ref:`topics-api-addonmanager`)::
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class MyAddon(object):
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name = 'myaddon'
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version = '1.0'
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def update_addons(self, config, addons):
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if 'httpcache' not in addons.enabled:
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addons.add('httpcache', {'expiration_secs': 60})
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Check configuration of fully initialized crawler (see
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:ref:`topics-api-crawler`)::
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class MyAddon(object):
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name = 'myaddon'
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version = '1.0'
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def update_settings(self, config, settings):
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settings.set('DNSCACHE_ENABLED', False, priority='addon')
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def check_configuration(self, config, crawler):
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if crawler.settings.getbool('DNSCACHE_ENABLED'):
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# The spider, some other add-on, or the user messed with the
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# DNS cache setting
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raise ValueError("myaddon is incompatible with DNS cache")
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Provide add-on interface through a module:
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.. No idea why just using '::' doesn't work for this one
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.. code-block:: python
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name = 'AddonModule'
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version = '1.0'
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class MyPipeline(object):
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# ...
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class MyDownloaderMiddleware(object):
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# ...
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def update_settings(config, settings):
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settings.set(
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'ITEM_PIPELINES',
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{MyPipeline(): 200},
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priority='addon',
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}
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settings.set(
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'DOWNLOADER_MIDDLEWARES',
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{MyDownloaderMiddleware(): 800},
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priority='addon',
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}
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Forward to other add-ons depending on Python version::
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# This could be a Python module, say project/pipelines/mypipeline.py, but
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# could also be done inside a class, etc.
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import six
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if six.PY3:
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# We're running Python 3
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_addon = 'path.to.addon'
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else:
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_addon = 'path.to.other.addon'
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@ -156,13 +156,15 @@ class AddonManager(Mapping):
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"""This class facilitates loading and storing :ref:`topics-addons`.
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You can treat it like a read-only dictionary in which keys correspond to
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add-on names and values correspond to the add-on objects::
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add-on names and values correspond to the add-on objects. Add-on
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configurations are saved in the :attr:`config` dictionary attribute::
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addons = AddonManager()
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# ... load some add-ons here
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print addons.enabled # prints names of all enabled add-ons
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print addons['TestAddon'].version # prints version of add-on with name
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# 'TestAddon'
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print addons.configs['TestAddon'] # prints configuration of 'TestAddon'
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"""
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