Plugins
*******

Beginning with version 3.0, Pelican supports plugins. Plugins are a
way to add features to Pelican without having to directly modify the
Pelican core.


How to use plugins
==================

Starting with version 4.5, Pelican moved to a new plugin structure
utilizing namespace packages that can be easily installed via Pip.
Plugins supporting this structure will install under the namespace
package "pelican.plugins" and can be automatically discovered by
Pelican. To see a list of Pip-installed namespace plugins that are
active in your environment, run:

   pelican-plugins

If you leave the "PLUGINS" setting as default ("None"), Pelican will
automatically discover namespace plugins and register them. If, on the
other hand, you specify a "PLUGINS" setting as a list of plugins, this
auto-discovery will be disabled. At that point, only the plugins you
specify will be registered, and you must explicitly list any namespace
plugins as well.

If you are using the "PLUGINS" setting, you can specify plugins in two
ways. The first method specifies plugins as a list of strings.
Namespace plugins can be specified either by their full names
("pelican.plugins.myplugin") or by their short names ("myplugin"):

   PLUGINS = ['package.myplugin',
              'namespace_plugin1',
              'pelican.plugins.namespace_plugin2']

Alternatively, you can import them in your settings file and pass the
modules:

   from package import myplugin
   from pelican.plugins import namespace_plugin1, namespace_plugin2
   PLUGINS = [myplugin, namespace_plugin1, namespace_plugin2]

Note:

  When experimenting with different plugins (especially the ones that
  deal with metadata and content) caching may interfere and the
  changes may not be visible. In such cases disable caching with
  "LOAD_CONTENT_CACHE = False" or use the "--ignore-cache" command-
  line switch.

If your plugins are not in an importable path, you can specify a list
of paths via the "PLUGIN_PATHS" setting. As shown in the following
example, paths in the "PLUGIN_PATHS" list can be absolute or relative
to the settings file:

   PLUGIN_PATHS = ["plugins", "/srv/pelican/plugins"]
   PLUGINS = ["assets", "liquid_tags", "sitemap"]


Where to find plugins
=====================

Namespace plugins can be found in the pelican-plugins organization as
individual repositories. Legacy plugins are located in the pelican-
plugins repository and will be gradually phased out in favor of the
namespace versions.

Please note that while we do our best to review and maintain these
plugins, they are submitted by the Pelican community and thus may have
varying levels of support and interoperability.


How to create plugins
=====================

Plugins are based on the concept of signals. Pelican sends signals,
and plugins subscribe to those signals. The list of available signals
is documented in a subsequent section.

The only rule to follow for plugins is to define a "register"
callable, in which you map the signals to your plugin logic. Let's
take a simple example:

   import logging

   from pelican import signals

   log = logging.getLogger(__name__)

   def test(sender):
       log.debug("%s initialized !!", sender)

   def register():
       signals.initialized.connect(test)

Note:

  Signal receivers are weakly-referenced and thus must not be defined
  within your "register" callable or they will be garbage-collected
  before the signal is emitted.

If multiple plugins connect to the same signal, there is no way to
guarantee or control in which order the plugins will be executed. This
is a limitation inherited from Blinker, the dependency Pelican uses to
implement signals.


Namespace plugin structure
--------------------------

Namespace plugins must adhere to a certain structure in order to
function properly. They need to be installable (i.e. contain
"setup.py" or equivalent) and have a folder structure as follows:

   myplugin
   ├── pelican
   │   └── plugins
   │       └── myplugin
   │           ├── __init__.py
   │           └── ...
   ├── ...
   └── setup.py

It is crucial that "pelican" or "pelican/plugins" folder **not**
contain an "__init__.py" file. In fact, it is best to have those
folders empty besides the listed folders in the above structure and
keep your plugin related files contained solely in the
"pelican/plugins/myplugin" folder to avoid any issues.

To easily set up the proper structure, a cookiecutter template for
plugins is provided. Refer to that project's README for instructions
on how to use it.


List of signals
===============

Here is the list of currently implemented signals:

+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| Signal                            | Arguments                    | Description                                                                 |
|===================================|==============================|=============================================================================|
| initialized                       | pelican object               |                                                                             |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| finalized                         | pelican object               | invoked after all the generators are executed and just before pelican exits |
|                                   |                              | useful for custom post processing actions, such as: - minifying js/css      |
|                                   |                              | assets. - notify/ping search engines with an updated sitemap.               |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| generator_init                    | generator                    | invoked in the Generator.__init__                                           |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| all_generators_finalized          | generators                   | invoked after all the generators are executed and before writing output     |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| readers_init                      | readers                      | invoked in the Readers.__init__                                             |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| article_generator_context         | article_generator, metadata  |                                                                             |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| article_generator_preread         | article_generator            | invoked before a article is read in ArticlesGenerator.generate_context; use |
|                                   |                              | if code needs to do something before every article is parsed                |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| article_generator_init            | article_generator            | invoked in the ArticlesGenerator.__init__                                   |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| article_generator_pretaxonomy     | article_generator            | invoked before categories and tags lists are created useful when e.g.       |
|                                   |                              | modifying the list of articles to be generated so that removed articles are |
|                                   |                              | not leaked in categories or tags                                            |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| article_generator_finalized       | article_generator            | invoked at the end of ArticlesGenerator.generate_context                    |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| article_generator_write_article   | article_generator, content   | invoked before writing each article, the article is passed as content       |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| article_writer_finalized          | article_generator, writer    | invoked after all articles and related pages have been written, but before  |
|                                   |                              | the article generator is closed.                                            |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| get_generators                    | pelican object               | invoked in Pelican.get_generator_classes, can return a Generator, or        |
|                                   |                              | several generators in a tuple or in a list.                                 |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| get_writer                        | pelican object               | invoked in Pelican.get_writer, can return a custom Writer.                  |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| page_generator_context            | page_generator, metadata     |                                                                             |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| page_generator_preread            | page_generator               | invoked before a page is read in PageGenerator.generate_context; use if     |
|                                   |                              | code needs to do something before every page is parsed.                     |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| page_generator_init               | page_generator               | invoked in the PagesGenerator.__init__                                      |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| page_generator_finalized          | page_generator               | invoked at the end of PagesGenerator.generate_context                       |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| page_generator_write_page         | page_generator, content      | invoked before writing each page, the page is passed as content             |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| page_writer_finalized             | page_generator, writer       | invoked after all pages have been written, but before the page generator is |
|                                   |                              | closed.                                                                     |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| static_generator_context          | static_generator, metadata   |                                                                             |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| static_generator_preread          | static_generator             | invoked before a static file is read in StaticGenerator.generate_context;   |
|                                   |                              | use if code needs to do something before every static file is added to the  |
|                                   |                              | staticfiles list.                                                           |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| static_generator_init             | static_generator             | invoked in the StaticGenerator.__init__                                     |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| static_generator_finalized        | static_generator             | invoked at the end of StaticGenerator.generate_context                      |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| content_object_init               | content_object               | invoked at the end of Content.__init__                                      |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| content_written                   | path, context                | invoked each time a content file is written.                                |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| feed_generated                    | context, feed                | invoked each time a feed gets generated. Can be used to modify a feed       |
|                                   |                              | object before it gets written.                                              |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+
| feed_written                      | path, context, feed          | invoked each time a feed file is written.                                   |
+-----------------------------------+------------------------------+-----------------------------------------------------------------------------+

Warning:

  Avoid "content_object_init" signal if you intend to read "summary"
  or "content" properties of the content object. That combination can
  result in unresolved links when Linking to internal content (see
  pelican-plugins bug #314). Use "_summary" and "_content" properties
  instead, or, alternatively, run your plugin at a later stage (e.g.
  "all_generators_finalized").

Note:

  After Pelican 3.2, signal names were standardized.  Older plugins
  may need to be updated to use the new names:

  +----------------------------+-----------------------------+
  | Old name                   | New name                    |
  |============================|=============================|
  | article_generate_context   | article_generator_context   |
  +----------------------------+-----------------------------+
  | article_generate_finalized | article_generator_finalized |
  +----------------------------+-----------------------------+
  | article_generate_preread   | article_generator_preread   |
  +----------------------------+-----------------------------+
  | pages_generate_context     | page_generator_context      |
  +----------------------------+-----------------------------+
  | pages_generate_preread     | page_generator_preread      |
  +----------------------------+-----------------------------+
  | pages_generator_finalized  | page_generator_finalized    |
  +----------------------------+-----------------------------+
  | pages_generator_init       | page_generator_init         |
  +----------------------------+-----------------------------+
  | static_generate_context    | static_generator_context    |
  +----------------------------+-----------------------------+
  | static_generate_preread    | static_generator_preread    |
  +----------------------------+-----------------------------+


Recipes
=======

We eventually realised some of the recipes to create plugins would be
best shared in the documentation somewhere, so here they are!


How to create a new reader
--------------------------

One thing you might want is to add support for your very own input
format. While it might make sense to add this feature in Pelican core,
we wisely chose to avoid this situation and instead have the different
readers defined via plugins.

The rationale behind this choice is mainly that plugins are really
easy to write and don't slow down Pelican itself when they're not
active.

No more talking — here is an example:

   from pelican import signals
   from pelican.readers import BaseReader

   # Create a new reader class, inheriting from the pelican.reader.BaseReader
   class NewReader(BaseReader):
       enabled = True  # Yeah, you probably want that :-)

       # The list of file extensions you want this reader to match with.
       # If multiple readers were to use the same extension, the latest will
       # win (so the one you're defining here, most probably).
       file_extensions = ['yeah']

       # You need to have a read method, which takes a filename and returns
       # some content and the associated metadata.
       def read(self, filename):
           metadata = {'title': 'Oh yeah',
                       'category': 'Foo',
                       'date': '2012-12-01'}

           parsed = {}
           for key, value in metadata.items():
               parsed[key] = self.process_metadata(key, value)

           return "Some content", parsed

   def add_reader(readers):
       readers.reader_classes['yeah'] = NewReader

   # This is how pelican works.
   def register():
       signals.readers_init.connect(add_reader)


Adding a new generator
----------------------

Adding a new generator is also really easy. You might want to have a
look at Pelican internals for more information on how to create your
own generator.

   def get_generators(pelican_object):
       # define a new generator here if you need to
       return MyGenerator

   def register():
       signals.get_generators.connect(get_generators)


Adding a new writer
-------------------

Adding a writer will allow you to output additional file formats to
disk, or change how the existing formats are written to disk. Note
that only one writer will be active at a time, so be sure to either
subclass the built-in Writer, or completely re-implement it.

Here is a basic example of how to set up your own writer:

   from pelican.writers import Writer
   from pelican import signals

   class MyWriter(Writer):
       # define new writer functionality
       pass


   def add_writer(pelican_object):
       # use pelican_instance to setup stuff if needed
       return MyWriter


   def register():
       signals.get_writer.connect(add_writer)


Using Plugins to Inject Content
-------------------------------

You can programmatically inject articles or pages using plugins. This
can be useful if you plan to fetch articles from an API, for example.

Following is a simple example of how one can build a plugin that
injects a custom article, using the "article_generator_pretaxonomy"
signal:

   import datetime

   from pelican import signals
   from pelican.contents import Article
   from pelican.readers import BaseReader

   def addArticle(articleGenerator):
       settings = articleGenerator.settings

       # Author, category, and tags are objects, not strings, so they need to
       # be handled using BaseReader's process_metadata() function.
       baseReader = BaseReader(settings)

       content = "I am the body of an injected article!"

       newArticle = Article(content, {
           "title": "Injected Article!",
           "date": datetime.datetime.now(),
           "category": baseReader.process_metadata("category", "fromAPI"),
           "tags": baseReader.process_metadata("tags", "tagA, tagB")
       })

       articleGenerator.articles.insert(0, newArticle)

   def register():
       signals.article_generator_pretaxonomy.connect(addArticle)
