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)