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# 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, plugins will be invoked in the order they are registered. When the `PLUGINS` setting is defined, plugin invocation order will be the order in which the plugins are listed in the `PLUGINS` setting. If you rely on auto-discovered namespace plugins and have no `PLUGINS` setting defined, plugins will be invoked in the same order that they are discovered (the same order as listed in the output of the `pelican-plugins` command). If you want to specify the order explicitly, disable auto-discovery by defining `PLUGINS` in the desired order.

### 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)