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Pelican internals
#################
This section describe how Pelican works internally. As you'll see, it's
quite simple, but a bit of documentation doesn't hurt. :)
You can also find in the :doc:`report` section an excerpt of a report the
original author wrote with some software design information.
.. _report: :doc:`report`
Overall structure
=================
What Pelican does is take a list of files and process them into some
sort of output. Usually, the input files are reStructuredText and Markdown
files, and the output is a blog, but both input and output can be anything you
want.
The logic is separated into different classes and concepts:
* **Writers** are responsible for writing files: .html files, RSS feeds, and so
on. Since those operations are commonly used, the object is created once and
then passed to the generators.
* **Readers** are used to read from various formats (Markdown and
reStructuredText for now, but the system is extensible). Given a file, they return
metadata (author, tags, category, etc.) and content (HTML-formatted).
* **Generators** generate the different outputs. For instance, Pelican comes with
``ArticlesGenerator`` and ``PageGenerator``. Given a configuration, they can do
whatever they want. Most of the time, it's generating files from inputs.
* Pelican also uses templates, so it's easy to write your own theme. The
syntax is `Jinja2 <http://jinja.pocoo.org/>`_ and is very easy to learn, so
don't hesitate to jump in and build your own theme.
How to implement a new reader?
==============================
Is there an awesome markup language you want to add to Pelican?
Well, the only thing you have to do is to create a class with a ``read``
method that returns HTML content and some metadata.
Take a look at the Markdown reader::
class MarkdownReader(Reader):
enabled = bool(Markdown)
def read(self, filename):
"""Parse content and metadata of markdown files"""
text = open(filename)
md = Markdown(extensions = ['meta', 'codehilite'])
content = md.convert(text)
metadata = {}
for name, value in md.Meta.items():
if name in _METADATA_FIELDS:
meta = _METADATA_FIELDS[name](value[0])
else:
meta = value[0]
metadata[name.lower()] = meta
return content, metadata
Simple, isn't it?
If your new reader requires additional Python dependencies, then you should wrap
their ``import`` statements in a ``try...except`` block. Then inside the reader's
class, set the ``enabled`` class attribute to mark import success or failure.
This makes it possible for users to continue using their favourite markup method
without needing to install modules for formats they don't use.
How to implement a new generator?
=================================
Generators have two important methods. You're not forced to create
both; only the existing ones will be called.
* ``generate_context``, that is called first, for all the generators.
Do whatever you have to do, and update the global context if needed. This
context is shared between all generators, and will be passed to the
templates. For instance, the ``PageGenerator`` ``generate_context`` method
finds all the pages, transforms them into objects, and populates the context
with them. Be careful *not* to output anything using this context at this
stage, as it is likely to change by the effect of other generators.
* ``generate_output`` is then called. And guess what is it made for? Oh,
generating the output. :) It's here that you may want to look at the context
and call the methods of the ``writer`` object that is passed as the first
argument of this function. In the ``PageGenerator`` example, this method will
look at all the pages recorded in the global context and output a file on
the disk (using the writer method ``write_file``) for each page encountered.
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