With my new blog setup, I decided to give Google AdSense a try. I don't have many regular readers, but I do have a few pages that attract search-engine traffic. So, to monetize those older pages without shoving ads in my regular readers' faces, I decided to show ads on posts that were over a month old. That should be easy enough to add to the article.html template:
{% if article.date|age_in_days > 30 %}
<!-- ... Include AdSense code ... -->
{% endif %}
Only one problem: there is no age_in_days filter provided by Pelican or Jinja. I'd have to write a custom filter myself.
A Jinja custom filter is just a Python function that takes an argument and produces a result. My filter function is pretty simple:
from datetime import datetime
def age_in_days(dt):
"""Return a number of days between now and a specified datetime."""
now = datetime.now(dt.tzinfo)
delta = now - dt
return delta.days
I had to figure out how to add that to Pelican's Jinja2 template-rendering environment. I found a How to add a custom Jinja filter to Pelican post that showed how to import a function and set the Pelican's JINJA_FILTERS variable in pelicanconf.py, but I didn't like the idea of adding imports and functions to a configuration file. I decided it would be cleaner (and more fun) to figure out how to make a Pelican plugin that provided my filter.
It wasn't hard to figure this out, because I found a Jinja Filters plugin that provides a set of filters, so I could just copy the little bits I needed from that for registering my filter. A Pelican plugin must provide a register() function that sets up a mapping of Pelican "signals" to functions that are to be called. My plugin handles the generator_init signal by adding the age_in_days filter to Pelican's Jinja environment:
from pelican import signals
from . import ageindays
def add_filter(pelican):
"""Add age_in_days filter to Pelican."""
pelican.env.filters.update({'age_in_days': ageindays.age_in_days})
def register():
"""Plugin registration."""
signals.generator_init.connect(add_filter)
Then I had to update my pelicanconf.py to load this plugin in addition to the other plugins I use:
PLUGIN_PATHS = ['../plugins', '../pelican-plugins']
PLUGINS = ['age_in_days', 'pelican_alias', 'neighbors', 'sitemap']
And that's it.
You can see the whole plugin here: https://github.com/kristopherjohnson/undefinedvalue-pelican/tree/master/plugins/age_in_days