Adding a Jinja2 Filter with a Pelican Plugin

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|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 =
    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, 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."""

Then I had to update my 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:

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