Django custom annotation function

You can't use python functions for annotations. Annotation is a computation that is done on a database level. Django provides you only a set of basic computations which can be processed by the database - SUM, AVERAGE, MIN, MAX and so on... For more complex stuffs only from version 1.8 we have an API for more complex query expressions. Before Django 1.8 the only way to achieve similar functionality was to use .extra which means to write plain SQL.

So you basically have two options.

First

Write your hotness computation in plain SQL using .extra or via the new API if your Django version is >= 1.8.

Second

Create hotness field inside you model, which will be calculated by a cron job once a day (or more often depending on your needs). And use it for your needs (the hottest list).

For those looking for an updated answer (Django 2.0+) it is possible to subclass Func to generate custom functions for aggregations as per the documentation . There is a good explanation and example here about 80% of the way through the post in the "Extending with custom database functions" section.