Filters are called once an object has been saved to the database, they build on the Django signal infrastrcture.
In the settings.py file filters are activated by specifying them within the POST_SAVE_FILTERS variable:
POST_SAVE_FILTERS = [
("tardis.tardis_portal.filters.exif.EXIFFilter", ["EXIF", "http://exif.schema"]),
]
The format they are specified in is:
(<filter class path>, [args], {kwargs})
Where args and kwargs are both optional.