These are some proposed filter configuration designs for Django Assetfiles. The goal is to have a natural API for defining the inputs and filters, while being able to determine the input files from given output files.

filters_1.py

from assetfiles.conf import filter, patterns
from assetfiles.filters import (CoffeeFilter, ConcatFilter,
                                SassFilter, UglifyFilter)


# Resembles Django's URL configuration where the filters are basically views.
# Apps could define their own filter patterns, but you would not need to
# explicitly `include` filter patterns.
#
# The big trick here is reversing the patterns in order to find the input files
# for given output files. Can this be done using Regex?
filterpatterns = patterns(
    # Simplest possible configuration. This should default to looking for files
    # with the .sass or .scss extension and converting them to .css files.
    SassFilter,

    # This works the same as above, but explicitly defines the pattern
    # for matching the file extension. The SassFilter automatically uses the
    # compressed style when running with `DEBUG = False`.
    filter(r'^.*\.s[ac]ss$', SassFilter),

    # Here we chain a few filters together. The UglifyFilter only compresses
    # with `DEBUG = False`.
    filter(r'^.*\.coffee$', CoffeeFilter, UglifyFilter),

    # When using the ConcatFilter (or any filter for that matter) you can
    # explicity set the output file.
    filter(r'^application/.*\.coffee$',
           ConcatFilter, CoffeeFilter, UglifyFilter,
           output='application.js'),
)

filters_2.py

from assetfiles.conf import filter, patterns
from assetfiles.filters import (CoffeeFilter, ConcatFilter,
                                SassFilter, UglifyFilter)


# Maybe it would be much simpler for the API design and implementation if
# we use a glob syntax for defining the inputs.
filterpatterns = patterns(
    filter('**/*.s[ac]ss', SassFilter),
    filter('application/**/*.coffee',
           ConcatFilter, CoffeeFilter, UglifyFilter,
           output='application.js'),
)

filters_3.py

# Maybe the filter definition should be within the SETTINGS. In this case, it
# should be definable without any imports.
#
# The problem with this is that it is not really clear what's happening. What's
# an input configuration? What's an output configuration? Also it gets really
# silly looking with all the nested tuples.
ASSETFILES_FILTERS = (
    # Again the simplest possible definition
    'assetfiles.filters.SassFilter',

    # Explicitly defining the input pattern.
    ('**/*.s[ac]ss', 'assetfiles.filters.SassFilter'),

    # More complicated example, with chained filters and input and output
    # configuration.
    (
        'application/**/*.coffee',
        (
            'assetfiles.filters.ConcatFilter',
            'assetfiles.filters.CoffeeFilter',
            'assetfiles.filters.UglifyFilter'
        ),
        'application.js'
    ),
)