How does django convert string to modules

Question

I am trying to understand an other magic thing about django: it can convert strings to modules.

In settings.py, INSTALLED_APPS is declared like that:

INSTALLED_APPS = (
    'django.contrib.auth',
    'django.contrib.contenttypes',
    'django.contrib.sessions',
)

All it contains is strings. But django will convert those strings to modules and import them later.

I want to do be able to do the same thing. but i don't know how. I have a dictionary of renderer dispatcher in settings.py:

RESOUCE_RENDERER = {
    'video': 'video_player',
    'audio': 'audio_player', 
}

I want to use it later like this: RESOURCE_RENDERER['video'](MyVideo).

I cannot assign directly the function name(eg video_player) because it lives in a module that needs settings.py.