I discovered this hacky way to use the local_settings trick to extend and/or override values in the main Django settings file today. Some projects use a "reverse" version of the local settings trick (which is explained below), whereby the main settings file becomes settings_local.py or something similar, which first imports settings.py, and then extends or overrides the values as required.

I didn't want to change the name of the project settings file to settings_local.py, however, as it would mean changing the WSGI file on every server that the project runs on.

The Local Settings Trick

It's a well-known trick to use a file called local_settings.py or something similar, with a piece of code at the bottom of the main settings file:

try:
    from local_settings import *
except:
    pass

This allows you to override the value of settings variables. It won't work, however, if you wish to extend a settings variable (for example, adding an app to INSTALLED_APPS). For this, I have found that the following ugly hack seems to do the job.

The Ugly Hack

For this hack, replace the snippet at the bottom of the main settings file with the following code:

try:
    LOCAL_SETTINGS
except NameError:
    try:
        from local_settings import *
    except ImportError:
        pass

What this is doing is effectively checking for the presence of a variable called LOCAL_SETTINGS. The local settings file then contains this code:

LOCAL_SETTINGS = True
from settings import *

This means that the local settings file will only be imported once, and it has all the variables in the main settings file available to extend at will. For example, to enable the Django debug toolbar:

MIDDLEWARE_CLASSES += (
    'debug_toolbar.middleware.DebugToolbarMiddleware',
)
INTERNAL_IPS = ('127.0.0.1',)
INSTALLED_APPS += ('debug_toolbar',)

The code is ugly (and results in the main settings file being parsed twice) but it works!