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How to configure Django settings in Aldryn applications

In Django applications, settings are handled via the settings module (usually, the settings.py file).

Settings can be added to this file in the usual way, but in Divio applications, some settings need to be inspected and manipulated programmatically, to allow the addons system to handle configuration automatically. See Working with Django addons (legacy) for more on how this works.

This can entail a little extra work when you need to change settings yourself, but the huge convenience it offers is more than worth the effort.

The correct way to manage settings such as INSTALLED_APPS is to manipulate the existing value, after having loaded the settings from the addons with aldryn_addons.settings.load(locals()). For example, in the default settings.py you will find:

import aldryn_addons.settings
aldryn_addons.settings.load(locals())

INSTALLED_APPS.extend([
    # add your application specific apps here
])

This allows you to add items to INSTALLED_APPS without overwriting existing items, by manipulating the list.

You will need to do the same for other configured settings, which will include:

  • MIDDLEWARE (or the older MIDDLEWARE_CLASSES)
  • TEMPLATES (or the older TEMPLATE_CONTEXT_PROCESSORS, TEMPLATE_DEBUG and other template settings)
  • application-specific settings, for example that belong to django CMS or Wagtail. See each application's aldryn_config.py file for the settings it will configure.

Inserting an item at a particular position​

Sometimes it's not enough just to add an application or class to a list. It may need to be added before another item. Say you need to add your application security just before cms. In this case you can target cms in the list like this:

INSTALLED_APPS.insert(
    INSTALLED_APPS.index("cms") + 0,
    "security"
)

(+ 0 will insert the new item "security" immediately before "cms" in the list).

Of course you can use Python to manipulate the collections in any way you require.

Manipulating more complex settings​

Note that in the case of more complex settings, like TEMPLATES, which is no longer a simple list, you can't just extend them directly with new items, you'll need to dive into them to target the right list in the right dictionary, for example:

TEMPLATES[0]["OPTIONS"]["context_processors"].append('my_application.some_context_processor')

Listing applied settings​

The Django diffsettings management command will show the differences between your settings and Django's defaults, for example with:

docker compose run web python manage.py diffsettings

In some applications (with addons that manipulate settings late in the start-up process), you may get an error: RuntimeError: dictionary changed size during iteration.

In this case you can run a script to print out your settings:

from django.conf import settings

settings.configure()

django_settings = {}

for attr in dir(settings):
    value = getattr(settings, attr)
    django_settings[attr] = value

for key, value in django_settings.items():
    if not key.startswith('_'):
        print('%s = %r' % (key, value))