Why is django's settings object a LazyObject?

问题:

Looking in django.conf I noticed that settings are implemented like this:

class LazySettings(LazyObject):     
...

What is the rationale behind making settings objects lazy?

回答1:

Check out this section of the Django coding style. The reason is explained in there (quoted below).

In addition to performance, third-party modules can modify settings when they are imported. Accessing settings should be delayed to ensure this configuration happens first.

Modules should not in general use settings stored in django.conf.settings at the top level (i.e. evaluated when the module is imported). The explanation for this is as follows:

Manual configuration of settings (i.e. not relying on the DJANGO_SETTINGS_MODULE environment variable) is allowed and possible as follows:

from django.conf import settings

settings.configure({}, SOME_SETTING='foo')

However, if any setting is accessed before the settings.configure line, this will not work. (Internally, settings is a LazyObject which configures itself automatically when the settings are accessed if it has not already been configured).

So, if there is a module containing some code as follows:

from django.conf import settings
from django.core.urlresolvers import get_callable

default_foo_view = get_callable(settings.FOO_EXAMPLE_VIEW)

...then importing this module will cause the settings object to be configured. That means that the ability for third parties to import the module at the top level is incompatible with the ability to configure the settings object manually, or makes it very difficult in some circumstances.

Instead of the above code, a level of laziness or indirection must be used, such as django.utils.functional.LazyObject, django.utils.functional.lazy() or lambda.

回答2:

Its a proxy object that abstracts the actual settings files, and makes it light weight until you actually access the settings you want. Once you start accessing the attributes, it will then load on demand. The idea is to reduce overhead in loading settings until you need them.

回答3:

I think that the purpose is to simplify settings from a developers point of view. So each project can have its own settings.py file without having the need to define all other Django settings as well. The LazySettings wrapper kind of combines everything from Django global_settings.py and your local settings. It lets the developer decide what settings he wants to overwrite, which he want to keep the defaults or which he wants to add.

The LazySettings class is maybe a wrong name for this, because I think it is not really lazy. Once you do something like from django.conf import settings the whole settings object is in your scope.