RuntimeError: Model class django.contrib.sites.models.Site doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS

The fix

Just add Django's Sites framework to your apps and set SITE_ID to 1 in your settings.

INSTALLED_APPS = [
    ...
    'django.contrib.sites',
]

SITE_ID = 1

Why does this happen?

Django's Sites Framework is a contributed module bundled with the core library that allows for the use of a single Django application/codebase with different sites (that can use different databases, logic in views, etc). The SITE_ID setting, as stated in the docs, "is used so that application data can hook into specific sites and a single database can manage content for multiple sites."

In this particular case AllAuth requires the Sites Framework in order to function properly. Many other third-party libraries are built to safely handle cases where multiple sites may be present and as such may be best .

I landed on this post via Google search. My problem was running tests that blew up with the error:

RuntimeError: Model class app.taxonomy.models.Term doesn't declare an explicit app_label and isn't in an application in INSTALLED_APPS.

This was running on Python 2.7.x with absolute imports. As mentioned by Colton Hicks in the comments, below, this can also happen with Python 3 (pytest 3.2.3 with Django 1.11.4).

In my tests.py:

from __future__ import absolute_import
[...]
from .models import Demographics, Term

After changing the relative import to an absolute import the problem went away:

from taxonomy.models import Demographics, Term

HTH

I got the error above. However my problem was the in the urls.py. I was following PyDanny cookiecutter django recipe. My error was to put in the urls.py this line:

    url(r'^demo/', include('project.demoapp.urls', namespace='demoapp')),

when I corrected to this:

    url(r'^demo/', include('demoapp.urls', namespace='demoapp')),

all was well. I also changed my local apps (I did this first and so the critical error was the url misconfiguration):

LOCAL_APPS = [
    # Your stuff: custom apps go here
    'demoapp.apps.DemoAppConfig',
]