Fix django.contrib.sites detection in set_default_site command when u…

…sing AppConfig in INSTALLED_APPS: (#1662)

django.contrib.sites application can be specified in installed
application list either via its module path, or via its AppConfig
path. This can be either the default
one (django.contrib.sites.apps.SitesConfig), or user-provided
AppConfig class.

Instead of relying on a fixed name in the settings files, use the
application registry to check if the sites application is installed.

django_extensions/management/commands/set_default_site.py

@@ -3,6 +3,7 @@
 
 from django.conf import settings
 from django.core.management.base import BaseCommand, CommandError
+from django.apps import apps
 
 from django_extensions.management.utils import signalcommand
 
@@ -29,7 +30,7 @@ def add_arguments(self, parser):
 
     @signalcommand
     def handle(self, *args, **options):
-        if 'django.contrib.sites' not in settings.INSTALLED_APPS:
+        if not apps.is_installed('django.contrib.sites'):
             raise CommandError('The sites framework is not installed.')
 
         from django.contrib.sites.models import Site

tests/management/commands/test_set_default_site.py

@@ -82,3 +82,10 @@ def test_should_set_domain_only(self):
 
         self.assertEqual(result.name, 'example.com')
         self.assertEqual(result.domain, 'bar')
+
+    def test_should_not_raise_if_sites_installed_through_appconfig(self):
+        with self.modify_settings(INSTALLED_APPS={
+            'append': 'django.contrib.sites.apps.SitesConfig',
+            'remove': 'django.contrib.sites',
+        }):
+            call_command('set_default_site', '--name=foo', '--domain=foo.bar')