Fixed #21829 -- Added default AppConfigs.

Thanks Russell for the report, Marc for the initial patch, Carl for the
final review, and everyone who contributed to the design discussion.

tests/admin_scripts/tests.py

@@ -1113,7 +1113,7 @@ class ManageCheck(AdminScriptTestCase):
             apps=[
                 'admin_scripts.complex_app',
                 'admin_scripts.simple_app',
-                'django.contrib.admin',
+                'django.contrib.admin.apps.SimpleAdminConfig',
                 'django.contrib.auth',
                 'django.contrib.contenttypes',
             ],

tests/apps/default_config_app/__init__.py

@@ -0,0 +1 @@
+default_app_config = 'apps.default_config_app.apps.CustomConfig'

tests/apps/default_config_app/apps.py

@@ -0,0 +1,5 @@
+from django.apps import AppConfig
+
+
+class CustomConfig(AppConfig):
+    name = 'apps.default_config_app'

tests/apps/tests.py

@@ -7,6 +7,7 @@ from django.db import models
 from django.test import TestCase, override_settings
 from django.utils import six
 
+from .default_config_app.apps import CustomConfig
 from .models import TotallyNormal, SoAlternative, new_apps
 
 
@@ -82,6 +83,11 @@ class AppsTests(TestCase):
             with self.settings(INSTALLED_APPS=['apps.apps.NoSuchConfig']):
                 pass
 
+    def test_default_app_config(self):
+        with self.settings(INSTALLED_APPS=['apps.default_config_app']):
+            config = apps.get_app_config('default_config_app')
+        self.assertIsInstance(config, CustomConfig)
+
     @override_settings(INSTALLED_APPS=SOME_INSTALLED_APPS)
     def test_get_app_configs(self):
         """

tests/i18n/tests.py

@@ -1049,7 +1049,7 @@ class AppResolutionOrderI18NTests(ResolutionOrderI18NTests):
             # Doesn't work because it's added later in the list.
             self.assertUgettext('Date/time', 'Datum/Zeit')
 
-            with self.modify_settings(INSTALLED_APPS={'remove': 'django.contrib.admin'}):
+            with self.modify_settings(INSTALLED_APPS={'remove': 'django.contrib.admin.apps.SimpleAdminConfig'}):
                 self.flush_caches()
                 activate('de')
 

tests/runtests.py

@@ -44,7 +44,7 @@ ALWAYS_INSTALLED_APPS = [
     'django.contrib.sessions',
     'django.contrib.messages',
     'django.contrib.comments',
-    'django.contrib.admin',
+    'django.contrib.admin.apps.SimpleAdminConfig',
     'django.contrib.admindocs',
     'django.contrib.staticfiles',
     'django.contrib.humanize',
@@ -168,12 +168,11 @@ def setup(verbosity, test_labels):
                 for label in test_labels_set)
 
         installed_app_names = set(get_installed())
-        if module_found_in_labels:
+        if module_found_in_labels and module_label not in installed_app_names:
             if verbosity >= 2:
                 print("Importing application %s" % module_name)
             # HACK.
-            if module_label not in installed_app_names:
-                settings.INSTALLED_APPS.append(module_label)
+            settings.INSTALLED_APPS.append(module_label)
             app_config = AppConfig.create(module_label)
             apps.app_configs[app_config.label] = app_config
             app_config.import_models(apps.all_models[app_config.label])