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.

django/apps/base.py

@@ -61,13 +61,12 @@ class AppConfig(object):
         Factory that creates an app config from an entry in INSTALLED_APPS.
         """
         try:
-            # If import_module succeeds, entry is a path to an app module.
+            # If import_module succeeds, entry is a path to an app module,
+            # which may specify an app config class with default_app_config.
             # Otherwise, entry is a path to an app config class or an error.
             module = import_module(entry)
 
         except ImportError:
-            # Avoid django.utils.module_loading.import_by_path because it
-            # masks errors -- it reraises ImportError as ImproperlyConfigured.
             mod_path, _, cls_name = entry.rpartition('.')
 
             # Raise the original exception when entry cannot be a path to an
@@ -75,39 +74,51 @@ class AppConfig(object):
             if not mod_path:
                 raise
 
-            mod = import_module(mod_path)
-            try:
-                cls = getattr(mod, cls_name)
-            except AttributeError:
-                # Emulate the error that "from <mod_path> import <cls_name>"
-                # would raise when <mod_path> exists but not <cls_name>, with
-                # more context (Python just says "cannot import name ...").
-                raise ImportError(
-                    "cannot import name '%s' from '%s'" % (cls_name, mod_path))
-
-            # Check for obvious errors. (This check prevents duck typing, but
-            # it could be removed if it became a problem in practice.)
-            if not issubclass(cls, AppConfig):
-                raise ImproperlyConfigured(
-                    "'%s' isn't a subclass of AppConfig." % entry)
-
-            # Obtain app name here rather than in AppClass.__init__ to keep
-            # all error checking for entries in INSTALLED_APPS in one place.
-            try:
-                app_name = cls.name
-            except AttributeError:
-                raise ImproperlyConfigured(
-                    "'%s' must supply a name attribute." % entry)
-
-            # Ensure app_name points to a valid module.
-            app_module = import_module(app_name)
-
-            # Entry is a path to an app config class.
-            return cls(app_name, app_module)
-
         else:
-            # Entry is a path to an app module.
-            return cls(entry, module)
+            try:
+                # If this works, the app module specifies an app config class.
+                entry = module.default_app_config
+            except AttributeError:
+                # Otherwise, it simply uses the default app config class.
+                return cls(entry, module)
+            else:
+                mod_path, _, cls_name = entry.rpartition('.')
+
+        # If we're reaching this point, we must load the app config class
+        # located at <mod_path>.<cls_name>.
+
+        # Avoid django.utils.module_loading.import_by_path because it
+        # masks errors -- it reraises ImportError as ImproperlyConfigured.
+        mod = import_module(mod_path)
+        try:
+            cls = getattr(mod, cls_name)
+        except AttributeError:
+            # Emulate the error that "from <mod_path> import <cls_name>"
+            # would raise when <mod_path> exists but not <cls_name>, with
+            # more context (Python just says "cannot import name ...").
+            raise ImportError(
+                "cannot import name '%s' from '%s'" % (cls_name, mod_path))
+
+        # Check for obvious errors. (This check prevents duck typing, but
+        # it could be removed if it became a problem in practice.)
+        if not issubclass(cls, AppConfig):
+            raise ImproperlyConfigured(
+                "'%s' isn't a subclass of AppConfig." % entry)
+
+        # Obtain app name here rather than in AppClass.__init__ to keep
+        # all error checking for entries in INSTALLED_APPS in one place.
+        try:
+            app_name = cls.name
+        except AttributeError:
+            raise ImproperlyConfigured(
+                "'%s' must supply a name attribute." % entry)
+
+        # Ensure app_name points to a valid module.
+        app_module = import_module(app_name)
+
+        # Entry is a path to an app config class.
+        return cls(app_name, app_module)
+
 
     def get_model(self, model_name):
         """

django/conf/project_template/project_name/settings.py

@@ -30,7 +30,7 @@ ALLOWED_HOSTS = []
 # Application definition
 
 INSTALLED_APPS = (
-    'django.contrib.admin.apps.AdminConfig',
+    'django.contrib.admin',
     'django.contrib.auth',
     'django.contrib.contenttypes',
     'django.contrib.sessions',

django/contrib/admin/__init__.py

@@ -1,6 +1,5 @@
 # ACTION_CHECKBOX_NAME is unused, but should stay since its import from here
 # has been referenced in documentation.
-from django.contrib.admin.checks import check_admin_app
 from django.contrib.admin.decorators import register
 from django.contrib.admin.helpers import ACTION_CHECKBOX_NAME
 from django.contrib.admin.options import ModelAdmin, HORIZONTAL, VERTICAL
@@ -9,7 +8,6 @@ from django.contrib.admin.filters import (ListFilter, SimpleListFilter,
     FieldListFilter, BooleanFieldListFilter, RelatedFieldListFilter,
     ChoicesFieldListFilter, DateFieldListFilter, AllValuesFieldListFilter)
 from django.contrib.admin.sites import AdminSite, site
-from django.core import checks
 from django.utils.module_loading import autodiscover_modules
 
 __all__ = [
@@ -24,4 +22,5 @@ __all__ = [
 def autodiscover():
     autodiscover_modules('admin', register_to=site)
 
-checks.register('admin')(check_admin_app)
+
+default_app_config = 'django.contrib.admin.apps.AdminConfig'

django/contrib/admin/apps.py

@@ -1,11 +1,22 @@
 from django.apps import AppConfig
-
+from django.core import checks
+from django.contrib.admin.checks import check_admin_app
 from django.utils.translation import ugettext_lazy as _
 
 
-class AdminConfig(AppConfig):
+class SimpleAdminConfig(AppConfig):
+    """Simple AppConfig which does not do automatic discovery."""
+
     name = 'django.contrib.admin'
     verbose_name = _("administration")
 
     def ready(self):
+        checks.register('admin')(check_admin_app)
+
+
+class AdminConfig(SimpleAdminConfig):
+    """The default AppConfig for admin which does autodiscovery."""
+
+    def ready(self):
+        super(AdminConfig, self).ready()
         self.module.autodiscover()

django/contrib/admin/sites.py

@@ -161,7 +161,7 @@ class AdminSite(object):
         installed, as well as the auth context processor.
         """
         if not apps.is_installed('django.contrib.admin'):
-            raise ImproperlyConfigured("Put 'django.contrib.admin.apps.AdminConfig' in "
+            raise ImproperlyConfigured("Put 'django.contrib.admin' in "
                 "your INSTALLED_APPS setting in order to use the admin application.")
         if not apps.is_installed('django.contrib.contenttypes'):
             raise ImproperlyConfigured("Put 'django.contrib.contenttypes' in "

django/contrib/admindocs/__init__.py

@@ -0,0 +1 @@
+default_app_config = 'django.contrib.admindocs.apps.AdminDocsConfig'

django/contrib/auth/__init__.py

@@ -2,8 +2,6 @@ import inspect
 import re
 
 from django.conf import settings
-from django.contrib.auth.checks import check_user_model
-from django.core import checks
 from django.core.exceptions import ImproperlyConfigured, PermissionDenied
 from django.utils.module_loading import import_by_path
 from django.middleware.csrf import rotate_token
@@ -15,10 +13,6 @@ BACKEND_SESSION_KEY = '_auth_user_backend'
 REDIRECT_FIELD_NAME = 'next'
 
 
-# Register the user model checks
-checks.register('models')(check_user_model)
-
-
 def load_backend(path):
     return import_by_path(path)()
 
@@ -164,3 +158,6 @@ def get_permission_codename(action, opts):
     Returns the codename of the permission for the specified action.
     """
     return '%s_%s' % (action, opts.model_name)
+
+
+default_app_config = 'django.contrib.auth.apps.AuthConfig'

django/contrib/auth/apps.py

@@ -1,4 +1,6 @@
 from django.apps import AppConfig
+from django.core import checks
+from django.contrib.auth.checks import check_user_model
 
 from django.utils.translation import ugettext_lazy as _
 
@@ -6,3 +8,6 @@ from django.utils.translation import ugettext_lazy as _
 class AuthConfig(AppConfig):
     name = 'django.contrib.auth'
     verbose_name = _("authentication and authorization")
+
+    def ready(self):
+        checks.register('models')(check_user_model)

django/contrib/comments/__init__.py

@@ -1,6 +1,6 @@
 from importlib import import_module
 import warnings
-from django.apps import apps
+from django.apps import apps as django_apps
 from django.conf import settings
 from django.core import urlresolvers
 from django.core.exceptions import ImproperlyConfigured
@@ -16,7 +16,7 @@ def get_comment_app():
     Get the comment app (i.e. "django.contrib.comments") as defined in the settings
     """
     try:
-        app_config = apps.get_app_config(get_comment_app_name().rpartition(".")[2])
+        app_config = django_apps.get_app_config(get_comment_app_name().rpartition(".")[2])
     except LookupError:
         raise ImproperlyConfigured("The COMMENTS_APP (%r) "
                                    "must be in INSTALLED_APPS" % settings.COMMENTS_APP)
@@ -85,3 +85,6 @@ def get_approve_url(comment):
     else:
         return urlresolvers.reverse("django.contrib.comments.views.moderation.approve",
                                     args=(comment.id,))
+
+
+default_app_config = 'django.contrib.comments.apps.CommentsConfig'

django/contrib/contenttypes/__init__.py

@@ -1,8 +1 @@
-# -*- coding: utf-8 -*-
-from __future__ import unicode_literals
-
-from django.contrib.contenttypes.checks import check_generic_foreign_keys
-from django.core import checks
-
-
-checks.register('models')(check_generic_foreign_keys)
+default_app_config = 'django.contrib.contenttypes.apps.ContentTypesConfig'

django/contrib/contenttypes/apps.py

@@ -1,8 +1,12 @@
 from django.apps import AppConfig
-
+from django.contrib.contenttypes.checks import check_generic_foreign_keys
+from django.core import checks
 from django.utils.translation import ugettext_lazy as _
 
 
 class ContentTypesConfig(AppConfig):
     name = 'django.contrib.contenttypes'
     verbose_name = _("content types")
+
+    def ready(self):
+        checks.register('models')(check_generic_foreign_keys)

django/contrib/flatpages/__init__.py

@@ -0,0 +1 @@
+default_app_config = 'django.contrib.flatpages.apps.FlatPagesConfig'

django/contrib/formtools/__init__.py

@@ -0,0 +1 @@
+default_app_config = 'django.contrib.formtools.apps.FormToolsConfig'

django/contrib/gis/__init__.py

@@ -4,3 +4,6 @@ if six.PY3:
     memoryview = memoryview
 else:
     memoryview = buffer
+
+
+default_app_config = 'django.contrib.gis.apps.GISConfig'

django/contrib/humanize/__init__.py

@@ -0,0 +1 @@
+default_app_config = 'django.contrib.humanize.apps.HumanizeConfig'

django/contrib/messages/__init__.py

@@ -1,2 +1,5 @@
 from django.contrib.messages.api import *  # NOQA
 from django.contrib.messages.constants import *  # NOQA
+
+
+default_app_config = 'django.contrib.messages.apps.MessagesConfig'

django/contrib/redirects/__init__.py

@@ -0,0 +1 @@
+default_app_config = 'django.contrib.redirects.apps.RedirectsConfig'

django/contrib/sessions/__init__.py

@@ -0,0 +1 @@
+default_app_config = 'django.contrib.sessions.apps.SessionsConfig'

django/contrib/sitemaps/__init__.py

@@ -133,3 +133,6 @@ class GenericSitemap(Sitemap):
         if self.date_field is not None:
             return getattr(item, self.date_field)
         return None
+
+
+default_app_config = 'django.contrib.sitemaps.apps.SiteMapsConfig'

django/contrib/sites/__init__.py

@@ -0,0 +1 @@
+default_app_config = 'django.contrib.sites.apps.SitesConfig'

django/contrib/staticfiles/__init__.py

@@ -0,0 +1 @@
+default_app_config = 'django.contrib.staticfiles.apps.StaticFilesConfig'

django/contrib/syndication/__init__.py

@@ -0,0 +1 @@
+default_app_config = 'django.contrib.syndication.apps.SyndicationConfig'

django/contrib/webdesign/__init__.py

@@ -0,0 +1 @@
+default_app_config = 'django.contrib.webdesign.apps.WebDesignConfig'

django/core/checks/registry.py

@@ -29,7 +29,8 @@ class CheckRegistry(object):
 
         def inner(check):
             check.tags = tags
-            self.registered_checks.append(check)
+            if check not in self.registered_checks:
+                self.registered_checks.append(check)
             return check
 
         return inner