Added get_app_config() to look up app configs by label.

Refactored get_app() to rely on that method.

get_app() starts by calling _populate(), which goes through
INSTALLED_APPS and, for each app, imports the app module and attempts to
import the models module. At this point, no further imports are
necessary to return the models module for a  given app. Therefore, the
implementation of get_app() can be simplified and the safeguards for
race conditions can be removed.

Besides, the emptyOK parameter isn't used anywhere in Django. It was
introduced in d6c95e93 but not actually used nor documented, and it has
just been carried around since then. Since it's an obscure private API,
it's acceptable to stop supporting it without a deprecation path. This
branch aims at providing first-class support for applications without a
models module eventually.

For backwards-compatibility, get_app() still raises ImproperlyConfigured
when an app isn't found, even though LookupError is technically more
correct. I haven't gone as far as to preserve the exact error messages.
I've adjusted a few tests instead.

django/apps/cache.py

@@ -153,6 +153,26 @@ class BaseAppCache(object):
         """
         return self.loaded
 
+    def get_app_config(self, app_label, only_installed=True):
+        """
+        Returns the application configuration for the given app_label.
+
+        Raises LookupError if no application exists with this app_label.
+
+        Raises UnavailableApp when set_available_apps() disables the
+        application with this app_label.
+
+        If only_installed is True (default), only applications explicitly
+        listed in INSTALLED_APPS are considered.
+        """
+        self._populate()
+        app_config = self.app_configs.get(app_label)
+        if app_config is None or (only_installed and not app_config.installed):
+            raise LookupError("No app with label %r." % app_label)
+        if self.available_apps is not None and app_config.label not in self.available_apps:
+            raise UnavailableApp("App with label %r isn't available." % app_label)
+        return app_config
+
     def get_apps(self):
         """
         Returns a list of all installed modules that contain models.
@@ -197,29 +217,18 @@ class BaseAppCache(object):
             app_paths.append(self._get_app_path(app))
         return app_paths
 
-    def get_app(self, app_label, emptyOK=False):
+    def get_app(self, app_label):
         """
         Returns the module containing the models for the given app_label.
 
-        Returns None if the app has no models in it and emptyOK is True.
-
         Raises UnavailableApp when set_available_apps() in in effect and
         doesn't include app_label.
         """
-        self._populate()
-        imp.acquire_lock()
         try:
-            for app_name in settings.INSTALLED_APPS:
+            return self.get_app_config(app_label).models_module
-                if app_label == app_name.split('.')[-1]:
+        except LookupError as exc:
-                    mod = self.load_app(app_name, False)
+            # Change the exception type for backwards compatibility.
-                    if mod is None and not emptyOK:
+            raise ImproperlyConfigured(*exc.args)
-                        raise ImproperlyConfigured("App with label %s is missing a models.py module." % app_label)
-                    if self.available_apps is not None and app_label not in self.available_apps:
-                        raise UnavailableApp("App with label %s isn't available." % app_label)
-                    return mod
-            raise ImproperlyConfigured("App with label %s could not be found" % app_label)
-        finally:
-            imp.release_lock()
 
     def get_models(self, app_mod=None,
                    include_auto_created=False, include_deferred=False,

tests/admin_scripts/tests.py

@@ -379,14 +379,14 @@ class DjangoAdminMinimalSettings(AdminScriptTestCase):
         args = ['sqlall', '--settings=test_project.settings', 'admin_scripts']
         out, err = self.run_django_admin(args)
         self.assertNoOutput(out)
-        self.assertOutput(err, 'App with label admin_scripts could not be found')
+        self.assertOutput(err, "No app with label 'admin_scripts'.")
 
     def test_builtin_with_environment(self):
         "minimal: django-admin builtin commands fail if settings are provided in the environment"
         args = ['sqlall', 'admin_scripts']
         out, err = self.run_django_admin(args, 'test_project.settings')
         self.assertNoOutput(out)
-        self.assertOutput(err, 'App with label admin_scripts could not be found')
+        self.assertOutput(err, "No app with label 'admin_scripts'.")
 
     def test_builtin_with_bad_settings(self):
         "minimal: django-admin builtin commands fail if settings file (from argument) doesn't exist"
@@ -815,21 +815,21 @@ class ManageMinimalSettings(AdminScriptTestCase):
         args = ['sqlall', 'admin_scripts']
         out, err = self.run_manage(args)
         self.assertNoOutput(out)
-        self.assertOutput(err, 'App with label admin_scripts could not be found')
+        self.assertOutput(err, "No app with label 'admin_scripts'.")
 
     def test_builtin_with_settings(self):
         "minimal: manage.py builtin commands fail if settings are provided as argument"
         args = ['sqlall', '--settings=test_project.settings', 'admin_scripts']
         out, err = self.run_manage(args)
         self.assertNoOutput(out)
-        self.assertOutput(err, 'App with label admin_scripts could not be found')
+        self.assertOutput(err, "No app with label 'admin_scripts'.")
 
     def test_builtin_with_environment(self):
         "minimal: manage.py builtin commands fail if settings are provided in the environment"
         args = ['sqlall', 'admin_scripts']
         out, err = self.run_manage(args, 'test_project.settings')
         self.assertNoOutput(out)
-        self.assertOutput(err, 'App with label admin_scripts could not be found')
+        self.assertOutput(err, "No app with label 'admin_scripts'.")
 
     def test_builtin_with_bad_settings(self):
         "minimal: manage.py builtin commands fail if settings file (from argument) doesn't exist"
@@ -964,7 +964,7 @@ class ManageMultipleSettings(AdminScriptTestCase):
         args = ['sqlall', 'admin_scripts']
         out, err = self.run_manage(args)
         self.assertNoOutput(out)
-        self.assertOutput(err, 'App with label admin_scripts could not be found.')
+        self.assertOutput(err, "No app with label 'admin_scripts'.")
 
     def test_builtin_with_settings(self):
         "multiple: manage.py builtin commands succeed if settings are provided as argument"
@@ -1442,13 +1442,13 @@ class CommandTypes(AdminScriptTestCase):
         "User AppCommands can execute when a single app name is provided"
         args = ['app_command', 'NOT_AN_APP']
         out, err = self.run_manage(args)
-        self.assertOutput(err, "App with label NOT_AN_APP could not be found")
+        self.assertOutput(err, "No app with label 'NOT_AN_APP'.")
 
     def test_app_command_some_invalid_appnames(self):
         "User AppCommands can execute when some of the provided app names are invalid"
         args = ['app_command', 'auth', 'NOT_AN_APP']
         out, err = self.run_manage(args)
-        self.assertOutput(err, "App with label NOT_AN_APP could not be found")
+        self.assertOutput(err, "No app with label 'NOT_AN_APP'.")
 
     def test_label_command(self):
         "User LabelCommands can execute when a label is provided"

tests/empty/tests.py

@@ -32,5 +32,5 @@ class NoModelTests(TestCase):
     @override_settings(INSTALLED_APPS=("empty.no_models",))
     def test_no_models(self):
         with six.assertRaisesRegex(self, ImproperlyConfigured,
-                    'App with label no_models is missing a models.py module.'):
+                    "No app with label 'no_models'."):
             app_cache.get_app('no_models')