Fixed #29768 -- Improved error message when an AppConfig has a typo in INSTALLED_APPS.

django/apps/config.py

@@ -118,8 +118,21 @@ class AppConfig:
             cls = getattr(mod, cls_name)
         except AttributeError:
             if module is None:
-                # If importing as an app module failed, that error probably
-                # contains the most informative traceback. Trigger it again.
+                # If importing as an app module failed, check if the module
+                # contains any valid AppConfigs and show them as choices.
+                # Otherwise, that error probably contains the most informative
+                # traceback, so trigger it again.
+                candidates = sorted(
+                    repr(name) for name, candidate in mod.__dict__.items()
+                    if isinstance(candidate, type) and
+                    issubclass(candidate, AppConfig) and
+                    candidate is not AppConfig
+                )
+                if candidates:
+                    raise ImproperlyConfigured(
+                        "'%s' does not contain a class '%s'. Choices are: %s."
+                        % (mod_path, cls_name, ', '.join(candidates))
+                    )
                 import_module(entry)
             else:
                 raise

tests/apps/tests.py

@@ -81,10 +81,18 @@ class AppsTests(SimpleTestCase):
                 pass
 
     def test_no_such_app_config(self):
-        """
-        Tests when INSTALLED_APPS contains an entry that doesn't exist.
-        """
-        with self.assertRaises(ImportError):
+        msg = "No module named 'apps.NoSuchConfig'"
+        with self.assertRaisesMessage(ImportError, msg):
+            with self.settings(INSTALLED_APPS=['apps.NoSuchConfig']):
+                pass
+
+    def test_no_such_app_config_with_choices(self):
+        msg = (
+            "'apps.apps' does not contain a class 'NoSuchConfig'. Choices are: "
+            "'BadConfig', 'MyAdmin', 'MyAuth', 'NoSuchApp', 'PlainAppsConfig', "
+            "'RelabeledAppsConfig'."
+        )
+        with self.assertRaisesMessage(ImproperlyConfigured, msg):
             with self.settings(INSTALLED_APPS=['apps.apps.NoSuchConfig']):
                 pass