Fixed #22920 -- Avoid masking some exceptions.

If loading an application trigger an ImportError, the details of that
error were lost in some cases. Thanks Ben Davis for the report.

django/apps/config.py

@@ -87,6 +87,10 @@ class AppConfig(object):
             module = import_module(entry)
 
         except ImportError:
+            # Track that importing as an app module failed. If importing as an
+            # app config class fails too, we'll trigger the ImportError again.
+            module = None
+
             mod_path, _, cls_name = entry.rpartition('.')
 
             # Raise the original exception when entry cannot be a path to an
@@ -104,8 +108,8 @@ class AppConfig(object):
             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>.
+        # If we're reaching this point, we must attempt to 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.
@@ -113,11 +117,12 @@ class AppConfig(object):
         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))
+            if module is None:
+                # If importing as an app module failed, that error probably
+                # contains the most informative traceback. Trigger it again.
+                import_module(entry)
+            else:
+                raise
 
         # Check for obvious errors. (This check prevents duck typing, but
         # it could be removed if it became a problem in practice.)

tests/apps/failing_app/__init__.py

@@ -0,0 +1 @@
+raise ImportError("Oops")

tests/apps/tests.py

@@ -166,6 +166,14 @@ class AppsTests(TestCase):
             with self.settings(INSTALLED_APPS=['apps.apps.RelabeledAppsConfig', 'apps']):
                 pass
 
+    def test_import_exception_is_not_masked(self):
+        """
+        App discovery should preserve stack traces. Regression test for #22920.
+        """
+        with six.assertRaisesRegex(self, ImportError, "Oops"):
+            with self.settings(INSTALLED_APPS=['apps.failing_app']):
+                pass
+
     def test_models_py(self):
         """
         Tests that the models in the models.py file were loaded correctly.