From c981d9140d569d489d338741b6a3d878c4e712da Mon Sep 17 00:00:00 2001
From: Aymeric Augustin <aymeric.augustin@m4x.org>
Date: Sun, 31 Aug 2014 18:51:55 +0200
Subject: [PATCH] [1.7.x] 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.

Backport of b161c01 from master
---
 django/apps/config.py              | 19 ++++++++++++-------
 tests/apps/failing_app/__init__.py |  1 +
 tests/apps/tests.py                |  8 ++++++++
 3 files changed, 21 insertions(+), 7 deletions(-)
 create mode 100644 tests/apps/failing_app/__init__.py

diff --git a/django/apps/config.py b/django/apps/config.py
index 9cfe4858aa..843013f30e 100644
--- a/django/apps/config.py
+++ b/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.)
diff --git a/tests/apps/failing_app/__init__.py b/tests/apps/failing_app/__init__.py
new file mode 100644
index 0000000000..4ec7f18d40
--- /dev/null
+++ b/tests/apps/failing_app/__init__.py
@@ -0,0 +1 @@
+raise ImportError("Oops")
diff --git a/tests/apps/tests.py b/tests/apps/tests.py
index 2c7629ac97..513ed32039 100644
--- a/tests/apps/tests.py
+++ b/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.