From 9a70e62b8d545c8de7e3728e544cebe289a39254 Mon Sep 17 00:00:00 2001
From: Armin Ronacher <armin.ronacher@active-4.com>
Date: Wed, 21 Sep 2011 03:47:36 +0200
Subject: [PATCH] No longer swallow important import errors.

This implements a simple but elegant method to find out if an import
error can be swallowed to try the next module or if the import error is
important and must be reraised.
---
 flask/ext/__init__.py      | 20 ++++++++++++++++++--
 scripts/flaskext_compat.py | 18 +++++++++++++++++-
 2 files changed, 35 insertions(+), 3 deletions(-)

diff --git a/flask/ext/__init__.py b/flask/ext/__init__.py
index 14c8d8d4..d030336f 100644
--- a/flask/ext/__init__.py
+++ b/flask/ext/__init__.py
@@ -47,7 +47,7 @@ class _ExtensionImporter(object):
             return self
 
     def load_module(self, fullname):
-        from sys import modules
+        from sys import modules, exc_info
         if fullname in modules:
             return modules[fullname]
         modname = fullname.split('.', self.prefix_cutoff)[self.prefix_cutoff]
@@ -56,12 +56,28 @@ class _ExtensionImporter(object):
             try:
                 __import__(realname)
             except ImportError:
+                exc_type, exc_value, tb = exc_info()
+                if self.is_important_traceback(realname, tb):
+                    raise exc_type, exc_value, tb
                 continue
             module = modules[fullname] = modules[realname]
             if '.' not in modname:
                 setattr(modules[__name__], modname, module)
             return module
-        raise ImportError(fullname)
+        raise ImportError('No module named %s' % fullname)
+
+    def is_important_traceback(self, important_module, tb):
+        """Walks a traceback's frames and checks if any of the frames
+        originated in the given important module.  If that is the case
+        then we were able to import the module itself but apparently
+        something went wrong when the module was imported.  (Eg: import
+        of an import failed).
+        """
+        while tb is not None:
+            if tb.tb_frame.f_globals.get('__name__') == important_module:
+                return True
+            tb = tb.tb_next
+        return False
 
 
 _ExtensionImporter()
diff --git a/scripts/flaskext_compat.py b/scripts/flaskext_compat.py
index 03601a1a..f0b1739d 100644
--- a/scripts/flaskext_compat.py
+++ b/scripts/flaskext_compat.py
@@ -44,12 +44,28 @@ class _ExtensionImporter(object):
             try:
                 __import__(realname)
             except ImportError:
+                exc_type, exc_value, tb = sys.exc_info()
+                if self.is_important_traceback(realname, tb):
+                    raise exc_type, exc_value, tb
                 continue
             module = sys.modules[fullname] = sys.modules[realname]
             if '.' not in modname:
                 setattr(ext_module, modname, module)
             return module
-        raise ImportError(fullname)
+        raise ImportError('No module named %s' % fullname)
+
+    def is_important_traceback(self, important_module, tb):
+        """Walks a traceback's frames and checks if any of the frames
+        originated in the given important module.  If that is the case
+        then we were able to import the module itself but apparently
+        something went wrong when the module was imported.  (Eg: import
+        of an import failed).
+        """
+        while tb is not None:
+            if tb.tb_frame.f_globals.get('__name__') == important_module:
+                return True
+            tb = tb.tb_next
+        return False
 
 
 def activate():