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

@@ -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()

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():