- fixed the import for entrypoint-driven dialects to

not rely upon silly tb_info trick to determine import
error status.  [ticket:1630]

1 files changed, 24 insertions, 7 deletions

diff --git a/lib/sqlalchemy/engine/url.py b/lib/sqlalchemy/engine/url.py
index c62f1cf10..5d658d7fc 100644
--- a/lib/sqlalchemy/engine/url.py
+++ b/lib/sqlalchemy/engine/url.py
@@ -102,13 +102,30 @@ class URL(object):
 
             return module.dialect
         except ImportError:
-            if sys.exc_info()[2].tb_next is None:
-                import pkg_resources
-                for res in pkg_resources.iter_entry_points('sqlalchemy.dialects'):
-                    if res.name == self.drivername:
-                        return res.load()
-            raise
-
+            module = self._load_entry_point()
+            if module is not None:
+                return module
+            else:
+                raise
+    
+    def _load_entry_point(self):
+        """attempt to load this url's dialect from entry points, or return None
+        if pkg_resources is not installed or there is no matching entry point.
+        
+        Raise ImportError if the actual load fails.
+        
+        """
+        try:
+            import pkg_resources
+        except ImportError:
+            return None
+            
+        for res in pkg_resources.iter_entry_points('sqlalchemy.dialects'):
+            if res.name == self.drivername:
+                return res.load()
+        else:
+            return None
+        
     def translate_connect_args(self, names=[], **kw):
         """Translate url attributes into a dictionary of connection arguments.