leave field as tuple in %-string formatting

parse.py

@@ -825,7 +825,7 @@ def _to_group_name(self, field):
             elif '_' in field:
                 group = field.replace('_', '_' * n)
             else:
-                raise KeyError('duplicated group name %r' % field)
+                raise KeyError('duplicated group name %r' % (field,))
 
         # save off the mapping
         self._group_to_name_map[group] = field