better name's validation

tiramisu/option.py

@@ -40,9 +40,7 @@ forbidden_names = ('iter_all', 'iter_group', 'find', 'find_first',
 
 def valid_name(name):
     "an option's name is a str and does not start with 'impl' or 'cfgimpl'"
-    try:
-        name = str(name)
-    except:
+    if not isinstance(name, str):
         return False
     if re.match(name_regexp, name) is None and not name.startswith('_') \
             and name not in forbidden_names \