has_callback options cannot be overrided

tiramisu/config.py

@@ -337,6 +337,9 @@ class Config(object):
                 else:
                     newowner = who 
         if type(child) != SymLinkOption:
+            if child.has_callback():
+                raise TypeError("trying to set a value to an option "
+                    "wich has a callback: {0}".format(name))
 #            if oldowner == who:
 #                oldvalue = getattr(self, name)
 #                if oldvalue == value: