diff --git a/tiramisu/option.py b/tiramisu/option.py
index a21a0fd..be1e81f 100644
--- a/tiramisu/option.py
+++ b/tiramisu/option.py
@@ -68,7 +68,7 @@ class Multi(list):
             super(Multi, self).__init__(lst)
 
     def __setitem__(self, key, value):
-        return self._setvalue(value, key, who=settings.get_owner())
+        self._setvalue(value, key, who=settings.get_owner())
 
     def append(self, value, add_master=True):
         """the list value can be updated (appened)
@@ -82,20 +82,16 @@ class Multi(list):
                 raise IndexError("in a group with a master, you mustn't add "
                         "a value in a slave's Multi value")
         except TypeError:
-            return self._setvalue(value, who=settings.get_owner())
+            self._setvalue(value, who=settings.get_owner())
         multis = []
-        ret = None
         for name, multi in self.config:
             multis.append(multi)
         for multi in multis:
             if master == multi.opt._name:
                 if add_master:
-                    ret = multi._setvalue(value, who=settings.get_owner())
-                else:
-                    ret = value
+                    multi._setvalue(value, who=settings.get_owner())
             else:
                 multi._append_default()
-        return ret
 
     def _append_default(self):
         default_value = self.opt.getdefault_multi()
@@ -107,22 +103,24 @@ class Multi(list):
                 raise ConfigError("invalid value {0} "
                     "for option {1}".format(str(value), self.opt._name))
         oldvalue = list(self)
-        ret = None
         if key is None:
-            ret = super(Multi, self).append(value)
+            super(Multi, self).append(value)
         else:
-            ret = super(Multi, self).__setitem__(key, value)
+            super(Multi, self).__setitem__(key, value)
         if who != None:
             if not isinstance(who, owners.Owner):
                 raise TypeError("invalid owner {0} for the value {1}".format(
                                 str(who), str(value)))
             self.opt.setowner(self.config, getattr(owners, who))
         self.config._cfgimpl_previous_values[self.opt._name] = oldvalue
-        return ret
 
     def pop(self, key):
         """the list value can be updated (poped)
         only if the option is a master
+
+        :param key: index of the element to pop
+        :return: the requested element
+
         """
         try:
             master = self.config._cfgimpl_descr.get_master_name()
@@ -133,7 +131,6 @@ class Multi(list):
             return self._pop(key)
 
         multis = []
-        ret = None
         for name, multi in self.config:
             multis.append(multi)
         for multi in multis:
@@ -147,13 +144,14 @@ class Multi(list):
                 if len(multi.opt.getdefault()) >= len(multi):
                     change_who = True
                 multi._pop(key, change_who=change_who)
+        if ret not in locals():
+            raise ConfigError('Unexpected multi pop error: ret must be defined')
         return ret
 
     def _pop(self, key, change_who=True):
-        oldvalue = list(self)
         if change_who:
             self.opt.setowner(self.config, settings.get_owner())
-        self.config._cfgimpl_previous_values[self.opt._name] = oldvalue
+        self.config._cfgimpl_previous_values[self.opt._name] = list(self)
         return super(Multi, self).pop(key)
 # ____________________________________________________________
 #