diff --git a/tiramisu/autolib.py b/tiramisu/autolib.py
index bc1bf86..8108849 100644
--- a/tiramisu/autolib.py
+++ b/tiramisu/autolib.py
@@ -156,7 +156,9 @@ class Calculation:
                 option_bag: OptionBag,
                 leadership_must_have_index: bool=False,
                 orig_value: Any=undefined,
-                allow_raises=False) -> Any:
+                allow_value_error=False,
+                force_value_warning=False,
+                ) -> Any:
         return await carry_out_calculation(option_bag.option,
                                            callback=self.function,
                                            callback_params=self.params,
@@ -164,7 +166,9 @@ class Calculation:
                                            config_bag=option_bag.config_bag,
                                            leadership_must_have_index=leadership_must_have_index,
                                            orig_value=orig_value,
-                                           allow_raises=allow_raises)
+                                           allow_value_error=allow_value_error,
+                                           force_value_warning=force_value_warning,
+                                           )
 
     async def help(self,
                    option_bag: OptionBag,
@@ -361,18 +365,18 @@ async def carry_out_calculation(option,
                                 config_bag: Optional[ConfigBag],
                                 orig_value=undefined,
                                 leadership_must_have_index: bool=False,
-                                allow_raises: int=False):
+                                allow_value_error: bool=False,
+                                force_value_warning: bool=False,
+                                ):
     """a function that carries out a calculation for an option's value
 
     :param option: the option
     :param callback: the name of the callback function
-    :type callback: str
     :param callback_params: the callback's parameters
                             (only keyword parameters are allowed)
-    :type callback_params: dict
     :param index: if an option is multi, only calculates the nth value
-    :type index: int
-    :param allow_raises: to know if carry_out_calculation is used to validate a value
+    :param allow_value_error: to know if carry_out_calculation can return ValueError or ValueWarning (for example if it's a validation)
+    :param force_value_warning: transform valueError to ValueWarning object
 
     The callback_params is a dict. Key is used to build args (if key is '')
     and kwargs (otherwise). Values are tuple of:
@@ -411,7 +415,8 @@ async def carry_out_calculation(option,
                 continue
     ret = calculate(option,
                     callback,
-                    allow_raises,
+                    allow_value_error,
+                    force_value_warning,
                     args,
                     kwargs)
     if isinstance(ret, list) and not option.impl_is_dynoptiondescription() and \
@@ -436,9 +441,11 @@ async def carry_out_calculation(option,
 
 def calculate(option,
               callback: Callable,
-              allow_raises: bool,
+              allow_value_error: bool,
+              force_value_warning: bool,
               args,
-              kwargs):
+              kwargs,
+              ):
     """wrapper that launches the 'callback'
 
     :param callback: callback function
@@ -448,12 +455,12 @@ def calculate(option,
     """
     try:
         return callback(*args, **kwargs)
-    except ValueError as err:
-        if allow_raises:
+    except (ValueError, ValueWarning) as err:
+        if allow_value_error:
+            if force_value_warning:
+                raise ValueWarning(str(err))
             raise err
         error = err
-    except ValueWarning as err:
-        raise err
     except Exception as err:
         # import traceback
         # traceback.print_exc()
diff --git a/tiramisu/option/baseoption.py b/tiramisu/option/baseoption.py
index 8440b2f..40a3909 100644
--- a/tiramisu/option/baseoption.py
+++ b/tiramisu/option/baseoption.py
@@ -106,7 +106,7 @@ class Base:
                           context_od) -> Set[str]:
         ret = set(getattr(self, '_dependencies', STATIC_TUPLE))
         if context_od and hasattr(context_od, '_dependencies'):
-            # if context is set in options, add those options
+            # add options that have context is set in calculation
             return set(context_od._dependencies) | ret
         return ret
 
diff --git a/tiramisu/option/option.py b/tiramisu/option/option.py
index 7b42d6e..fde3fa0 100644
--- a/tiramisu/option/option.py
+++ b/tiramisu/option/option.py
@@ -84,7 +84,9 @@ class Option(BaseOption):
             is_multi = True
             _multi = submulti
         else:
-            raise ValueError(_('invalid multi type "{}"').format(multi))
+            raise ValueError(_('invalid multi type "{}" for "{}"').format(multi,
+                                                                          name,
+                                                                          ))
         if _multi != 1:
             _setattr(self, '_multi', _multi)
         if multi is not False and default is None:
@@ -335,7 +337,9 @@ class Option(BaseOption):
                 if ((check_error and not calc_is_warnings_only) or
                         (not check_error and calc_is_warnings_only)):
                     try:
-                        kwargs = {'allow_raises': True}
+                        kwargs = {'allow_value_error': True,
+                                  'force_value_warning': calc_is_warnings_only,
+                                  }
                         if _index is not None and option_bag.index == _index:
                             soption_bag = option_bag
                         else:
@@ -346,17 +350,6 @@ class Option(BaseOption):
                         await validator.execute(soption_bag,
                                                 leadership_must_have_index=True,
                                                 **kwargs)
-                    except ValueError as err:
-                        if calc_is_warnings_only:
-                            warnings.warn_explicit(ValueWarning(val,
-                                                                self._display_name,
-                                                                self,
-                                                                '{0}'.format(err),
-                                                                _index),
-                                                   ValueWarning,
-                                                   self.__class__.__name__, 306)
-                        else:
-                            raise err
                     except ValueWarning as warn:
                         warnings.warn_explicit(ValueWarning(val,
                                                             self._display_name,
@@ -364,7 +357,7 @@ class Option(BaseOption):
                                                             '{0}'.format(warn),
                                                             _index),
                                                ValueWarning,
-                                               self.__class__.__name__, 316)
+                                               self.__class__.__name__, 356)
 
         async def do_validation(_value,
                                 _index):
diff --git a/tiramisu/value.py b/tiramisu/value.py
index 73c81ce..f492d0f 100644
--- a/tiramisu/value.py
+++ b/tiramisu/value.py
@@ -241,7 +241,8 @@ class Values:
         await option_bag.config_bag.context.cfgimpl_reset_cache(option_bag)
 
     async def calculate_value(self,
-                              option_bag: OptionBag) -> Any:
+                              option_bag: OptionBag,
+                              ) -> Any:
 
         # if value has callback, calculate value
         callback, callback_params = option_bag.option.impl_get_callback()