add global 'empty' property, this property raise mandatory PropertiesOptionError if multi or master have empty value

tiramisu/setting.py

@@ -75,6 +75,9 @@ everything_frozen
     whole option in config are frozen (even if option have not frozen
     property)
 
+empty
+    raise mandatory PropertiesOptionError if multi or master have empty value
+
 validator
     launch validator set by user in option (this property has no effect
     for internal validator)
@@ -97,10 +100,10 @@ read_write
     you can set all variables not frozen
 """
 ro_append = set(['frozen', 'disabled', 'validator', 'everything_frozen',
-                'mandatory'])
+                'mandatory', 'empty'])
 ro_remove = set(['permissive', 'hidden'])
 rw_append = set(['frozen', 'disabled', 'validator', 'hidden'])
-rw_remove = set(['permissive', 'everything_frozen', 'mandatory'])
+rw_remove = set(['permissive', 'everything_frozen', 'mandatory', 'empty'])
 
 
 forbidden_set_properties = set(['force_store_value'])
@@ -468,8 +471,13 @@ class Settings(object):
         else:
             if 'mandatory' in properties and \
                     not self._getcontext().cfgimpl_get_values()._isempty(
-                        opt_or_descr, value, opt_or_descr.impl_allow_empty_list()):
+                        opt_or_descr, value):
                 properties.remove('mandatory')
+            elif not is_write and 'empty' in forced_properties and \
+                    not opt_or_descr.impl_is_master_slaves('slave') and \
+                    self._getcontext().cfgimpl_get_values()._isempty(
+                        opt_or_descr, value, force_allow_empty_list=True):
+                properties.add('mandatory')
             if is_write and 'everything_frozen' in forced_properties:
                 properties.add('frozen')
             elif 'frozen' in properties and not is_write: