preferences: refactor to check consistently input values

searx/preferences.py

 class EnumStringSetting(Setting):
     """Setting of a value which can only come from the given choices"""
 
+    def _validate_selection(self, selection):
+        if selection not in self.choices:
+            raise ValidationException('Invalid value: "{0}"'.format(selection))
+
     def _post_init(self):
         if not hasattr(self, 'choices'):
             raise MissingArgumentException('Missing argument: choices')
-
-        if self.value != '' and self.value not in self.choices:
-            raise ValidationException('Invalid default value: {0}'.format(self.value))
+        self._validate_selection(self.value)
 
     def parse(self, data):
-        if data not in self.choices and data != self.value:
-            raise ValidationException('Invalid choice: {0}'.format(data))
+        self._validate_selection(data)
         self.value = data
 
 
 class MultipleChoiceSetting(EnumStringSetting):
     """Setting of values which can only come from the given choices"""
 
+    def _validate_selections(self, selections):
+        for item in selections:
+            if item not in self.choices:
+                raise ValidationException('Invalid value: "{0}"'.format(selections))
+
     def _post_init(self):
         if not hasattr(self, 'choices'):
             raise MissingArgumentException('Missing argument: choices')
-        for item in self.value:
-            if item not in self.choices:
-                raise ValidationException('Invalid default value: {0}'.format(self.value))
+        self._validate_selections(self.value)
 
     def parse(self, data):
         if data == '':
             return
 
         elements = data.split(',')
-        for item in elements:
-            if item not in self.choices:
-                raise ValidationException('Invalid choice: {0}'.format(item))
+        self._validate_selections(elements)
         self.value = elements
 
     def parse_form(self, data):
         self.key_value_settings = {'categories': MultipleChoiceSetting(['general'], choices=categories),
                                    'language': EnumStringSetting('all', choices=LANGUAGE_CODES),
                                    'locale': EnumStringSetting(settings['ui']['default_locale'],
-                                                               choices=settings['locales'].keys()),
+                                                               choices=settings['locales'].keys() + ['']),
                                    'autocomplete': EnumStringSetting(settings['search']['autocomplete'],
                                                                      choices=autocomplete.backends.keys() + ['']),
                                    'image_proxy': MapSetting(settings['server']['image_proxy'],