Fixed #21421 -- Added level_tag attribute on messages.

Exposing the level name (e.g. "info") makes it possible to prepend
something to the class name. For example, Twitter Bootstrap has
an alert-info class. This class can now be added to the message
using `class="alert-{{ message.level_tag }}".
Because the level_tag was on the end of the `tags` property, it
could not be used in this fashion when extra_tags were given.

django/contrib/messages/storage/base.py

@@ -40,18 +40,20 @@ class Message(object):
         return force_text(self.message)
 
     def _get_tags(self):
-        label_tag = force_text(LEVEL_TAGS.get(self.level, ''),
-                                  strings_only=True)
         extra_tags = force_text(self.extra_tags, strings_only=True)
-        if extra_tags and label_tag:
-            return ' '.join([extra_tags, label_tag])
+        if extra_tags and self.level_tag:
+            return ' '.join([extra_tags, self.level_tag])
         elif extra_tags:
             return extra_tags
-        elif label_tag:
-            return label_tag
+        elif self.level_tag:
+            return self.level_tag
         return ''
     tags = property(_get_tags)
 
+    @property
+    def level_tag(self):
+        return force_text(LEVEL_TAGS.get(self.level, ''), strings_only=True)
+
 
 class BaseStorage(object):
     """

django/contrib/messages/tests/base.py

@@ -361,6 +361,15 @@ class BaseTests(object):
                          ['info', '', 'extra-tag debug', 'warning', 'error',
                           'success'])
 
+    def test_level_tag(self):
+        storage = self.get_storage()
+        storage.level = 0
+        add_level_messages(storage)
+        tags = [msg.level_tag for msg in storage]
+        self.assertEqual(tags,
+                         ['info', '', 'debug', 'warning', 'error',
+                          'success'])
+
     @override_settings_tags(MESSAGE_TAGS={
         constants.INFO: 'info',
         constants.DEBUG: '',