diff --git a/‎gitlab/base.py‎
Lines changed: 18 additions & 1 deletion b/‎gitlab/base.py‎
Lines changed: 18 additions & 1 deletion
diff --git a/‎gitlab/v4/objects.py‎
Lines changed: 6 additions & 0 deletions b/‎gitlab/v4/objects.py‎
Lines changed: 6 additions & 0 deletions
@@ -564,7 +564,24 @@ def __getattr__(self, name):
             return self.__dict__['_updated_attrs'][name]
         except KeyError:
             try:
-                return self.__dict__['_attrs'][name]
+                value = self.__dict__['_attrs'][name]
+
+                # If the value is a list, we copy it in the _updated_attrs dict
+                # because we are not able to detect changes made on the object
+                # (append, insert, pop, ...). Without forcing the attr
+                # creation __setattr__ is never called, the list never ends up
+                # in the _updated_attrs dict, and the update() and save()
+                # method never push the new data to the server.
+                # See https://github.com/python-gitlab/python-gitlab/issues/306
+                #
+                # note: _parent_attrs will only store simple values (int) so we
+                # don't make this check in the next except block.
+                if isinstance(value, list):
+                    self.__dict__['_updated_attrs'][name] = value[:]
+                    return self.__dict__['_updated_attrs'][name]
+
+                return value
+
             except KeyError:
                 try:
                     return self.__dict__['_parent_attrs'][name]
 
@@ -960,6 +960,12 @@ class ProjectIssueManager(CRUDMixin, RESTManager):
                                'milestone_id', 'labels', 'created_at',
                                'updated_at', 'state_event', 'due_date'))
 
+    def _sanitize_data(self, data, action):
+        new_data = data.copy()
+        if 'labels' in data:
+            new_data['labels'] = ','.join(data['labels'])
+        return new_data
+
 
 class ProjectMember(SaveMixin, ObjectDeleteMixin, RESTObject):
     _short_print_attr = 'username'