Add note on mutable types and traitlets

Author: jtpio

docs/source/examples/Widget Custom.ipynb

@@ -280,7 +280,13 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "Traitlets is an IPython library for defining type-safe properties on configurable objects.  For this tutorial you do not need to worry about the *configurable* piece of the traitlets machinery.   The `sync=True` keyword argument tells the widget framework to handle synchronizing that value to the browser.  Without `sync=True`, attributes of the widget won't be synchronized with the front-end."
+    "Traitlets is an IPython library for defining type-safe properties on configurable objects. For this tutorial you do not need to worry about the *configurable* piece of the traitlets machinery. The `sync=True` keyword argument tells the widget framework to handle synchronizing that value to the browser. Without `sync=True`, attributes of the widget won't be synchronized with the front-end.\n",
+    "\n",
+    "<div class=\"alert alert-info\">\n",
+    "Syncing mutable types\n",
+    "    \n",
+    "Please keep in mind that mutable types will not necessarily be synced when they are modified. For example appending an element to a `list` will not cause the changes to sync. Instead a new list must be created and assigned to the trait for the changes to be synced.\n",
+    "</div>"
    ]
   },
   {
@@ -749,7 +755,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.8.1"
+   "version": "3.9.1"
   }
  },
  "nbformat": 4,