A JupyterLab architecture walkthrough from June 16, 2016, provides an overview of the notebook architecture.

The most complicated plugin included in the JupyterLab application is the Notebook plugin.

The NotebookWidgetFactory constructs a new NotebookPanel from a model and populates the toolbar with default widgets.

Structure of the Notebook plugin

The Notebook plugin provides a model and widgets for dealing with notebook files.


The NotebookModel contains an observable list of cells.

A cell model can be:

  • a code cell

  • a markdown cell

  • raw cell

A code cell contains a list of output models. The list of cells and the list of outputs can be observed for changes.

Cell operations

The NotebookModel cell list supports single-step operations such as moving, adding, or deleting cells. Compound cell list operations, such as undo/redo, are also supported by the NotebookModel. Right now, undo/redo is only supported on cells and is not supported on notebook attributes, such as notebook metadata. Currently, undo/redo for individual cell input content is supported by the CodeMirror editor’s undo feature. (Note: CodeMirror editor’s undo does not cover cell metadata changes.)


The notebook model and the cell model (i.e. notebook cells) support getting and setting metadata through an IObservableJSON object. You can use this to get and set notebook/cell metadata, as well as subscribe to changes to it.

Notebook widget

After the NotebookModel is created, the NotebookWidgetFactory constructs a new NotebookPanel from the model. The NotebookPanel widget is added to the DockPanel. The NotebookPanel contains:

The NotebookPanel also adds completion logic.

The NotebookToolbar maintains a list of widgets to add to the toolbar. The Notebook widget contains the rendering of the notebook and handles most of the interaction logic with the notebook itself (such as keeping track of interactions such as selected and active cells and also the current edit/command mode).

The NotebookModel cell list provides ways to do fine-grained changes to the cell list.

Higher level actions using NotebookActions

Higher-level actions are contained in the NotebookActions namespace, which has functions, when given a notebook widget, to run a cell and select the next cell, merge or split cells at the cursor, delete selected cells, etc.

Widget hierarchy

A Notebook widget contains a list of cell widgets, corresponding to the cell models in its cell list.

A CodeCell also contains an OutputArea. An OutputArea is responsible for rendering the outputs in the OutputAreaModel list. An OutputArea uses a notebook-specific RenderMimeRegistry object to render display_data output messages.

Rendering output messages

A Rendermime plugin provides a pluggable system for rendering output messages. Default renderers are provided for markdown, html, images, text, etc. Extensions can register renderers to be used across the entire application by registering a handler and mimetype in the rendermime registry. When a notebook is created, it copies the global Rendermime singleton so that notebook-specific renderers can be added. The ipywidgets widget manager is an example of an extension that adds a notebook-specific renderer, since rendering a widget depends on notebook-specific widget state.

How to extend the Notebook plugin

We’ll walk through two notebook extensions:

  • adding a button to the toolbar

  • adding a widget to the notebook header

  • adding an ipywidgets extension

Adding a button to the toolbar

Since JupyterLab 3.2, adding toolbar item can be done using a Toolbar Registry and settings. In particular for the notebook, if the button is linked to a new command, you can add a button in the toolbar using the following JSON snippet in your extension settings file:

"jupyter.lab.toolbars": {
  "Notebook": [ // Widget factory name for which you want to add a toolbar item.
    // Item with default button widget triggering a command
    { "name": "run", "command": "runmenu:run" }

You may add a rank attribute to modify the item position (the default value is 50).

Adding a widget to the notebook header

Start from the cookie cutter extension template.

pip install cookiecutter
cd my_cookie_cutter_name

Install the dependencies. Note that extensions are built against the released npm packages, not the development versions.

jlpm add -D @jupyterlab/notebook @jupyterlab/application @jupyterlab/ui-components @jupyterlab/docregistry @lumino/disposable @lumino/widgets --legacy-peer-deps

Copy the following to src/index.ts:

import { IDisposable, DisposableDelegate } from '@lumino/disposable';

import { Widget } from '@lumino/widgets';

import {
} from '@jupyterlab/application';

import {
} from '@jupyterlab/docregistry';

import { NotebookPanel, INotebookModel } from '@jupyterlab/notebook';

* The plugin registration information.
const plugin: JupyterFrontEndPlugin<void> = {
  id: 'my-extension-name:widgetPlugin',
  autoStart: true

* A notebook widget extension that adds a widget in the notebook header (widget below the toolbar).
export class WidgetExtension
  implements DocumentRegistry.IWidgetExtension<NotebookPanel, INotebookModel>
  * Create a new extension object.
    panel: NotebookPanel,
    context: DocumentRegistry.IContext<INotebookModel>
  ): IDisposable {
    const widget = new Widget({ node: Private.createNode() });

    panel.contentHeader.insertWidget(0, widget);
    return new DisposableDelegate(() => {

* Activate the extension.
function activate(app: JupyterFrontEnd): void {
  app.docRegistry.addWidgetExtension('Notebook', new WidgetExtension());

* Export the plugin as default.
export default plugin;

* Private helpers
namespace Private {
  * Generate the widget node
  export function createNode(): HTMLElement {
    const span = document.createElement('span');
    span.textContent = 'My custom header';
    return span;

And the following to style/base.css:

.jp-myextension-myheader {
    min-height: 20px;
    background-color: lightsalmon;

Run the following commands:

pip install -e .
pip install jupyter-packaging
jupyter labextension develop . --overwrite
jupyter lab

Open a notebook and observe the new “Header” widget.

The ipywidgets third party extension

This discussion will be a bit confusing since we’ve been using the term widget to refer to lumino widgets. In the discussion below, ipython widgets will be referred to as ipywidgets. There is no intrinsic relation between lumino widgets and ipython widgets.

The ipywidgets extension registers a factory for a notebook widget extension using the Document Registry. The createNew() function is called with a NotebookPanel and DocumentContext. The plugin then creates a ipywidget manager (which uses the context to interact the kernel and kernel’s comm manager). The plugin then registers an ipywidget renderer with the notebook instance’s rendermime (which is specific to that particular notebook).

When an ipywidget model is created in the kernel, a comm message is sent to the browser and handled by the ipywidget manager to create a browser-side ipywidget model. When the model is displayed in the kernel, a display_data output is sent to the browser with the ipywidget model id. The renderer registered in that notebook’s rendermime is asked to render the output. The renderer asks the ipywidget manager instance to render the corresponding model, which returns a JavaScript promise. The renderer creates a container lumino widget which it hands back synchronously to the OutputArea, and then fills the container with the rendered ipywidget when the promise resolves.