Bokeh Extensions

Bokeh Extensions supports efficient, bidirectional communication from the server (Python) to the browser (Javascript) and back. It also gives you access to all the super powers of modern front end framework languages (js or typescript), tooling and frameworks like React, Vue and Angular. The layouts, panes and widgets that ships with Panel are Bokeh extensions.

Please note that in order for Bokeh Extensions to compile you will need to have node.js installed. You can install it directly from their web site or via conda install -c conda-forge nodejs.

Before you read on I would ask you to quickly study the offical Bokeh documentation Extending Bokeh. You don’t need to code and run the examples. But you need to get a basic understanding of

  • the existence and location of official Bokeh documentation
  • what a Bokeh extension is and how it is developed.

We will now focus on Bokeh Extensions in a Panel context.


In this example we will create a Panel HTMLButton extension that enables a user to catch a click event from any HTML element he/ she would like as shown below.


The implentation consists of 3 files

This is the Panel specific file. We need to import the Bokeh python extension and wrap that into a Panel extension.

import panel as pn
from panel.widgets.base import Widget
from . import html_button_model
import param

class HTMLButton(Widget):
    # Set the Bokeh model to use
    _widget_type = html_button_model.HTMLButton

    # Rename Panel Parameters -> Bokeh Model properties
    # Parameters like title that does not exist on the Bokeh model should be renamed to None
    _rename = {
        "title": None,

    # Parameters to be mapped to Bokeh model properties
    object = param.String(default=html_button_model.DEFAULT_OBJECT)
    clicks = param.Integer(default=0)

import pathlib

from import Int, String
from bokeh.layouts import column
from bokeh.models import HTMLBox

CUSTOM_TS = pathlib.Path(__file__).parent / "html_button_model.ts"
CUSTOM_TS_STR = str(CUSTOM_TS.resolve())

DEFAULT_OBJECT = "<button style='width:100%'>Click Me</button>"

class HTMLButton(HTMLBox):
    """Example implementation of a Custom Bokeh Model"""

    __implementation__ = CUSTOM_TS_STR

    object = String(default=DEFAULT_OBJECT)
    clicks = Int(default=0)


// See
import { HTMLBox, HTMLBoxView } from "models/layouts/html_box"

// See
import * as p from "core/properties"

// The view of the Bokeh extension/ HTML element
// Here you can define how to render the model as well as react to model changes or View events.
export class HTMLButtonView extends HTMLBoxView {
    model: HTMLButton
    objectElement: any // Element

    connect_signals(): void {

        this.connect(, () => {

    render(): void {
        this.el.innerHTML = this.model.object
        this.objectElement = this.el.firstElementChild

        this.objectElement.addEventListener("click", () => {this.model.clicks+=1;}, false)

export namespace HTMLButton {
    export type Attrs = p.AttrsOf<Props>
    export type Props = HTMLBox.Props & {
        object: p.Property<string>,
        clicks: p.Property<number>,

export interface HTMLButton extends HTMLButton.Attrs { }

// The Bokeh .ts model corresponding to the Bokeh .py model
export class HTMLButton extends HTMLBox {
    properties: HTMLButton.Props

    constructor(attrs?: Partial<HTMLButton.Attrs>) {

    static init_HTMLButton(): void {
        this.prototype.default_view = HTMLButtonView;

            object: [p.String, "<button style='width:100%'>Click Me</button>"],
            clicks: [p.Int, 0],

Finally we can use the new Widget in an example app.

def _example_app():
    # Default Button
    html_button = HTMLButton()

    # Material Button
    material_js = (
    material_html = """\
<link href=",400,500" rel="stylesheet">
<link href="" rel="stylesheet">
mwc-button {
    --mdc-theme-primary: #4CAF50;
    --mdc-theme-on-primary: white;
    material_html_pane = pn.pane.HTML(material_html, width=0, height=0, margin=0, sizing_mode="fixed")
    material_button = HTMLButton(object="<mwc-button style='width:100%' raised label='Panel' icon='favorite'></mwc-button>", height=40)

    # Image Button
    src = ""
    image_style = "height:95%;cursor: pointer;border: 1px solid #ddd;border-radius: 4px;padding: 5px;"
    image_html = f"<img class='image-button' src='{src}' style='{image_style}'>"
    image_button = HTMLButton(object=image_html, height=100, align="center")

    # Bar
    bar = pn.pane.Markdown(
        "## Panel Extension: HTMLButton",
        style={"color": "white", "padding-left": "25px", "padding-top": "10px"},

    app = pn.Column(
    return app


Other Examples

Click the images below to see the code.

../../_images/custom-bokeh-model.gifCustom Bokeh Model


Official Panel Examples

Every layout, pane or widget in Panel is essentially a Bokeh Extension so a good place to get inspiration is to navigate the Panel Reference Gallery to find an extension similar to the one you would like to implement and then study the code

../../_images/panel-reference-gallery.gifPanel Reference Gallery

You can find the code of the Panel components on Github via

and the underlying Bokeh extensions via

Prebuilt Bokeh Extensions

There are two ways in which the Bokeh .ts models can be built.

  • Automatically when you run the code.
    1. If you instantiate your extension before running .servable then the extension will automatically be built and registered by Panel/ Bokeh.
  • Manually up front using the panel build or bokeh buildcommand. 2. This is referred to as prebuilt Bokeh extensions.

In this document I will describe how I setup the awesome-panel-extensions package for prebuilt bokeh extensions. This was nescessary to distribute the extensions as a package.

I hope this description can help others who would like to develop and share Bokeh Extensions for Panel.

Setting up prebuilt extensions using Bokeh init --interactive is described in the Bokeh Docs. See Bokeh Pre-built extensions.

Steps for the awesome-panel-extensions Package as of 20200721 Panel 0.9.7/ Bokeh 2.1.1

I navigated to the awesome_panel_extensions inside the project.

cd awesome_panel_extensions

I ran bokeh init --interactive

$ bokeh init --interactive
Working directory: ...\awesome_panel_extensions
Wrote ...\awesome_panel_extensions\bokeh.ext.json
Create package.json? This will allow you to specify external dependencies. [y/n] y
  What's the extension's name? [awesome_panel_extensions]
  What's the extension's version? [0.0.1]
  What's the extension's description? [] A collection of awesome extensions for Panel
Wrote ...\awesome_panel_extensions\package.json
Create tsconfig.json? This will allow for customized configuration and improved IDE experience. [y/n] y
Wrote ...\awesome_panel_extensions\tsconfig.json
Created empty index.ts. This is the entry point of your extension.
You can build your extension with bokeh build
All done.

In the package.json I replaced

"dependencies": {
    "bokehjs": "^2.1.1"


"dependencies": {
    "@bokeh/bokehjs": "^2.1.1"

in order to import from bokehjs in the same way as Panel does. See bokeh init issue for more info.

I also replaced the tsconfig.json contents with

  "compilerOptions": {
    "noImplicitAny": true,
    "noImplicitThis": true,
    "noImplicitReturns": true,
    "noUnusedLocals": true,
    "noUnusedParameters": true,
    "strictNullChecks": true,
    "strictBindCallApply": false,
    "strictFunctionTypes": false,
    "strictPropertyInitialization": false,
    "alwaysStrict": true,
    "noErrorTruncation": true,
    "noEmitOnError": false,
    "declaration": true,
    "sourceMap": true,
    "importHelpers": false,
    "experimentalDecorators": true,
    "module": "esnext",
    "moduleResolution": "node",
    "esModuleInterop": true,
    "resolveJsonModule": true,
    "skipLibCheck": true,
    "target": "ES2017",
    "lib": ["es2017", "dom", "dom.iterable"],
    "baseUrl": ".",
    "outDir": "./dist/lib",
    "paths": {
      "@bokehjs/*": [
  "include": ["./**/*.ts"]

At least including the paths section is needed to be able to import { div, label } from "@bokehjs/core/dom" like @philippjfr does in Panel.

In the index.ts file I imported my models

import * as AwesomePanelExtensions from "./bokeh_extensions/"
export {AwesomePanelExtensions}

import {register_models} from "@bokehjs/base"
register_models(AwesomePanelExtensions as any)

In the bokeh_extensions/index.ts file I exported the WebComponent.

export {WebComponent} from "./web_component"

Then I could build my extension

$ panel build
Working directory: C:\repos\private\awesome-panel\package\awesome_panel
Using C:\repos\private\awesome-panel\package\awesome_panel\tsconfig.json
Compiling TypeScript (3 files)
Linking modules
Output written to C:\repos\private\awesome-panel\package\awesome_panel\dist
All done.

The result is in the dist folder.

I discovered I did not even have to do anything special to serve the awesome_panel_extensions.js file. It just works.

Finally I added awesome_panel_extensions/node_modules/*to my .gitignore file.


Should I define default values on the Bokeh .ts, Bokeh .py or Panel .py Model