Modern Web Meets Jupyter

By Trevor Manz

TL;DR: anywidget v0.2 brings modern web development to Jupyter. You can now use a file path to enable anywidget’s integrated Hot Module Replacement (HMR):

pip install --upgrade "anywidget[dev]"
import pathlib
import anywidget
import traitlets

class Counter(anywidget.AnyWidget):
    _esm = pathlib.Path("index.js") # path to an existing file
    value = traitlets.Int(0).tag(sync=True)

… the contents of _esm are read automatically from disk, and any changes made to index.js are instantly reflected in the front end without refreshing or re-executing notebook cells. The release also includes various bug fixes and improved support for rendering widgets in Google Colab. Check the video previewing some of these features or read on to learn more.

anywidget v0.2

anywidget v0.2 is a significant update that brings new features to dramatically improve the widget development experience. Some of these features have been on my wishlist for Jupyter for a while, and I am thrilled to finally share them!

Native Hot Module Replacement (HMR)

Hot Module Replacement (HMR) is a powerful tool implemented in modern frameworks like Vite that enables developers to see changes in their application user interface without requiring full page load or clearing state. HMR works by updating only modules that have been modified, rather than refreshing the entire application. After experiencing web development with HMR, working with frameworks that lack this capability can be frustrating.

The Vite plugin for anywidget currently enables HMR for widget developers, but at the cost of adding project complexity. While Vite offers a best-in-class developer experience, its full feature set is not essential for numerous anywidget projects. This tension motivated us to develop HMR support directly within anywidget, exposing a framework for live-reloading without additional configuration.

We included an initial implementation of HMR on top of the Jupyter Widgets framework in a prior anywidget release, but in v0.2 it is fully integrated in the top-level API. This feature provides a seamless (opt-in) front-end development experience on par with modern front-end frameworks.

Effortless Front-End Integration

anywidget previously required the _esm and _css attributes (i.e., the widget front-end code) to be Python strings. Therefore, separate files had to be read manually from disk:

class Counter(anywidget.AnyWidget):
    _esm = pathlib.Path("index.js").read_text()

in v0.2, you can now drop the read_text() and pass a file path string or a pathlib.Path object directly:

class Counter(anywidget.AnyWidget):
    _esm = pathlib.Path("index.js")

and anywidget will automatically read the file contents from disk for you.

Now for the magic 🪄. Passing a file path not only offers a convenience to loading your front-end code, but also opts in to anywidget’s native HMR during developement. anywidget will start listening for modifications to the referenced files and instantly apply changes to the front-end, offering an integrated development experience within Jupyter like never before.

What sets this implementation apart is the ability to deliver integrated HMR without the need for a separate front-end development server (e.g., Vite, Webpack, Parcel). In anywidget, we reuse the active IPython session and its Comm framework as our devlopement server, delivering a familiar front-end developer experience to these tools, but entirely from within Jupyter and tailored towards creating custom widgets.

You can watch this video to see this modern development workflow in action entirely from within JupyterLab, or try it our yourself.

Embracing Type Safety in the Front End

This release also introduces the type-only anywidget/types submodule on NPM, enabling developers who use TypeScript to benefit from enhanced type safety in their front-end widget code. Widget authors can specify their model’s types in front-end take advantage of autocomplete and IntelliSense features within their editor. For example, for the following Python module:

import pathlib

import anywidget
import traitlets

class Counter(anywidget.AnyWidget):
    _esm = pathlib.Path("index.js")
    value = traitlets.Int(0).tag(sync=True)

the associated widget code may also define the expected types on the Model (i.e., value) though JSDoc comments:

// @ts-check
 * @typedef Model
 * @prop {number} value - the current count value

/** @type {import("anywidget/types").Render<Model>} */
export function render({ model, el }) {
	let value = model.get("value");
	//^? number

	// type error, `nope` is not defined on Model

	model.set("value", "not a number");
	//^? type error, must be a number

The import("anywidget/widget").Render<Model> utilty strictly types the render function such that model.get and model.set are typed based on the user-defined Model.

This feature brings Jupyter Widgets one step closer to having end-to-end type safety, and the use of TypeScript within JSDoc comments means that widget front-end code can benefit from static analysis without additional compilation, much like Python’s builtin type hints.

Defining Custom Cleanup Logic

Prior versions of anywidget did not expose a dedicated API for defining cleanup logic for a view. In v0.2, we’re introducing an API (inspired by React’s useEffect hook) to allow developers to define a callback that is executed any time the a view is removed from the DOM. This enhancement is useful when dealing with complex event listeners, subscriptions, or third-party libraries that require proper teardown.

Although this feature might not be essential for all use cases, it provides a flexible and more declarative way to ensure proper cleanup when needed:

export function render({ model, el }) {
	// Create DOM elements and set up subscribers
	return () => {
		// Optionally cleanup

This new API is particularly useful when working with third-party libraries like React that take control of the DOM and require proper cleanup to prevent exceptions when elements are unmounted. For example:

import * as React from "";
import * as ReactDOM from "";

function App(props) {
	return <h1>Hello, world</h1>;

export function render({ model, el }) {
	let root = ReactDOM.createRoot(el);
	root.render(<App />);
	return () => root.unmount();

Note: The above front-end code requires transformation with tool like esbuild to allow for the special JSX syntax (e.g., <App />).

Getting Started

To start using anywidget v0.2, upgrade your package using pip:

pip install --upgrade "anywidget[dev]"

I encourage you to explore the new features in anywidget v0.2 and experience the modern web development practices it brings to the Jupyter ecosystem. Happy coding!