Experimental Features

This page describes experimental features and APIs which are subject to change in future releases.

A brief overview of widgets in Jupyter

In order to understand the motivation and function of some features in the experimental module, it is helpful to have a brief overview of how widgets work in Jupyter. This is summarized below, but for a more in-depth explanation, see the jupyter-widgets messaging protocol.

1. The _repr_mimebundle_ method

When representing a python object, many front-ends REPLs, including IPython, will look for a _repr_mimebundle_ method on the object. If found, this method must return a dictionary of data, keyed by MIME type, that can be used to render the object in the front-end. A super simple plain-text representation of an object might look like this:

class SomeDisplayableObject:
    def _repr_mimebundle_(self, **kwargs):
        return {"text/plain": repr(self)}

2. The application/vnd.jupyter.widget-view+json MIME type

The application/vnd.jupyter.widget-view+json MIME type is a special MIME type that may be included in the _repr_mimebundle_ in order to display a widget for an object. In this message, the model_id is the comm channel id of the widget to display.

  "application/vnd.jupyter.widget-view+json": {
    "model_id": "some-uuid",

3. The Comm object

A comm.base_comm.BaseComm object manages communication between the front end (Javascript model) and the backend (Python model). When the user modifies the state of the python object, the comm must send a message containing the updated JSON state to the front end, so that the widget view can be update. Similarly, when the user interacts with the widget in the browser, the front end must send a message to the backend, so that the python model can be updated.


In summary, we need the following to display a javascript-backed widget for a python object:

  1. A communication object to send messages between the backend python kernel and the frontend javascript code running in the browser.
  2. A python object that has the following properties:
    • A _repr_mimebundle_ method that returns a application/vnd.jupyter.widget-view+json MIME type with a model_id key pointing to the comm object from step 1.
    • The ability to serialize/deserialize the state of the python object to/from JSON.
    • An observer pattern that sends messages to the comm object when the state of the python object changes.
  3. A javascript object that is also aware of the model_id and can send messages to the comm object when the state of the widget changes, or update the view when the state of the python object changes (this is provided by the @jupyter-widgets/base package).

MimeBundle Descriptor

In the traditional ipywidgets setup, steps 1 and 2 above are provided by the ipywidgets package (with the comm package providing the communication object, and traitlets providing the observer pattern and serialization.)

However, there are now many data-class patterns in python that one might want to use to represent a serializeable python object (such as pydantic, dataclasses, msgspec, etc).

Anywidget’s MimeBundleDescriptor class is an experimental attempt to add the bare minimum widget communication functionality to any python object that implements a known data-class pattern and observer pattern (not just those that inherit from ipywidgets.Widget).:

from anywidget.experimental import MimeBundleDescriptor

class Foo:
  _repr_mimebundle_ = MimeBundleDescriptor()

When this descriptor is accessed on an instance, it will

  1. create a new Comm channel for this object.
  2. determine what data-class pattern the object uses, and set-up serialization of the object currently supporting:
  3. determine what observer pattern the object uses, and set-up two-way data binding between the object and the comm, currently supporting:

The widget decorator

In practice, we’d like the MimeBundleDescriptor to be a low-level API that most users don’t need to interact with directly. The anywidget.experimental.widget decorator provides a higher-level API that can be used to create a widget from any python object that implements a known data-class pattern and observer.

Here’s an example of how to use the widget decorator to create a widget from a dataclasses.dataclass, that uses psygnal’s evented-dataclass pattern for observers:

esm = "export function render({ model, el }) {}"
css = ".foo { color: red;}"

@widget(esm=esm, css=css)
class Foo:
    bar: str = "baz"

foo = Foo()

In the future, @widget may automatically add the @psygnal.evented decorator and/or the dataclass decorator if a pattern isn’t automatically detected, but for now all must be explicitly added.