from __future__ import print_function

Building a Custom Widget - Hello World

The widget framework is built on top of the Comm framework (short for communication). The Comm framework is a framework that allows the kernel to send/receive JSON messages to/from the front end (as seen below).

Widget layer

To create a custom widget, you need to define the widget both in the browser and in the python kernel.

Building a Custom Widget

To get started, you’ll create a simple hello world widget. Later you’ll build on this foundation to make more complex widgets.

Python Kernel

DOMWidget and Widget

To define a widget, you must inherit from the Widget or DOMWidget base class. If you intend for your widget to be displayed in the Jupyter notebook, you’ll want to inherit from the DOMWidget. The DOMWidget class itself inherits from the Widget class. The Widget class is useful for cases in which the Widget is not meant to be displayed directly in the notebook, but instead as a child of another rendering environment. For example, if you wanted to create a three.js widget (a popular WebGL library), you would implement the rendering window as a DOMWidget and any 3D objects or lights meant to be rendered in that window as Widgets.

_view_name

Inheriting from the DOMWidget does not tell the widget framework what front end widget to associate with your back end widget.

Instead, you must tell it yourself by defining specially named trait attributes, _view_name and _view_module (as seen below) and optionally _model_name and _model_module.

import ipywidgets as widgets
from traitlets import Unicode, validate


class HelloWidget(widgets.DOMWidget):
    _view_name = Unicode('HelloView').tag(sync=True)
    _view_module = Unicode('hello').tag(sync=True)

sync=True traitlets

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, the browser would have no knowledge of _view_name or _view_module.

Other traitlet types

Unicode, used for _view_name, is not the only Traitlet type, there are many more some of which are listed below:

• Any
• Bool
• Bytes
• CBool
• CBytes
• CComplex
• CFloat
• CInt
• CLong
• CRegExp
• CUnicode
• CaselessStrEnum
• Complex
• Dict
• DottedObjectName
• Enum
• Float
• FunctionType
• Instance
• InstanceType
• Int
• List
• Long
• Set
• TCPAddress
• Tuple
• Type
• Unicode
• Union

Not all of these traitlets can be synchronized across the network, only the JSON-able traits and Widget instances will be synchronized.

Front end (JavaScript)

Models and views

The IPython widget framework front end relies heavily on Backbone.js. Backbone.js is an MVC (model view controller) framework. Widgets defined in the back end are automatically synchronized with generic Backbone.js models in the front end. The traitlets are added to the front end instance automatically on first state push. The _view_name trait that you defined earlier is used by the widget framework to create the corresponding Backbone.js view and link that view to the model.

Import jupyter-js-widgets

You first need to import the jupyter-js-widgets module. To import modules, use the define method of require.js (as seen below).

%%javascript
define('hello', ["jupyter-js-widgets"], function(widgets) {

});

Define the view

Next define your widget view class. Inherit from the DOMWidgetView by using the .extend method.

%%javascript
require.undef('hello');

define('hello', ["jupyter-js-widgets"], function(widgets) {

    // Define the HelloView
    var HelloView = widgets.DOMWidgetView.extend({

    });

    return {
        HelloView: HelloView
    }
});

Render method

Lastly, override the base render method of the view to define custom rendering logic. A handle to the widget’s default DOM element can be acquired via this.el. The el property is the DOM element associated with the view.

%%javascript
require.undef('hello');

define('hello', ["jupyter-js-widgets"], function(widgets) {

    var HelloView = widgets.DOMWidgetView.extend({

        // Render the view.
        render: function() {
            this.el.textContent = 'Hello World!';
        },
    });

    return {
        HelloView: HelloView
    };
});

Test

You should be able to display your widget just like any other widget now.

HelloWidget()

Making the widget stateful

There is not much that you can do with the above example that you can’t do with the IPython display framework. To change this, you will make the widget stateful. Instead of displaying a static “hello world” message, it will display a string set by the back end. First you need to add a traitlet in the back end. Use the name of value to stay consistent with the rest of the widget framework and to allow your widget to be used with interact.

class HelloWidget(widgets.DOMWidget):
    _view_name = Unicode('HelloView').tag(sync=True)
    _view_module = Unicode('hello').tag(sync=True)
    value = Unicode('Hello World!').tag(sync=True)

Accessing the model from the view

To access the model associate with a view instance, use the model property of the view. get and set methods are used to interact with the Backbone model. get is trivial, however you have to be careful when using set. After calling the model set you need call the view’s touch method. This associates the set operation with a particular view so output will be routed to the correct cell. The model also has an on method which allows you to listen to events triggered by the model (like value changes).

Rendering model contents

By replacing the string literal with a call to model.get, the view will now display the value of the back end upon display. However, it will not update itself to a new value when the value changes.

%%javascript
require.undef('hello');

define('hello', ["jupyter-js-widgets"], function(widgets) {

    var HelloView = widgets.DOMWidgetView.extend({

        render: function() {
            this.el.textContent = this.model.get('value');
        },
    });

    return {
        HelloView : HelloView
    };
});

Dynamic updates

To get the view to update itself dynamically, register a function to update the view’s value when the model’s value property changes. This can be done using the model.on method. The on method takes three parameters, an event name, callback handle, and callback context. The Backbone event named change will fire whenever the model changes. By appending :value to it, you tell Backbone to only listen to the change event of the value property (as seen below).

%%javascript
require.undef('hello');

define('hello', ["jupyter-js-widgets"], function(widgets) {

    var HelloView = widgets.DOMWidgetView.extend({

        render: function() {
            this.value_changed();
            this.model.on('change:value', this.value_changed, this);
        },

        value_changed: function() {
            this.el.textContent = this.model.get('value');
        },
    });

    return {
        HelloView : HelloView
    };
});

Test

w = HelloWidget()
w

w.value = 'test'

Conclusion

The example above dumps the value directly into the DOM. There is no way for you to interact with this dumped data in the front end. To create an example that accepts input, you will have to do something more than blindly dumping the contents of value into the DOM.

In the next section of the tutorial, you will build a date picker to display and accept input in the front end.

More advanced uses: Packaging and distributing Jupyter widgets

A template project is available in the form of a cookie cutter: https://github.com/jupyter/widget-cookiecutter

This project is meant to help custom widget authors get started with the packaging and the distribution of Jupyter interactive widgets.

It produces a project for a Jupyter interactive widget library following the current best practices for using interactive widgets. An implementation for a placeholder “Hello World” widget is provided.