Creating a minimal custom Jupyter widget

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)
    _view_module_version = Unicode('0.1.0').tag(sync=True)
    value = Unicode('Hello World!').tag(sync=True)

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

define('hello', ["@jupyter-widgets/base"], function(widgets) {

    var HelloView = widgets.DOMWidgetView.extend({

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

    return {
        HelloView : HelloView
    };
});

HelloWidget()