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).
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.
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.
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_module_version (as seen below) and optionally
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)
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
Other traitlet types¶
Unicode, used for
_view_name, is not the only Traitlet type, there are many more some of which are listed below:
Not all of these traitlets can be synchronized across the network, only the JSON-able traits and Widget instances will be synchronized.
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.
You first need to import the
@jupyter-widgets/base module. To import modules, use the
define method of require.js (as seen below).
Define the view¶
Next, define your widget view class. Inherit from the
DOMWidgetView by using the
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
el property is the DOM element associated with the view.
You should be able to display your widget just like any other widget now.
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) _view_module_version = Unicode('0.1.0').tag(sync=True) value = Unicode('Hello World!').tag(sync=True)
Accessing the model from the view¶
To access the model associated with a view instance, use the
model property of the view.
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.
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).
w = HelloWidget() w
w.value = 'test'
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.