将小部件对象状态存储到ipython内核对象实例而不是对象类

Question

Following the stateful widget tutorial , we can create a simple DOM widget. Here is the python code:

import ipywidgets.widgets as widgets
from traitlets import Unicode

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)

And the javascript code:

%%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
    };
});

This works as advertised in the notebook:

In [1]: HelloWidget()
Out [1]: Hello World!

Now, if I want to store the widget value state to the object instance, I can change the python code so it looks like the following:

import ipywidgets.widgets as widgets
from traitlets import Unicode

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)
    def __init__(self, s):
        super().__init__()
        self.value = Unicode(s).tag(sync=True)

However, this does not work; the state is not rendered to the output cell as expected (no output):

In [1]: HelloWidget("Hello World!")
Out [1]: 

How can this be done?

Answer 1

I figured this out (and feel a bit dumb).

The trailets.Unicode object is a descriptor, so it has to be attached to the class object, HelloWidget . So the correct code is as follows:

import ipywidgets.widgets as widgets
from traitlets import Unicode

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().tag(sync=True) # value is set to an empty string as placeholder

    def __init__(self, s):
        super().__init__()
        self.value = s # initialize with a value here