Callbacks not triggered when using observe

Hi I have the following problem.

When I use:

import ipywidgets as widgets

toggle = widgets.ToggleButton(description='click me')

def on_click(change):
    print(change['new'])

toggle.observe(on_click, 'value')

the callback does not get triggered, but if I use

import ipywidgets as widgets

@interact
def clicked(a=widgets.ToggleButton()):
    print(a)

the call back gets triggered.

I am using Jupyter Lab: Jupyter Notebook version: 5.6.0 ipywidgets version: 7.2.1

[jasongrout]

You’ll need to use an output widget to capture the printed output: http://ipywidgets.readthedocs.io/en/stable/examples/Output Widget.html.

out = Output()
display(out)

@out.capture()
def on_click(change):
    print(change['new'])

[DavidJVitale]

@vidartf Thanks for your help. That code snippet functions the same in the live notebook on Classic server and JupyterLab: The problem is that it still stores all those <h3> HTML elements inside of a dynamic JS widget. If you run your code and export to HTML, you need JS enabled (and some libraries loaded from CDN) to view the Arbitrary HTML text. Try running your cell, exporting to HTML, and doing an ‘Inspect Element’. You’ll just see <script> tags in the output, and no text.

Luckily, I have a found a solution for my problem, using the display_handler = IPython.display.display("foo", display_id="id") and display_handler.update() functionality. Below is the code snippet that works in both JupyterLab and Classic Notebook Server. If you export after running it, you will see the raw <h3> element in the output cell.

from ipywidgets import *
from IPython.display import display, HTML
from uuid import uuid4
w = IntSlider()
pure_html_display_hander = display(HTML('starting text'),
                                   display_id = str(uuid4()))

out = Output()
display(out)
display(w)

@out.capture()
def callback(change):
    display_html = HTML(f'<h3>Arbitrary HTML {uuid4()}</h3>')
    pure_html_display_hander.update(display_html)

w.observe(callback, names=['value'])

It’s a bit hacky updating a global from inside a callback, but it fits my need. I am not sure if anyone else has ran into this issue, but this is the best way to dynamically update output cell HTML of the underlying notebook file asyncronously in both JupyterLab and Classic Notebook Server.