jupyter notebook - ipywidgets dropdown widgets: what is the onchange event?

Question

I can register a handler to button.on_click in ipython notebook widgets, but I don't know how to do the same for a dropdown widget

import ipywidgets as widgets
from IPython.display import display

def on_button_clicked(b):
    print("Button clicked.")

button = widgets.Button(description="Click Me!")
display(button)

button.on_click(on_button_clicked)

But for

choose_task = widgets.Dropdown(
    options=['Addition', 'Multiplication', 'Subtraction'],
    value='Addition',
    description='Task:',
)

there seems to be only

on_trait_change(...)

if I register a handler with this, can I use it to access the value of the widget? I have seen examples with the handler and the widget belong to a subclass, and the handler can use self to introspect. But if I don't want to use a subclass, how does the handler know what widget was the target of the event.?

Answer 1

Between this link and the traitlet docs on github and just playing around, I finally figured this out:

w = widgets.Dropdown(
    options=['Addition', 'Multiplication', 'Subtraction', 'Division'],
    value='Addition',
    description='Task:',
)

def on_change(change):
    if change['type'] == 'change' and change['name'] == 'value':
        print("changed to %s" % change['new'])

w.observe(on_change)

display(w)

Overall this looks a lot richer than the deprecated interface, but it could definitely use more examples.