Description
Problem
I found the response in this ticket to be inaccurate when implemented myself. When I use observe to register a validation handler, changes are made to the widget value before the change dict is passed for validation. This wouldn't be a problem except that returning change['old'] still triggers the handler again.
If I want to display a message to the user, which will clear after any value change occurs, this is not possible under the current pattern. Using the example from the ticket linked above:
from ipywidgets import FloatText
text = FloatText(value=50, continuous_update=False)
def validate(change):
# Put your validation condition here
print(f'validating {change["new"]}')
if change['new'] > 50:
change['owner'].value = change['old']
text.observe(validate, 'value')
text
Changing the value to 51 (by clicking into the text box with my mouse and hitting the up arrow key once) produces the following output:
validating 51.0
validating 50.0
Assuming that the validating is not this simple (e.g. requires a call to a method, as in my case), it's not suitable to require that the previous input, which has already presumably been validated, undergo the validation again.
Suggested Improvement
I think a full working example of validation outlining how we might extend a Widget using a custom handler would be extremely helpful. If the behavior demonstrated above is a bug, I'd be happy to file a bug report instead.