Dropdown ipywidget after unobserve_all() does not accept any further observe()

I use ipywidget 7.0.5 (py35_0) The dropdown ipywidget after unobserve_all() does not work for any further observe(). This is easily reproducible with the following code:

import ipywidgets.widgets as widgets
from IPython.display import display
def helloWorld(_=None):
    print("Hello World!")

# the dropdown is not working properly
widget = widgets.Dropdown( 
            options=[('One', 1), 
                     ('Two', 2),
                     ('Three', 3)],
            description='dropdown_w:',
            value = 2
        )
widget.observe(helloWorld, 'value')
display(widget)
widget.unobserve_all()
widget.observe(helloWorld, 'value')


## instead same code works for other widgets (as it should)
w = widget = widgets.FloatSlider(value = 0.5, min = 0, max = 1, step = 0.1, description = "w")
w.observe(helloWorld, 'value')
display(w)
w.unobserve_all()
w.observe(helloWorld, 'value')

[NimSed]

Not sure if this is still relevant. But it may help some one. w.unobserve(None,...) does something like what unobserve_all is expected to do. Of course you’d still need to pass names whenever needed. But at least you don’t need to know the exact name of the handler. Specially if you have used a partial when setting the observer.

[thomasaarholt]

I just ran into this with select as well. Got a minimum working example below for future reference:

import ipywidgets as widgets

def add(value):
    select.options = select.options + (1,) # adds "1" for each event triggered
    select2.unobserve_all() # Three appear
    #select2.unobserve(add) # Five appear
    select2.observe(add)
    
select = widgets.Select(options=('Cheese', 'Onions'), rows=15, value=None)
select2 = widgets.Select(options=('CLICK ME', 'OR ME'), rows=15, value=None)
select2.observe(add)
widgets.HBox([select, select2])