ipywidgets dropdown onclick - jupyter-notebook

Question

It seems that there is no on_click option with dropdown widgets, I was wondering if there is some sort of workaround. One method I was thinking was, everytime an option is chosen, to flush the options and start the dropdown from the top again, where the top option would be the empty "".
For instance suppose I have:
from IPython.html import widgets
from IPython.display import display
def dropdown_event_handler(change):
print(change.new)
# flush the options and start from "" again
options = ["", "A", "B"]
dropdown = widgets.Dropdown(options=options, description="Categories")
dropdown.observe(dropdown_event_handler, names="value")
display(dropdown)
So the desired behaviour is that if I press "A" and "A" again, A would be printed out twice.

Answer 1

As you already suggested, you could set the value of the widget to "" after each change:
from IPython.html import widgets
from IPython.display import display
def dropdown_event_handler(change):
print(change.new)
dropdown.value = ""
options = ["", "A", "B"]
dropdown = widgets.Dropdown(options=options, description="Categories")
dropdown.observe(dropdown_event_handler, names='value')
display(dropdown)
And I fear that is your only option. The Dropdown widget has no other type than "change". You can see all available types by printing them with type=All.
from IPython.html import widgets
from IPython.display import display
from traitlets import All
def dropdown_event_handler(change):
print(change)
options = ["", "A", "B"]
dropdown = widgets.Dropdown(options=options, description="Categories")
dropdown.observe(dropdown_event_handler, type=All)
display(dropdown)
Output:
{'name': '_property_lock', 'old': traitlets.Undefined, 'new': {'index': 1}, 'owner': Dropdown(description='Categories', options=('', 'A', 'B'), value=''), 'type': 'change'}
{'name': 'label', 'old': '', 'new': 'A', 'owner': Dropdown(description='Categories', index=1, options=('', 'A', 'B'), value=''), 'type': 'change'}
{'name': 'value', 'old': '', 'new': 'A', 'owner': Dropdown(description='Categories', index=1, options=('', 'A', 'B'), value='A'), 'type': 'change'}
{'name': 'index', 'old': 0, 'new': 1, 'owner': Dropdown(description='Categories', index=1, options=('', 'A', 'B'), value='A'), 'type': 'change'}
{'name': '_property_lock', 'old': {'index': 1}, 'new': {}, 'owner': Dropdown(description='Categories', index=1, options=('', 'A', 'B'), value='A'), 'type': 'change'}
So you can't observe a value in a Dropdown widget if it did not change. For more information see the Traitlets documentation.