Dynamically changing dropdowns in IPython notebook widgets and Spyre

Question

I have a dropdown in an IPython notebook (as part of the HTML widgets) and in a Spyre app (as a dropdown element), say to pick a continent and I'd like to add a second dropdown to select the country within the continent. Now obviously the options within the second dropdown are dependent on the value of the first one. I'm struggling to find a convenient way to have a callback function that would update this UI element.

I have almost done this in the IPython notebook where I had one interact function and within the called function, I'd create a second interact element with the second dropdown. But whenever I'd change the first dropdown a new dropdown element would be created, so I'd end up with one additional dropdown with each change. But I only want one dropdown to be updated, that's all.

Hope the issue is clear. Thank you.

Answer 1

Use interactive instead of interact and update your widget:

from IPython.html import widgets
from IPython.display import display

geo={'USA':['CHI','NYC'],'Russia':['MOW','LED']}

def print_city(city):
    print city

def select_city(country):
    cityW.options = geo[country]


scW = widgets.Select(options=geo.keys())
init = scW.value
cityW = widgets.Select(options=geo[init])
j = widgets.interactive(print_city, city=cityW)
i = widgets.interactive(select_city, country=scW)
display(i)
display(j)

Answer 2

The answer with the most up-votes is useful but seems a bit clumsy for me. After searching for a while, I found the answer here based on Jupyter docs is more preferred for me. I adapted them and provide the following.

from ipywidgets import interact, Dropdown

geo = {'USA':['CHI','NYC'],'Russia':['MOW','LED']}
countryW = Dropdown(options = geo.keys())
cityW = Dropdown()

def update_cityW_options(*args): # *args represent zero (case here) or more arguments.
    cityW.options = geo[countryW.value]
cityW.observe(update_cityW_options) # Here is the trick, i.e. update cityW.options based on countryW.value.

@interact(country = countryW, city = cityW)
def print_city(country, city):
    print(country, city)

As an alternative, I also found that I can just update the cityW.options within print_city function, an even clearer practice!

from ipywidgets import interact, Dropdown

geo = {'USA':['CHI','NYC'],'Russia':['MOW','LED']}
countryW = Dropdown(options = geo.keys())
cityW = Dropdown()

@interact(country = countryW, city = cityW)
def print_city(country, city):
    cityW.options = geo[country] # Here is the trick, i.e. update cityW.options based on country, namely countryW.value.
    print(country, city)