Interactive Widget Session State:How can it return data index by session state,PLEASE?

Hello everyone
I am a noob and I just meet such a problem:
(code below)
I want to get data from dataset by some query method, and use RADIO(or whatever) as interactive widget.
When users select a label, another method(callback) is activated and the codes go on.
The question is:
as the pic shows, st.session_state.“keyname” only return the label of selection
(eg. Japan - Osaka) and I want it return the index(3) or “other_data_3” as return value.
How to make it?

By the way!! I don’t know how to use the callback parameters(on_change,*args,**kwargs)
Can anyone teach me?
I do really appreciate!