Best method to keep the state of a widget

Using Streamlit
1

Hi there
I finally have how to keep the state of a widget like for example selectbox if I change section with a sidebar for example

what surprises me is that the only valid method is to go through a callback and especially why the key of the widget here ‘value’ disappears when I change section
The disappearance of the ‘value’ key is an official feature of streamlit ?
wouldn’t it be wiser to keep this key as long as there is no clear cache and thus avoid switching from one key to another via a callback?

Attached is the sample code

import streamlit as st

options = ['a','b']
if 'index' not in st.session_state:
    st.session_state['index'] = 0

def func_():
    st.session_state['index'] = options.index(st.session_state['value'])
        
section  = st.sidebar.radio('',['S1','S2'])

if section =='S1':   
    st.write(st.session_state)
    print(st.session_state)
    index  = st.session_state['index']
    side = st.selectbox("test" ,options = options,index = index, key = 'value', on_change = func_)
    st.write(st.session_state)
    print(st.session_state)
    
if section =='S2':
    st.write(st.session_state)
    print(st.session_state)

1 change selectbox to ‘b’
2 pass in section ‘S2’
3 return to section ‘S1’

we find the state of the widget on ‘b’ but the key value has disappeared

thank you in advance

2

update
no one have any idea or my question is not clear ?

3

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