Example of how to access widget state in a callback.

streamlit_app.py

import streamlit as st

with st.echo():
    def on_change():
        st.write(f"my_selection **(1)**: `{st.session_state.my_selection}`")

    my_selection = st.selectbox(
        "Select something", ["a", "b"], on_change=on_change, key="my_selection"
    )
    st.write(f"my_selection **(2)**: `{st.session_state.my_selection}`")

To see this in action run:

streamlit run https://gist.githubusercontent.com/treuille/427a7359c591adf4685110e34f3af4a9/raw/18c19de9e058d943710308f88d4d16053395c47e/streamlit_app.py