Do something when selectbox has an option selected

Using Streamlit
1

Hello:
I have a code like this:

selected_option = st.selectbox(“Select an option”, options, key=“search_option”,index=None,)

How can I make a contitional sentence that is only execute when a selection is made, and don’t do nothing when no selection is made?

I’ve tred something like:

if selected_option!=“None”:
#dosometing

But it doesn’t work…

2

Depending on what kind of thing you’re trying to do, you can check for None or use a callback.

Check for None instead of "None".

import streamlit as st

selected_option = st.selectbox("Select an option", ["A","B","C"], key="search_option", index=None,)
if selected_option is not None:
    st.write(f"You selected {selected_option}.")

st.button("Rerun")

Use a callback:

import streamlit as st

def notification():
    st.toast(f"You selected {st.session_state.search_option}.")

selected_option = st.selectbox("Select an option", ["A","B","C"], key="search_option", index=None, on_change=notification)

st.button("Rerun")

I’ve include another button in both examples to highlight the difference: when you use the conditional to check for None, the code in the if block will execute on every rerun. (The message persists.) If you use a callback, the callback code will only execute in reaction to the changed selection, but not again if the user clicks elsewhere in the app. (The message is only one-time, in reaction to a change.)