selectbox does not play well with session state persistence

Summary

When using selectbox together with session state persistence code found at https://gist.github.com/tvst/036da038ab3e999a64497f42de966a92, I need to select the same item two times in row for it to be actually selected. The first time I select it, the selectbox automatically restores the previous value.

Steps to reproduce

Code snippet:

import SessionState
import streamlit as st


options = ["option 1", "option 2"]
session_state = SessionState.get(option='-')
try:
    index = options.index(session_state.option)
except ValueError:
    index = 0
session_state.option = st.sidebar.selectbox('Options', options, index=index)

1. At startup, the select box displays “option 1”.
2. Select “option 2”, it works, the first selection after startup always works.
3. Select “option 1”, the select box displays “option 1” during a very short period of time and then restores the select box content to “option 2”.
4. Select “option 1” again, it works.

Expected behavior:

I expect step 3 to provide “option 1”

Actual behavior:

Step 3 provides “option 2”.

Is this a regression?

That is, did this use to work the way you expected in the past? I don’t know. I observed the same behavior in Streamlit version 0.81.1.

Debug info

• Streamlit version: version 0.82.0
• Python version: Python 3.8.9
• pyenv 1.2.27
• OS version: macOS Big Sur 11.3.1
• Browser version: Firefox 88.0.1 / Safari 14.1

[AnOctopus]

@vincbo

With the recent release of session state in core streamlit, the snippet you provided can now be written in a cleaner and more concise way that does not have this issue. Since widgets are integrated into the session state api, there is no longer a need for manually syncing the widget and session state values. This snippet works and I think does what the original wanted to do:

import streamlit as st

state = st.session_state


options = ["option 1", "option 2"]
st.sidebar.selectbox("Options", options, key="option")
st.write(state.option)  # to show us retrieving the value from state

The selected value resetting used to be caused by the changing index argument giving the widget a new identity, but now we do it deliberately to facilitate live-editing scripts. I’m starting a discussion internally to evaluate that use case versus this being a potential footgun. The way to make it work correctly is to only ever set a value or index to a constant, and use the session state api to get/set the widget value instead of trying to manually sync them.

[kmcgrady]

Going to close this issue with a new, preferred, solution.