Diagnosing Streamlit Widget State Loss and Fixing with st.session_state

Condition

A Streamlit widget (e.g., st.selectbox, st.slider) shows the user‑selected value immediately after interaction, but on the next script rerun it reverts to the widget’s default argument.

Cause and quick diagnostic table

Check	Expected observation if cause is present
Insert st.write(st.session_state) before and after the widget	The widget’s key is absent or its value resets to the default after the rerun
Run the app and interact with the widget	Value changes in the UI but disappears after the next automatic rerun

Ordered checks

1. Start the app locally: streamlit run app.py (no special permissions needed).
2. Open the browser console or watch the Streamlit output for the st.write(st.session_state) lines.
3. Note whether a key matching the widget appears and retains the selected value after you change it.
4. If the key is missing or its value equals the default, the cause is confirmed.

Fixes tied to findings

Bind the widget manually to st.session_state

import streamlit as st

# Ensure a default exists in state
if 'my_slider' not in st.session_state:
    st.session_state['my_slider'] = 0

# Read current state, show widget, write back
current = st.session_state['my_slider']
new_val = st.slider('Select a value', 0, 10, current)
st.session_state['my_slider'] = new_val
st.write('Current slider value:', st.session_state['my_slider'])

Use the widget’s key argument for automatic handling

import streamlit as st

# The widget will read/write its value to st.session_state['my_slider'] automatically
val = st.slider('Select a value', 0, 10, key='my_slider')
st.write('Current slider value:', val)

Escalation criteria

• After applying either fix, the widget still reverts to its default.
• You observe calls to st.experimental_rerun() or st.rerun() that may be clearing state.
• Callbacks associated with the widget modify st.session_state in a way that overwrites the value.

Verification steps

1. Apply one of the fixes above and save the file.
2. Rerun the app: streamlit run app.py.
3. Interact with the widget and watch the st.session_state output (or the st.write line).
4. Confirm that the selected value persists across multiple interactions and automatic reruns without returning to the default.

Limitations and practical check

If you use mutable default arguments (e.g., a list or dict) directly in a widget call, they are shared across all sessions and can cause unexpected state changes. Prefer initializing such objects in st.session_state before the widget runs. The practical way to confirm a fix is to observe that the widget’s value shown in the UI matches the value printed from st.session_state after each rerun.