Streamlit

Core Workflow

1. Inspect the installed Streamlit version and the local API signature before relying on memory:

uv run python - <<'PY'
import inspect
import streamlit as st
print(st.__version__)
print(inspect.signature(st.data_editor))
PY

1. Prefer the repository's package manager and app entrypoint. For Python projects using uv, run Streamlit through uv run ....
2. Treat widget key values as part of the state model. Avoid changing keys casually; when a key must change to force remounting, make the remount intentional and bounded.
3. Validate with both Python checks and a running app smoke check when possible:

uv run python -m compileall path/to/file.py
uv run pytest
uv run streamlit run path/to/app.py --server.headless true --server.port <free-port>
curl -sS http://localhost:<free-port>/_stcore/health

Version-Sensitive APIs

• Check local signatures for parameters that changed across Streamlit releases.
• Prefer width="stretch" on supported recent versions instead of older usecontainerwidth=True patterns. Do not blindly convert without confirming the installed signature.
• If maintaining compatibility with older Streamlit versions, keep the compatibility choice explicit in code or in the final note. Hidden compatibility branches can mask UI bugs.
• For widget callbacks, remember that the callback runs before the script rerun completes. Use st.session_state as the handoff surface between the callback and the next render.

State Rules

• Initialize session state once, then normalize existing state on later renders instead of overwriting it from the source model every rerun.
• Keep editable widget state separate from derived display data. Recompute derived columns from the editable source after every edit.
• Use stable widget keys for normal editing; use versioned keys only when a widget must be remounted to consume a freshly normalized baseline.
• Delete stale versioned widget keys after remounting to avoid accumulating obsolete state.
• When a widget returns partial state rather than full values, reconstruct the full value from a stored baseline before validating or saving.

Complex Components

For st.dataeditor state synchronization, derived columns, dynamic rows, and one-rerun lag bugs, read [references/dataeditor.md](references/data_editor.md).

UI Checks

• After significant Streamlit UI changes, run the app locally and hit /_stcore/health.
• Prefer browser verification when available for visual or interaction regressions.
• If browser automation is unavailable, report that only server-level smoke checks were performed.