@thalesgroup/agilab-streamlit-pages

---
name: agilab-streamlit-pages
description: Streamlit page authoring patterns for AGILAB (session_state safety, keys, rerun, UX).
license: BSD-3-Clause (see repo LICENSE)
metadata:
  updated: 2026-02-27
---

# Streamlit Pages Skill (AGILAB)

Use this skill when editing:
- `src/agilab/About_agilab.py`
- `src/agilab/pages/*.py`
- `src/agilab/apps-pages/*/src/*/*.py`

## Session State Rules (Avoid Common Crashes)

- Never assign `st.session_state["k"] = …` **after** a widget with `key="k"` was created.
  - Prefer `st.session_state.setdefault("k", default)` before the widget.
  - Or use widget return values and compute derived state separately.

- If you need to “reset” a widget value:
  - Use a different key (versioned key pattern), or
  - Gate the reset behind a rerun and only mutate state before widget creation.

## Recommended Pattern

1. Initialize defaults with `setdefault` at the top of the page.
2. Render widgets.
3. Read values from widgets, compute derived state, store under *different* keys.

## Rerun API

- Do not use `st.experimental_rerun()`; use `st.rerun()`.

## Key Hygiene

- Every widget must have a stable, unique key.
- Prefer namespaced keys: `f\"{page_id}:df_files\"`, not `"df_files"`.