st.expander expanded/collapsed state

Problem

I’d like to be able to know whether a st.expander is expanded or collapsed (e.g. to show something on the main layout only when an expander in the sidebar is in expanded state).

Solution

MVP: Something like st.expander().expanded as a bool would be nice.

---

Community voting on feature requests enables the Streamlit team to understand which features are most important to our users.

If you’d like the Streamlit team to prioritize this feature request, please use the 👍 (thumbs up emoji) reaction in response to the initial post.

[enzedonline]

Would like to see this. I want to hide charts etc not in use which are quite processor intensive and only run the code when the expander is expanded. Something like:

exp = st.expander("Something Heavy")
with exp:
    if exp.expanded:
        st.write(heavy_function())

At the moment, everything is run, even in the hidden content.

Could also be included as an attribute for the expander:

with st.expander("Something Heavy", run_when_hidden=False):
    st.write(heavy_function())

(Psuedo-code, but you get the idea 😉)

[YoungXu06]

Getting expander state is a must especially when I need to monitor if users visit the content inside, looking forward to seeing an update!