empty() does not remove cleared elements?

Summary

Not sure if this is expected behaviour (at least from docs on empty it doesn’t seem so). Calling .empty() on st.empty keeps children elements in the Streamlit state, so their ids cannot be re-cycled even though the widgets dissappear.

Steps to reproduce

import streamlit as st

placeholder = st.empty()

with placeholder.container():
  st.number_input("One", value=1)
  st.number_input("Two", value=2)

if st.button('Clear & Add'):
  placeholder.empty()
  with placeholder.container():
    st.number_input("One", value=1)
    st.number_input("Two", value=2)
    st.number_input("Three", value=3)

Run the app and click Clear & Add button.

Expected behavior:

2 number inputs are replaced with 3 number inputs as specified in code snippet.

Actual behavior:

streamlit.errors.DuplicateWidgetID: There are multiple identical `st.number_input` widgets with the
same generated key.

When a widget is created, it's assigned an internal key based on
its structure. Multiple widgets with an identical structure will
result in the same internal key, which causes this error.

To fix this error, please pass a unique `key` argument to
`st.number_input`.

Debug info

• Streamlit version: 1.10.0
• Python version: 3.7.13
• OS version: macOS Big Sur 11.6.8
• Browser version: 103

Additional information

I’m using modified example from https://docs.streamlit.io/library/api-reference/layout/st.empty with different set of widgets. It would be nice for empty to actually remove the widgets so that it’s possible to re-draw bits of UI from scratch.

Thanks.

[blackary]

See the app here to see issue for container entries in general, and how it can be fixed by adding a sleep after emptying

[LukasMasuch]

It just seems a bit wasteful to keep all those objects when they are not visible.

I’m not so sure if there is even a lot of waste created by this since the check is mainly based on an str set of already added widget keys. But I also haven’t looked closely into this, and there could be some wasteful stuff happening on the frontend part. This code part might be a good point to start looking into this:

https://github.com/streamlit/streamlit/blob/756072522ee4a473158355f025f36e1dbd7b74a2/lib/streamlit/state/widgets.py#L195