KeyError (only in terminal) when accessing session_state key in a multipage app

[luxiaolei]

Checklist

• I have searched the existing issues for similar issues.
• I added a very descriptive title to this issue.
• I have provided sufficient information below to help reproduce this issue.

Summary

I have encountered an issue when using st.session_state in a multipage Streamlit app. The app runs without any errors in the browser, and I can successfully access and modify the st.session_state values. However, in the terminal where I run the app, I get a KeyError indicating that the key is missing from st.session_state.

Reproducible Code Example

No response

Steps To Reproduce

1. Create a Streamlit app with the following directory structure
   ├── app.py
   └── pages
   └── mypage.py

2. In app.py, add the following code:

import streamlit as st

if 'my_val' not in st.session_state:
    st.session_state.my_val = 1

3. In mypage.py , add the following code:

import streamlit as st

st.session_state.my_val = st.session_state.my_val + 1
st.write(st.session_state.my_val)

4. Start the Streamlit app using the command: streamlit run app.py
5. Navigate between the main page and mypage.py in the browser.
6. After a while, check the error message in the terminal.

Expected Behavior

The terminal should not display any KeyError

Current Behavior

The app runs fine in the browser, and my_val increments as expected when navigating between pages.
However, the terminal displays a KeyError indicating that the key "my_val" is missing from st.session_state.

Is this a regression?

• Yes, this used to work in a previous version.

Debug info

• Streamlit version: 1.34.0
• Python version: 3.11
• Operating System: macOS
• Browser: Safari

Additional Information

No response

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[Asaurus1]

I suspect this will go away if you add a if "my_val" in st.session_state guard to your subpage, but agree that doesn't explain why you would get an issue only in your terminal.

[Asaurus1]

I'm not able to reproduce this error with the latest version of streamlit. I am able to produce key errors if I refresh and go directly to the "mypage.py" page (e.g. start a session on that page) which makes sense, since my_val doesn't exist yet. But those key errors also show up in the app.

[raethlein]

Thanks @Asaurus1 for responding to this! I agree with your assessment that in general if "my_val" in st.session_state should guard against this behavior. Indeed, if you start the app and go directly to /mypage, I get a key error, which is expected as explained above. As I am also not able to reproduce any other behavior, I am going to close the issue. @luxiaolei feel free to re-open the issue if you have more details on how this can be reproduced or feel the original issue was misunderstood 🙂