Expander inside Form raises `Missing Submit Button`

[H4dr1en]

Summary

st.expander inside st.form raises Missing Submit Button when folded

Steps to reproduce

Code snippet:

import streamlit as st

with st.form("my_form"):
    with st.expander("test expander"):
        st.write("Inside the form")
        slider_val = st.slider("Form slider")
        checkbox_val = st.checkbox("Form checkbox")

        # Every form must have a submit button.
        submitted = st.form_submit_button("Submit")
        if submitted:
            st.write("slider", slider_val, "checkbox", checkbox_val)

st.write("Outside the form")

Expected behavior:

No bug

Actual behavior:

The following error is raised:

Missing Submit Button

This form has no submit button, which means that user interactions will never be sent to your Streamlit app.

To create a submit button, use the st.form_submit_button() function.

For more information, refer to the [documentation for forms](https://docs.streamlit.io/library/api-reference/control-flow/st.form).

Is this a regression?

That is, did this use to work the way you expected in the past?
No

Debug info

• Streamlit version: (get it with $ streamlit version) 1.7.0
• Python version: (get it with $ python --version) 3.6.0
• Using Conda? PipEnv? PyEnv? Pex? Conda
• OS version: macOS 12.2.1
• Browser version: Firefox 98.0.1

[mayagbarnes]

Hi @H4dr1en! 😃
Thanks for your submission! The following code snippet, which switches the expander to be the outermost element with the form inside, should address your error. Please let me know if you still encounter the same problem!

with st.expander("test expander"):
    with st.form("my_form"):
        st.write("Inside the form")
        slider_val = st.slider("Form slider")
        checkbox_val = st.checkbox("Form checkbox")

        # Every form must have a submit button.
        submitted = st.form_submit_button("Submit")
    if submitted:
        st.write("slider", slider_val, "checkbox", checkbox_val)

st.write("Outside the form")

[H4dr1en]

Hi @mayagbarnes , yes that way it works, but it's not fitting my use case: I have many options inside a form and I would like to hide some of them using an expander, so switching the expander and the form doesn't some my issue

[mayagbarnes]

@H4dr1en appreciate the clarification on the issue / use case!
In that case, I would ask if you need the submit button to be within the expander (like in your original code snippet) given that it is being used for additional options. If not, the changed indentation of code below should resolve the error & I believe meet your use case:

import streamlit as st

with st.form("my_form"):
    st.write("Inside the form")
    with st.expander("test expander"):
        slider_val = st.slider("Form slider")
        checkbox_val = st.checkbox("Form checkbox")

    # Every form must have a submit button.
    submitted = st.form_submit_button("Submit")
    if submitted:
        st.write("slider", slider_val, "checkbox", checkbox_val)

st.write("Outside the form")

[H4dr1en]

Yes, that would work!