Demonstrate 3 different ways to run operations with form values in Streamlit

form-callback-demo.py

import streamlit as st

st.header('Experiments with forms')

st.markdown('There is no connection between the forms here. The purpose is to demonstrate 3 different ways to run operations with form values.')

state = st.session_state
if 'word_choices' not in state:
    state.word_choices = []

en_words = ['streamlit', 'has', 'you', 'covered', 'and', 'just', 'like', 'guinness', 'is', 'good', 'for', 'you']
es_words = ['streamlit', 'tiene', 'usted', 'cubierto', 'y', 'solo', 'como', 'guinness', 'es', 'bueno', 'para', 'usted']

lang = st.radio('Choose language', ['English', 'Spanish'])
words = []
if lang == 'English':
    words = en_words
elif lang == 'Spanish':
    words = es_words

word_choices = []

st.write('---')
st.markdown('## FORM 1')
st.markdown('#### This form simply submits and the external function uses values from the execution scope')

with st.form(key='words_form_1', clear_on_submit=True):
    word_choices = st.multiselect(f'Choose some word options in {lang}', options=words)
    st.form_submit_button('Apply')

def expensive_operation():
    st.markdown(f'''
        ### `expensive_operation` on words choices

        * Assuming this is an expensive operation that should only occur when
          the word choices are finalized.
        * The operation is declared AFTER the form, so is called with updated 
          form values after the form is submitted.
        * If the operation is called INSIDE the form it must be declared BEFORE the form.
        * This list is held in session state.

        {word_choices}
    ''')
expensive_operation()

st.write('---')
st.markdown('## FORM 2')
st.markdown('#### This form submits and calls the external function directly, in the form itself, with form values')

def another_expensive_operation(word_choices):
    st.markdown(f'''
        ### `another_expensive_operation` on words choices

        * Assuming this is an expensive operation that should only occur when
          the word choices are finalized.
        * The operation is declared BEFORE the form declaration, so is called with updated 
          form values within the form when it is submitted.
        * If the operation is called OUTSIDE the form it must be declared AFTER the form.
        * If the operation is declared BEFORE the form, then the form submit must issue a `rerun`
          after form values have been stored in session state AND the operation should use session
          state to retrieve these values, OR
        * This list is held in session state.

        {word_choices}
    ''')

with st.form(key='words_form_2', clear_on_submit=True):
    word_choices = st.multiselect(f'Choose some word options in {lang}', options=words)
    if st.form_submit_button('Apply'):
        another_expensive_operation(word_choices)


st.write('---')
st.markdown('## FORM 3')
st.markdown('#### This form saves and reruns on submit so external functions above and below can run with state values')

def yet_another_expensive_operation():
    st.markdown(f'''
        ### `yet_another_expensive_operation` on words choices

        * Assuming this is an expensive operation that should only occur when
          the word choices are finalized.
        * The operation is declared BEFORE the form declaration, so is called with updated 
          form values within the form when it is submitted.
        * If the operation is called OUTSIDE the form it must be declared AFTER the form.
        * If the operation is declared BEFORE the form, then the form submit must issue a `rerun`
          after form values have been stored in session state AND the operation should use session
          state to retrieve these values, OR
        * This list is held in session state.

        {state.word_choices}
    ''')
yet_another_expensive_operation()

with st.form(key='words_form_3', clear_on_submit=True):
    word_choices = st.multiselect(f'Choose some word options in {lang}', options=words)
    if st.form_submit_button('Apply'):
        state.word_choices = word_choices
        st.experimental_rerun()