app.py

import streamlit as st

# query params exist
try:
    options = ['cat', 'dog', 'mouse', 'bat', 'duck']

    query_params = st.experimental_get_query_params()
    query_option = query_params['option'][0] #throws an exception when visiting http://host:port

    option_selected = st.sidebar.selectbox('Pick option',
                                            options,
                                            index=options.index(query_option))
    if option_selected:
        st.experimental_set_query_params(option=option_selected)

# run when query params don't exist. e.g on first launch
except: # catch exception and set query param to predefined value
    options = ['cat', 'dog', 'mouse', 'bat', 'duck']
    st.experimental_set_query_params(option=options[1]) # defaults to dog

    query_params = st.experimental_get_query_params()
    query_option = query_params['option'][0]

    option_selected = st.sidebar.selectbox('Pick option',
                                            options,
                                            index=options.index(query_option))
    if option_selected:
        st.experimental_set_query_params(option=option_selected)