Session State is a way to share variables between reruns, for each user session. In addition to the ability to store and persist state, Streamlit also exposes the ability to manipulate state using Callbacks.
Initialize values in Session State
The Session State API follows a field-based API, which is very similar to Python dictionaries:
# Initialization if 'key' not in st.session_state: st.session_state['key'] = 'value' # Session State also supports attribute based syntax if 'key' not in st.session_state: st.session_state.key = 'value'
Reads and updates
Read the value of an item in Session State by passing the item to
# Reads st.write(st.session_state.key) # Outputs: value
Update an item in Session State by assigning it a value:
st.session_state.key = 'value2' # Attribute API st.session_state['key'] = 'value2' # Dictionary like API
Curious about what is in Session State? Use
st.write or magic:
st.write(st.session_state) # With magic: st.session_state
Streamlit throws a handy exception if an uninitialized variable is accessed:
st.write(st.session_state['value']) # Throws an exception!
Delete items in Session State using the syntax to delete items in any Python dictionary:
# Delete a single key-value pair del st.session_state[key] # Delete all the items in Session state for key in st.session_state.keys(): del st.session_state[key]
Session State can also be cleared by going to Settings → Clear Cache, followed by Rerunning the app.
Session State and Widget State association
Every widget with a key is automatically added to Session State:
st.text_input("Your name", key="name") # This exists now: st.session_state.name
Use Callbacks to update Session State
A callback is a python function which gets called when an input widget changes.
Order of execution: When updating Session state in response to events, a callback function gets executed first, and then the app is executed from top to bottom.
Callbacks can be used with widgets using the parameters
- on_change or on_click - The function name to be used as a callback
- args (tuple) - List of arguments to be passed to the callback function
- kwargs (dict) - Named arguments to be passed to the callback function
Widgets which support the
Widgets which support the
To add a callback, define a callback function above the widget declaration and pass it to the widget via the
on_click ) parameter.
Forms and Callbacks
Widgets inside a form can have their values be accessed and set via the Session State API.
st.form_submit_button can have a callback associated with it. The callback gets executed upon clicking on the submit button. For example:
def form_callback(): st.write(st.session_state.my_slider) st.write(st.session_state.my_checkbox) with st.form(key='my_form'): slider_input = st.slider('My slider', 0, 10, 5, key='my_slider') checkbox_input = st.checkbox('Yes or No', key='my_checkbox') submit_button = st.form_submit_button(label='Submit', on_click=form_callback)
Caveats and limitations
st.form_submit_buttonhas a callback in forms. Other widgets inside a form are not allowed to have callbacks.
on_clickevents are only supported on input type widgets.
Modifying the value of a widget via the Session state API, after instantiating it, is not allowed and will raise a
StreamlitAPIException. For example:
slider = st.slider( label='My Slider', min_value=1, max_value=10, value=5, key='my_slider') st.session_state.my_slider = 7 # Throws an exception!
Setting the widget state via the Session State API and using the
valueparameter in the widget declaration is not recommended, and will throw a warning on the first run. For example:
st.session_state.my_slider = 7 slider = st.slider( label='Choose a Value', min_value=1, max_value=10, value=5, key='my_slider')
Setting the state of button-like widgets:
st.file_uploadervia the Session State API is not allowed. Such type of widgets are by default False and have ephemeral True states which are only valid for a single run. For example:
if 'my_button' not in st.session_state: st.session_state.my_button = True st.button('My button', key='my_button') # Throws an exception!