dataprofessor / st.session_state

st.session_state

We define access to a Streamlit app in a browser tab as a session. For each browser tab that connects to the Streamlit server, a new session is created. Streamlit reruns your script from top to bottom every time you interact with your app. Each reruns takes place in a blank slate: no variables are shared between runs.

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.

In this tutorial, we will illustrate the usage of Session State and Callbacks as we build a weight conversion app.

st.session_state allows the implementation of session state in a Streamlit app.

Demo app

Code

Here's how to use st.session_state:

import streamlit as st

st.title('st.session_state')

def lbs_to_kg():
  st.session_state.kg = st.session_state.lbs/2.2046
def kg_to_lbs():
  st.session_state.lbs = st.session_state.kg*2.2046

st.header('Input')
col1, spacer, col2 = st.columns([2,1,2])
with col1:
  pounds = st.number_input("Pounds:", key = "lbs", on_change = lbs_to_kg)
with col2:
  kilogram = st.number_input("Kilograms:", key = "kg", on_change = kg_to_lbs)

st.header('Output')
st.write("st.session_state object:", st.session_state)

Line-by-line explanation

The very first thing to do when creating a Streamlit app is to start by importing the streamlit library as st like so:

import streamlit as st

Firstly, we'll start by creating the title of the app:

st.title('st.session_state')

Next, we define custom functions for the weight conversion from lbs to kg and vice versa:

def lbs_to_kg():
  st.session_state.kg = st.session_state.lbs/2.2046
def kg_to_lbs():
  st.session_state.lbs = st.session_state.kg*2.2046

Here, we use st.number_input to accept numerical inputs of the weight values:

st.header('Input')
col1, spacer, col2 = st.columns([2,1,2])
with col1:
  pounds = st.number_input("Pounds:", key = "lbs", on_change = lbs_to_kg)
with col2:
  kilogram = st.number_input("Kilograms:", key = "kg", on_change = kg_to_lbs)

The above 2 custom functions will be called upon as soon as a numerical value is entered into the number box created using the st.number_input command. Notice how the on_change option specifies the 2 custom functions lbs_to_kg and kg_to_lbs).

In a nutshell, upon entering a number into the st.number_input box the number is converted by these custom functions.

Finally, the weight values in kg and lbs units as stored in the session state as st.session_state.kg and st.session_state.lbs will be printed out via st.write:

st.header('Output')
st.write("st.session_state object:", st.session_state)

Further reading

• Session State
• Add statefulness to apps