Session State in Streamlit

When you build apps with Streamlit, the script reruns from top to bottom every time a user interacts (clicks a button, changes input, etc.). Without state management, variables reset on each rerun.

👉 Session State solves this by storing data per user session.

What is Session State?

Session State is a dictionary-like storage that persists values across reruns for a single user.

Think of it as:

✔ Memory for each user session
✔ Stores variables between interactions
✔ Enables multi-step workflows

Why Session State is Needed

❌ Without Session State

Python

import streamlit as st

count = 0

if st.button("Increase"):
    count += 1

st.write(count)

👉 Output always shows 0 because the script reruns.

✅ With Session State

Python

import streamlit as st

# Initialize session variable
if "count" not in st.session_state:
    st.session_state.count = 0

# Update state
if st.button("Increase"):
    st.session_state.count += 1

st.write("Count:", st.session_state.count)

✔ Count persists
✔ Value increases correctly

How Session State Works

Internally:

Each user gets a unique session
Streamlit stores values in memory
State survives script reruns
State resets when session ends (refresh or browser close)

Basic Syntax

Initialize

if "key" not in st.session_state:
    st.session_state.key = value

Read

st.session_state.key

Update

st.session_state.key = new_value

Delete

# Delete a single key in session state
del st.session_state.key

# Delete all items in session state
for key in st.session_state.keys():
    del st.session_state[key]

Example: User Name Persistence

Python

import streamlit as st

# Initialize
if "name" not in st.session_state:
    st.session_state.name = ""

# Input
name_input = st.text_input("Enter your name")

# Save to session
if st.button("Save"):
    st.session_state.name = name_input

st.write("Saved Name:", st.session_state.name)

✔ What Happens

User enters name
Click Save → value stored
Value remains after rerun

Callback Functions with Session State

Callbacks update state before rerun.

Python

import streamlit as st

def update_count():
    st.session_state.count += 1

if "count" not in st.session_state:
    st.session_state.count = 0

st.button("Increase", on_click=update_count)

st.write(st.session_state.count)