using-streamlit-session-state

name	using-streamlit-session-state
description	Using st.session_state to manage state across Streamlit reruns. Use when persisting data, handling widget state, implementing callbacks, or debugging state issues. Covers initialization patterns, widget-state association, and common gotchas.
license	Apache-2.0

Using Streamlit session state

Streamlit reruns scripts top-to-bottom on every interaction. Without session state, variables reset each time. Use st.session_state to persist values across reruns.

Basic usage

Session state is a dictionary-like object supporting attribute and bracket notation:

# Initialize with setdefault (preferred)
st.session_state.setdefault("count", 0)

# Alternative: check before setting
if "count" not in st.session_state:
    st.session_state.count = 0

# Read
current = st.session_state.count

# Update
st.session_state.count += 1
st.session_state["count"] = 5  # Bracket notation also works

# Delete
del st.session_state.count

Accessing uninitialized keys raises KeyError. Use st.session_state.get("key", default) for safe access.

Widget-state association

Every widget with a key parameter automatically syncs to session state:

name = st.text_input("Name", key="user_name")
# st.session_state.user_name contains the same value as `name`

Callbacks

Callbacks execute before the script reruns, allowing immediate state changes. Use on_change or on_click with optional args and kwargs:

def increment(amount):
    st.session_state.count += amount

st.button("Add 5", on_click=increment, args=(5,))

Access a widget's value in its own callback via st.session_state.key, not the return variable.

Initialization patterns

Initialize all state at the top of your app for clarity:

st.session_state.setdefault("user", None)
st.session_state.setdefault("page", "home")
st.session_state.setdefault("filters", {})

Multipage state

Widgets are NOT stateful across pages. Their values reset when navigating between pages.

Sharing state

Use session state variables (not widget keys) to share data:

# Page 1: Store value
st.session_state.selected_user = st.selectbox("User", users)

# Page 2: Read stored value
if "selected_user" in st.session_state:
    st.write(f"Selected: {st.session_state.selected_user}")

Shared widgets pattern

Put common widgets in the entrypoint file (before nav.run()):

# app.py (entrypoint)
with st.sidebar:
    st.session_state.theme = st.selectbox("Theme", ["Light", "Dark"])

nav = st.navigation(pages)
nav.run()

Common mistakes

Module-level mutable state

# BAD: In imported modules, this is shared across ALL users
# utils.py
cache = {}  # Persists across reruns AND users!

# GOOD: Use session state for per-user data
st.session_state.setdefault("cache", {})

Modifying state after widget creation

Cannot assign to a widget's state after the widget has rendered:

st.slider("Value", key="my_slider")
st.session_state.my_slider = 50  # Raises StreamlitAPIException!

Mixing value parameter and session state

Don't set both—it causes warnings:

# BAD: Conflicting sources
st.session_state.setdefault("name", "Alice")
st.text_input("Name", value="Bob", key="name")  # Warning!

# GOOD: Use one or the other
st.session_state.setdefault("name", "Alice")
st.text_input("Name", key="name")

Session characteristics

• Per-user, per-tab: Each browser tab has its own session
• Temporary: Lost when tab closes or server restarts
• Not suitable for persistence: Use databases for permanent storage

References

• st.session_state API
• Session State concepts
• Widget behavior