st.toast alerts without rerun duplicates: choosing persistence and dedup strategy

Implementing Condition-Based Toast Alerts

To fire an st.toast exactly once per condition occurrence rather than once per session, you must track the state of the trigger rather than the state of the notification. Instead of a simple boolean flag (e.g., toast_shown = True), store a unique identifier or a version timestamp of the event in st.session_state.

The Sentinel Pattern for Deduplication

By comparing the current event ID against the last recorded ID in the session state, you ensure the toast only triggers when the condition actually changes, allowing it to fire again if the condition clears and recurs.

import streamlit as st

# Assume 'data_version' is a value that changes when a refresh completes
current_version = get_data_version()

if "last_toast_version" not in st.session_state:
    st.session_state.last_toast_version = None

if current_version != st.session_state.last_toast_version:
    st.toast(f"Data refreshed to version {current_version}")
    st.session_state.last_toast_version = current_version

Batching and Stacking

When multiple events occur in a single rerun, batching is the recommended approach. Streamlit's st.toast implementation stacks notifications, but rapid-fire toasts can create visual noise and may be dismissed by the user before they are read.

• Batching: Collect all events into a list during the script execution and trigger a single summary toast (e.g., "3 background tasks completed").
• Stacking: Use only for distinct, high-importance events. If more than 3-4 toasts fire simultaneously, the UI becomes cluttered, and the transient nature of toasts makes them unreliable for critical itemized lists.

Choosing Between st.toast and Persistent Elements

The decision to switch from st.toast to st.warning or st.error depends on the required user acknowledgment and criticality.

Feature	st.toast	st.warning / st.error
Persistence	Transient (disappears)	Persistent (inline)
Interruption	Non-blocking	Visual anchor in layout
Use Case	Status updates, confirmations	Action required, critical failures

Switch to persistent elements when the alert contains information the user must see to proceed or when the alert describes a state that requires manual intervention to resolve.

Verification Note

This pattern assumes a recent version of Streamlit where st.toast is available. To verify, initialize your session state key, trigger the condition, and interact with any other widget; the toast should not reappear until the version/ID changes.