Flash Messages

Flash messages are one-time messages stored in the user’s session.

They’re commonly used after:

• successful form submission
• login/logout
• errors like “invalid password”

diagram a flash message lives in the session until something reads it mermaid

  flowchart LR
  A["POST /subscribe"] --> B["flash('Subscribed!', 'success')"]
  B --> C[("session cookie")]
  C --> D["redirect to /"]
  D --> E["GET /"]
  E --> F["get_flashed_messages() in the template"]
  F --> G["message rendered"]
  F --> H["and removed from the session"]
  H --> I["a reload shows nothing -- it was consumed"]

flash writes into the session, so the message survives the redirect that follows it. The template reads it on the next request and the read CONSUMES it -- which is why the message appears exactly once and is gone on reload.

Consumed on read, verified

Flashed messages are removed the moment they are read. With Flask 3.1.3:

text

GET /set   -> flash("hello", "success")
GET /show  -> [('success', 'hello')]
GET /show  -> []

The second read returns an empty list. That is the behaviour you want — a message shown once, not one that reappears on every page until it is manually cleared — but it means calling get_flashed_messages() in your view and in the template will make the message vanish before the template runs. Read it in exactly one place, normally the base layout.

Flashing a message

python

from flask import flash, redirect, url_for
 
@app.route("/subscribe", methods=["POST"])
def subscribe():
    # ...do work...
    flash("Subscribed successfully!", "success")
    return redirect(url_for("home"))

The second argument ("success") is a category.

Display flash messages in templates

In your base template:

html

{% with messages = get_flashed_messages(with_categories=true) %}
  {% if messages %}
    <ul>
      {% for category, message in messages %}
        <li class="flash {{ category }}">{{ message }}</li>
      {% endfor %}
    </ul>
  {% endif %}
{% endwith %}

Requirements

Flashing uses sessions, so you need SECRET_KEY configured.

Best practice

Use flash + redirect (PRG pattern) so the message shows on the GET page.