Flash Messaging System & User Feedback UI

Beginner12 min read•Updated: Jul 11, 2026

Flash Messaging System & User Feedback UI

Web applications frequently redirect users following state-changing operations (such as login, registration, or profile updates) according to the Post/Redirect/Get (PRG) pattern. Because HTTP is stateless, transmitting feedback messages across redirects requires session storage.

Flask provides a lightweight, built-in mechanism called Flashing to record a message in the session and consume it on the next rendered page.

1. How Flask Flash Messaging Works

1
In your Python route, call flash(message, category):

Flask cryptographically signs and stores the message in the client's session cookie.

1
The user is redirected (return redirect(...)).
2
In the destination template, call get_flashed_messages():

The messages are extracted, rendered on the page, and immediately purged from the session.

1
If the user refreshes the page, the messages are gone!

Output

1[ POST /profile ] ---> flash("Profile updated!", "success") ---> 302 Redirect
2                                                                      |
3                                                                      v
4[ GET /dashboard ] <-- get_flashed_messages() renders alert <--- (Cookie Cleared)

2. Flashing Messages in Route Handlers

Python

1from flask import Flask, flash, redirect, url_for, render_template
2 
3app = Flask(__name__)
4app.config["SECRET_KEY"] = "super-secret-session-key"
5 
6@app.route("/update-settings", methods=["POST"])
7def update_settings():
8    # Perform business logic...
9    success = True
10    
11    if success:
12        # Category allows styling (success, danger, warning, info)
13        flash("Your profile settings were saved successfully!", "success")
14        flash("An email confirmation has been dispatched.", "info")
15    else:
16        flash("Could not update settings. Please verify your inputs.", "danger")
17        
18    return redirect(url_for("dashboard"))

3. Rendering Flashed Messages in Jinja2

Use get_flashed_messages(with_categories=True) in your master base.html layout so messages render consistently across every page on your site:

HTML

1<!-- templates/base.html -->
2<main class="container mx-auto px-4 py-6">
3 
  <!-- Flash Message Notification Container -->
  {% with messages = get_flashed_messages(with_categories=true) %}
      {% if messages %}
          <div class="space-y-3 mb-6">
          {% for category, message in messages %}
              <div class="p-4 rounded-xl border flex items-center justify-between 
                  {% if category == 'success' %}
                      bg-emerald-50 border-emerald-200 text-emerald-800
                  {% elif category == 'danger' or category == 'error' %}
                      bg-red-50 border-red-200 text-red-800
                  {% elif category == 'warning' %}
                      bg-amber-50 border-amber-200 text-amber-800
                  {% else %}
                      bg-blue-50 border-blue-200 text-blue-800
                  {% endif %}
              ">
                  <span class="text-sm font-medium">{{ message }}</span>
                  <button onclick="this.parentElement.remove()" class="text-xs font-bold opacity-60 hover:opacity-100">
                      &times;
                  </button>
              </div>
          {% endfor %}
          </div>
      {% endif %}
  {% endwith %}
29 
  {% block content %}{% endblock %}
31</main>

4. Filtering Messages by Category

In specific modular templates, you may want to extract only messages of a single category:

HTML

1<!-- Extract only error messages -->
2{% for message in get_flashed_messages(category_filter=["danger", "error"]) %}
3    <div class="alert-box error">{{ message }}</div>
4{% endfor %}

Interactive Quiz• Knowledge Check

Practice Quiz

Question 1

Where does Flask store flashed messages between the initial request and the subsequent redirect?

Question 2

What parameter must be passed to get_flashed_messages() to receive tuples of (category, message)?

Question 3

What happens to flashed messages after they are read by get_flashed_messages() during page rendering?

Question 4

Why is the Post/Redirect/Get (PRG) pattern combined with flash messaging considered a web design best practice?

Question 5

How do you filter flashed messages to display only "success" alerts in a specific template?

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