Flashing Messages

Flashing is Flask's built-in way to show a user a short, one-time message — such as "Login successful" or "Form has errors" — on the next page they visit, then automatically clear it so it never appears twice.

Learn Flashing Messages in our free Flask course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick reference.

Part of the free Flask course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

In this lesson you'll set a secret_key , call flash() to queue messages, and read them back with get_flashed_messages() to give users instant, friendly feedback.

A flash message is a one-time notification you store during one request and display on the next page the user loads. After it's shown once, Flask throws it away automatically.

This solves a classic problem: after a user submits a form you usually redirect them to a fresh page. A normal variable would be lost across that redirect — but a flashed message survives exactly one hop, making it perfect for "Saved!" or "Invalid password" banners.

Call flash("your message") inside a route to queue a notification. Then redirect to another page — the message waits in the session until that page reads it.

The example below flashes a confirmation after a fake "save", then redirects home. Start the server with flask run to see the banner appear once.

Notice we redirect with url_for("home") after flashing. This redirect-then-show pattern prevents the form from being resubmitted if the user refreshes.

get_flashed_messages() returns the pending messages and clears them. You usually call it in a Jinja template, but it works in routes too. Add a second argument to flash() to give each message a category like "success" or "error" .

Read categories back with get_flashed_messages(with_categories=True) , which returns pairs you can use to color the banner. Here's a typical Jinja loop you'd place in your base template:

Complete the app so it sets a secret key, flashes a success message, and reads it back. Replace each ___ .

❌ RuntimeError: The session is unavailable because no secret key was set

You called flash() without setting app.secret_key . Add a line like app.secret_key = "change-me" near where you create the app.

You're calling get_flashed_messages() more than once on the same page. Reading the messages clears them, so call it a single time — ideally in your base template only.

Build a route that checks a username and flashes a success or error message based on the result.

Lesson 15 complete — your app talks back to users!

You can set a secret_key , queue notifications with flash() , and display them once with get_flashed_messages() — even across redirects.

🚀 Up next: Building a REST API — move beyond HTML and serve structured data with proper HTTP verbs and status codes.

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