# How to add error handling to a Flask app

TIP

List of all code changes made in this lecture: https://diff-store.com/diff/section12__10_add_error_handling_to_flask_app (opens new window)

Adding error handling to a Flask app is fairly straightforward when we use the @app.errorhandler decorator.

Let's add this to our __init__.py:

@app.errorhandler(404)
def page_not_found(error):
    return render_template("404.html"), 404

This function is like a normal Flask route, but it only triggers when we raise an exception of type HTTPException with a response code of 404.

If we define an errorhandler(500), then that will also trigger when there is an uncaught exception in our code.

Flask provides the abort() function, which is a shorthand for raising an HTTPException with a given response code. Therefore, abort(404) will trigger this error handler.

Now let's create the 404.html template so that we can show our users a nicer error page:

<!DOCTYPE html>
<html lang="en">

<head>
    <title>Bob Smith | Portfolio</title>
    <link rel="preconnect" href="https://fonts.googleapis.com">
    <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
    <link href="https://fonts.googleapis.com/css2?family=Lato:wght@400;700&display=swap" rel="stylesheet">
    <link rel="stylesheet" href="{{ url_for('static', filename='css/styles.css') }}" />
</head>

<body>
    <main class="main main--error">
        <h1>404 - Page Not Found</h1>
        <p>
            Sorry, we couldn't find the page you're looking for.
            It may have been mis-spelled, or it doesn't exist anymore.
        </p>
        <a href="{{ url_for('home') }}">Go to home</a>
    </main>
</body>

</html>

Notice I'm not extending base.html here because I don't want the navbar showing up in the error page, but feel free to change this!

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