Register errorhandler Functions
Customize responses for 404 and 500.
Register errorhandler Functions is a free Flask Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Flask Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Beyond the Default Page
Flask shows a plain error page out of the box. You can replace it by registering your own errorhandler for any status code.
The errorhandler Decorator
You attach a function to a code with a decorator. This handler runs whenever that error happens anywhere in your app.
@app.errorhandler(404)
def not_found(e):
return "Nothing here", 404Return a Tuple
A handler returns a body and a status code, just like a view. The status code keeps the response honest about what failed.
return "Not found", 404The Error Argument
Each handler receives the error object as its argument. You can read its description or code to shape your reply.
def not_found(e):
return str(e.description), 404Handling 500 Errors
Unexpected crashes become a 500. Register a handler so users see a calm message instead of a raw stack trace.
@app.errorhandler(500)
def boom(e):
return "Server error", 500Render a Template Instead
Most apps return a styled page on error. Just call render_template in the handler and pass the status code along.
return render_template("404.html"), 404Return JSON for APIs
An API should answer errors in JSON, not HTML. Use jsonify so clients can parse the failure cleanly.
return jsonify(error="Not found"), 404Handlers Catch abort Too
Calling abort with a code triggers the matching handler. So one errorhandler covers both crashes and your own aborts.
Handle Exception Classes
You can register a handler by exception type, not just by number. Pass an exception class to catch a whole family at once.
@app.errorhandler(ValueError)
def bad(e):
return "Bad value", 400App-Wide vs Blueprint
Handlers on app apply everywhere, while a blueprint can scope its own. This lets each part of a big app fail its own way.
Why Custom Handlers Help
Consistent error pages keep your brand and your API tidy. A central handler means you fix the message in one place.
Quick Check
Spot the decorator that customizes a missing-page response.
Recap
You can now register errorhandler functions, read the error object, and return HTML or JSON with the right code. Great progress!
Frequently asked questions
Is the “Register errorhandler Functions” lesson free?
Yes — the full text of “Register errorhandler Functions” is free to read here on the web, and the Flask Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Flask Academy course, upgrade to CoddyKit PRO.
What will I learn in “Register errorhandler Functions”?
Customize responses for 404 and 500. You practise Flask Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Flask Academy?
No prior experience is required. Flask Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Register errorhandler Functions” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Flask Academy lesson?
Yes. Every Flask Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- abort and HTTP Error Codes
- Register errorhandler Functions
- Raise Custom Exception Classes
- Uniform JSON Error Envelopes