Default error pages are fine for development, but real apps should have friendly error pages.

Flask lets you register error handlers.

Basic error handlers

from flask import Flask
 
app = Flask(__name__)
 
 
@app.errorhandler(404)
def not_found(error):
    return "Page not found", 404
 
 
@app.errorhandler(500)
def server_error(error):
    return "Something went wrong", 500

from flask import Flask
 
app = Flask(__name__)
 
 
@app.errorhandler(404)
def not_found(error):
    return "Page not found", 404
 
 
@app.errorhandler(500)
def server_error(error):
    return "Something went wrong", 500

Using templates (recommended)

Once you start using templates (Phase 3), you can do:

from flask import render_template
 
@app.errorhandler(404)
def not_found(error):
    return render_template("errors/404.html"), 404

from flask import render_template
 
@app.errorhandler(404)
def not_found(error):
    return render_template("errors/404.html"), 404

Typical template structure:

templates/
  errors/
    404.html
    500.html

templates/
  errors/
    404.html
    500.html

Important: 500 handler and debugging

In debug mode, Flask shows an interactive debugger instead of your custom 500 handler.

That’s normal.

To see production-like behavior, disable debug.

Bonus: handling exceptions consistently

As your app grows, you’ll likely:

• create custom exception types
• return JSON errors for APIs
• return HTML pages for browser routes

That’s a key reason to keep API routes and page routes organized (often via Blueprints).

🧪 Try It Yourself

Exercise 1 – Create a Flask App

Exercise 2 – Dynamic Route

Exercise 3 – Return JSON