Exception handling in routes in Flask

Introduction

Exception handling in routes helps your web app stay friendly and stable when something goes wrong. It catches errors and shows helpful messages instead of crashing.

When a user requests a page that does not exist.

When a database query fails inside a route.

When input data from a form is invalid or missing.

When an external service your route depends on is down.

When you want to log errors and show a custom error page.

Syntax

Flask

from flask import Flask, jsonify

app = Flask(__name__)

@app.errorhandler(ExceptionType)
def handle_exception(e):
    return jsonify({'error': 'message'}), status_code

Replace ExceptionType with the specific error you want to catch, like 404 or ValueError.

The handler function returns a response with a message and an HTTP status code.

Examples

This catches 404 errors and returns a JSON message with status 404.

Flask

from flask import Flask, jsonify

app = Flask(__name__)

@app.errorhandler(404)
def not_found(e):
    return jsonify({'error': 'Page not found'}), 404

This catches ValueError exceptions and returns the error message with status 400.

Flask

from flask import Flask, jsonify

app = Flask(__name__)

@app.errorhandler(ValueError)
def handle_value_error(e):
    return jsonify({'error': str(e)}), 400

This catches all other exceptions and returns a generic error message with status 500.

Flask

from flask import Flask, jsonify

app = Flask(__name__)

@app.errorhandler(Exception)
def handle_all_errors(e):
    return jsonify({'error': 'Something went wrong'}), 500

Sample Program

This Flask app has a route /divide that divides two numbers from query parameters. It handles division by zero and invalid inputs with clear error messages. It also handles 404 errors with a JSON message.

Flask

from flask import Flask, jsonify, request

app = Flask(__name__)

@app.route('/divide')
def divide():
    try:
        x = float(request.args.get('x', ''))
        y = float(request.args.get('y', ''))
        result = x / y
        return jsonify({'result': result})
    except ZeroDivisionError:
        return jsonify({'error': 'Cannot divide by zero'}), 400
    except (ValueError, TypeError):
        return jsonify({'error': 'Invalid input, please provide numbers'}), 400

@app.errorhandler(404)
def not_found(e):
    return jsonify({'error': 'Route not found'}), 404

if __name__ == '__main__':
    app.run(debug=True)

Important Notes

Use specific exception handlers to give clear feedback to users.

Always return proper HTTP status codes with error messages.

Use @app.errorhandler to catch errors globally for your app.

Summary

Exception handling in routes keeps your app stable and user-friendly.

Use @app.errorhandler to catch and respond to errors.

Return clear messages and correct HTTP status codes for errors.