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.

Practice

1. What is the main purpose of using @app.errorhandler in a Flask route?

easy

A. To connect to a database automatically

B. To define a new route in the Flask app

C. To catch specific errors and return custom responses

D. To start the Flask application server

Solution

1. Step 1: Understand the role of @app.errorhandler

   This decorator is used to catch exceptions or errors that happen during request handling.

2. Step 2: Purpose in routes

   It allows you to return a custom message or page instead of a default error, improving user experience.

3. Final Answer:

   To catch specific errors and return custom responses -> Option C

4. Quick Check:

   Error handling = catch and respond [OK]

Hint: Errorhandler decorates functions to catch errors in routes [OK]

Common Mistakes:

• Confusing errorhandler with route decorator
• Thinking it starts the server
• Assuming it connects to databases

2. Which of the following is the correct syntax to handle a 404 error in Flask?

easy

A. @app.errorhandler(404)
def not_found(error):
return 'Page not found', 404

B. @app.route(404)
def not_found():
return 'Page not found'

C. @app.errorhandler('404')
def not_found(error):
return 'Page not found', 404

D. @app.errorhandler(404)
def not_found():
return 'Page not found'

Solution

1. Step 1: Check the decorator usage

   The correct decorator is @app.errorhandler(404) with the status code as an integer.

2. Step 2: Verify function signature and return

   The handler function should accept an error argument and return a response with the status code.

3. Final Answer:

   @app.errorhandler(404)\ndef not_found(error):\n return 'Page not found', 404 -> Option A

4. Quick Check:

   Correct decorator and function signature = A [OK]

Hint: Use integer status code and error parameter in handler [OK]

Common Mistakes:

• Using @app.route instead of @app.errorhandler
• Passing status code as string '404'
• Missing error parameter in handler function

3. Given this Flask code snippet, what will be the output when accessing /divide/10/0?

from flask import Flask
app = Flask(__name__)

@app.route('/divide/<int:a>/<int:b>')
def divide(a, b):
    try:
        result = a / b
        return str(result)
    except ZeroDivisionError:
        return 'Cannot divide by zero', 400

medium

A. ZeroDivisionError exception traceback

B. 0

C. 10

D. Cannot divide by zero with HTTP status 400

Solution

1. Step 1: Analyze the route and input

   The route divides a by b. Input is 10 and 0, so division by zero occurs.

2. Step 2: Check exception handling

   The ZeroDivisionError is caught, returning 'Cannot divide by zero' with status 400.

3. Final Answer:

   Cannot divide by zero with HTTP status 400 -> Option D

4. Quick Check:

   ZeroDivisionError caught returns message and 400 [OK]

Hint: Try-except catches zero division, returns error message [OK]

Common Mistakes:

• Expecting a crash or traceback
• Ignoring the exception handling block
• Assuming division returns 0

4. Identify the error in this Flask route error handling code:

@app.errorhandler(500)
def internal_error():
    return 'Internal Server Error', 500

medium

A. The handler function must accept an error parameter

B. The status code 500 is invalid for errorhandler

C. The return statement should not include status code

D. The decorator should be @app.route, not @app.errorhandler

Solution

1. Step 1: Check function signature for errorhandler

   Functions decorated with @app.errorhandler must accept one parameter for the error.

2. Step 2: Verify other parts

   Status code 500 is valid, return with status code is correct, and decorator usage is correct.

3. Final Answer:

   The handler function must accept an error parameter -> Option A

4. Quick Check:

   Errorhandler function needs error argument [OK]

Hint: Errorhandler functions always take an error argument [OK]

Common Mistakes:

• Omitting the error parameter in handler function
• Using wrong decorator for error handling
• Returning without status code

5. You want to handle both 404 and 500 errors in your Flask app with custom JSON responses. Which approach correctly implements this?

hard

A. Use one function with @app.errorhandler([404, 500]) to handle both errors

B. Define two separate functions with @app.errorhandler(404) and @app.errorhandler(500), returning JSON and status codes

C. Handle errors inside each route function with try-except blocks only

D. Set app.config['ERROR_404_JSON'] = True and Flask will auto-handle JSON errors

Solution

1. Step 1: Understand Flask errorhandler usage

   Flask requires separate functions for different error codes with their own decorators.

2. Step 2: Confirm JSON response and status code

   Each handler should return a JSON response and the correct HTTP status code for clarity.

3. Final Answer:

   Define two separate functions with @app.errorhandler(404) and @app.errorhandler(500), returning JSON and status codes -> Option B

4. Quick Check:

   Separate handlers per error code = A [OK]

Hint: Use separate errorhandler functions for each error code [OK]

Common Mistakes:

• Trying to handle multiple errors in one function decorator
• Relying only on try-except inside routes
• Assuming Flask auto-converts errors to JSON