Error handler decorators in Flask

Introduction

Error handler decorators help you catch and respond to errors in your web app in a simple way.

You want to show a friendly message when a user visits a page that does not exist.

You want to handle server errors and log them while showing a custom error page.

You want to return a specific response when a user sends a bad request.

You want to keep your app running smoothly by managing unexpected errors gracefully.

Syntax

Flask

@app.errorhandler(error_code_or_exception)
def function_name(error):
    # handle the error
    return response, status_code

Use the @app.errorhandler decorator with an error code like 404 or an exception class.

The decorated function receives the error object and returns a response and status code.

Examples

This handles 404 errors by returning a simple message and status code.

Flask

@app.errorhandler(404)
def page_not_found(error):
    return "Page not found", 404

This handles server errors with a custom message.

Flask

@app.errorhandler(500)
def server_error(error):
    return "Server error, try later", 500

This handles a specific exception type for bad requests.

Flask

from werkzeug.exceptions import BadRequest

@app.errorhandler(BadRequest)
def handle_bad_request(error):
    return "Bad request!", 400

Sample Program

This Flask app has two error handlers: one for 404 errors and one for 500 errors. When a user visits a page that does not exist, they see a friendly message. If the server has an error, they see a different message.

Flask

from flask import Flask

app = Flask(__name__)

@app.route('/')
def home():
    return "Welcome to the home page!"

@app.errorhandler(404)
def page_not_found(error):
    return "Sorry, this page does not exist.", 404

@app.errorhandler(500)
def server_error(error):
    return "Oops! Something went wrong on the server.", 500

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

Important Notes

Use error handler decorators to keep your app user-friendly and stable.

Remember to return both a message and the correct HTTP status code.

Testing your error handlers by visiting wrong URLs or causing errors helps ensure they work.

Summary

Error handler decorators catch errors and let you respond nicely.

Use @app.errorhandler with error codes or exceptions.

Return a message and status code to inform users clearly.

Practice

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

easy

A. To catch specific errors and return custom responses

B. To define routes for the application

C. To start the Flask server

D. To connect to a database

Solution

1. Step 1: Understand the role of error handlers

   Error handlers catch errors like 404 or 500 and let you respond with custom messages.

2. Step 2: Recognize the decorator's purpose

   @app.errorhandler is used to register functions that handle specific errors gracefully.

3. Final Answer:

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

4. Quick Check:

   Error handler = Custom error response [OK]

Hint: Error handlers catch errors and customize replies [OK]

Common Mistakes:

• Confusing errorhandler with route decorator
• Thinking it starts the server
• Assuming it manages database connections

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

easy

A. @app.errorhandler('404')
def handle_404(error):
return 'Not Found', 404

B. @app.errorhandler(404)
def handle_404(error):
return 'Not Found', 404

C. @app.route(404)
def handle_404():
return 'Not Found'

D. @app.errorhandler(HTTP_404)
def handle_404(error):
return 'Not Found', 404

Solution

1. Step 1: Identify correct decorator usage

   The @app.errorhandler decorator takes an integer error code like 404, not a string or undefined constant.

2. Step 2: Check function signature and return

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

3. Final Answer:

   @app.errorhandler(404) def handle_404(error): return 'Not Found', 404 -> Option B

4. Quick Check:

   Use integer error code with errorhandler [OK]

Hint: Use integer error code, not string or constants [OK]

Common Mistakes:

• Using @app.route instead of @app.errorhandler
• Passing error code as string '404'
• Using undefined constants like HTTP_404

3. What will be the output when a 500 error occurs with this code?

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

@app.route('/')
def index():
    raise Exception('Oops')

medium

A. Server Error Occurred with status code 500

B. Exception traceback shown in browser

C. 404 Not Found error page

D. Empty response with status code 200

Solution

1. Step 1: Understand error raising in route

   The index route raises a generic Exception, which triggers a 500 Internal Server Error.

2. Step 2: Check error handler for 500

   The @app.errorhandler(500) catches this error and returns the custom message with status 500.

3. Final Answer:

   Server Error Occurred with status code 500 -> Option A

4. Quick Check:

   500 error triggers custom handler message [OK]

Hint: Exception triggers 500 error handler message [OK]

Common Mistakes:

• Expecting default Flask error page instead of custom
• Confusing 404 with 500 error
• Assuming no response is returned

4. Identify the error in this Flask error handler code:

@app.errorhandler(403)
def forbidden():
    return 'Forbidden Access', 403

medium

A. Incorrect error code used in decorator

B. Return statement missing status code

C. Missing error parameter in handler function

D. Decorator should be @app.route instead

Solution

1. Step 1: Check function signature for error handlers

   Error handler functions must accept one parameter for the error object.

2. Step 2: Identify missing parameter

   The function forbidden has no parameters, so Flask will raise a TypeError.

3. Final Answer:

   Missing error parameter in handler function -> Option C

4. Quick Check:

   Error handler needs error argument [OK]

Hint: Error handler functions must accept error argument [OK]

Common Mistakes:

• Omitting the error parameter in handler function
• Using @app.route instead of @app.errorhandler
• Forgetting to return status code with response

5. You want to handle both 404 and 405 errors with the same function in Flask. Which code correctly achieves this?

hard

A. @app.errorhandler(404, 405)
def handle_errors(error):
return f'Error {error.code} occurred', error.code

B. @app.errorhandler([404, 405])
def handle_errors(error):
return f'Error {error.code} occurred', error.code

C. @app.errorhandler(404 or 405)
def handle_errors(error):
return f'Error {error.code} occurred', error.code

D. @app.errorhandler(404)
@app.errorhandler(405)
def handle_errors(error):
return f'Error {error.code} occurred', error.code

Solution

1. Step 1: Understand Flask errorhandler decorator usage

   Flask does not accept a list or multiple codes in one decorator call; you must stack decorators for multiple errors.

2. Step 2: Identify correct stacking syntax

   Using two @app.errorhandler decorators stacked on the same function handles both errors with one function.

3. Final Answer:

   @app.errorhandler(404) @app.errorhandler(405) def handle_errors(error): return f'Error {error.code} occurred', error.code -> Option D

4. Quick Check:

   Stack decorators for multiple error codes [OK]

Hint: Stack multiple @app.errorhandler decorators for several errors [OK]

Common Mistakes:

• Passing list or multiple codes in one decorator
• Using logical OR inside decorator argument
• Trying to pass multiple arguments to one decorator