Error handler decorators in Flask - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of error handler decorators in Flask?

Error handler decorators in Flask let you define functions that run automatically when specific errors happen. They help you show friendly messages or handle problems smoothly.

beginner

How do you use the @app.errorhandler decorator in Flask?

You put @app.errorhandler(error_code) above a function. This function will run when that error happens. For example, @app.errorhandler(404) runs when a page is not found.

intermediate

What does the function decorated with @app.errorhandler receive as input?

The function receives the error object that caused the problem. You can use it to get details about the error and decide what to show the user.

intermediate

Can you handle multiple error codes with one error handler decorator in Flask?

No, you cannot handle multiple error codes with one @app.errorhandler by passing a tuple like (404, 500). You need to define separate functions for each error code or use base exception classes for broader coverage.

intermediate

What is the benefit of using error handler decorators instead of try-except blocks in Flask routes?

Error handler decorators keep your route code clean and separate error handling logic. They automatically catch errors app-wide, so you don't repeat try-except in every route.

What does @app.errorhandler(404) do in a Flask app?

A Defines a new route

B Runs a function when a page is not found

C Handles user login

D Runs a function when the server starts

The decorator @app.errorhandler(404) runs the decorated function when a 404 Not Found error occurs.

What argument does the function decorated with @app.errorhandler receive?

A The request object

B No arguments

C The error object

D The response object

The error handler function receives the error object that triggered the error.

Can one @app.errorhandler decorator handle multiple error codes?

A No, it handles one error code or exception type

B Yes, it can handle any number of errors

C Only if errors are related

D Only for HTTP errors

@app.errorhandler handles one error code or exception type per decorated function. To handle multiple errors, define multiple handlers.

Why use error handler decorators instead of try-except in routes?

A To avoid writing functions

B Because try-except is not allowed in Flask

C To speed up the app

D To keep routes clean and handle errors app-wide

Error handler decorators separate error logic and apply globally, keeping routes simpler.

Which of these is a correct way to define a 500 error handler in Flask?

A @app.errorhandler(500)\ndef server_error(error):\n return 'Server error', 500

B @app.route('/500')\ndef server_error():\n return 'Server error'

C @app.errorhandler('500')\ndef server_error(error):\n return 'Server error'

D @app.errorhandler(404)\ndef server_error(error):\n return 'Server error'

Option A correctly uses @app.errorhandler with integer 500 and defines a function with error argument.

Explain how to create a custom error page for a 404 error using Flask error handler decorators.

Think about how to catch the 404 error and show a friendly page.

Describe the advantages of using error handler decorators in a Flask web application.

Consider how error handling affects code organization and user feedback.

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