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.