Error handler decorators in Flask - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create an error handler for 404 errors in Flask.

Flask

from flask import Flask, render_template
app = Flask(__name__)

@app.errorhandler([1])
def not_found_error(error):
    return render_template('404.html'), 404

Drag options to blanks, or click blank then click option'

A500

B404

C403

D400

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong error code like 500 or 403.

Forgetting to use the @app.errorhandler decorator.

The @app.errorhandler(404) decorator tells Flask to use this function when a 404 Not Found error occurs.

2fill in blank

medium

Complete the code to handle 500 Internal Server Error using a decorator.

Flask

from flask import Flask, render_template
app = Flask(__name__)

@app.errorhandler([1])
def internal_error(error):
    return render_template('500.html'), 500

Drag options to blanks, or click blank then click option'

A403

B404

C500

D400

Attempts:

3 left

💡 Hint

Common Mistakes

Using 404 instead of 500.

Not returning the correct status code with the response.

The @app.errorhandler(500) decorator is used to catch server errors and display a custom page.

3fill in blank

hard

Fix the error in the code to correctly register a handler for 403 Forbidden errors.

Flask

from flask import Flask, render_template
app = Flask(__name__)

@app.errorhandler([1])
def forbidden_error(error):
    return render_template('403.html'), 403

Drag options to blanks, or click blank then click option'

A403

Berror403

CForbidden

D'403'

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around the number.

Using a string or variable name instead of the code.

The error code must be an integer, not a string or a name. So 403 is correct.

4fill in blank

hard

Fill both blanks to create a handler for 400 Bad Request errors that returns a JSON response.

Flask

from flask import Flask, jsonify
app = Flask(__name__)

@app.errorhandler([1])
def bad_request(error):
    response = jsonify({'error': 'Bad Request'})
    response.status_code = [2]
    return response

Drag options to blanks, or click blank then click option'

A400

B404

C500

D403

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatching the decorator code and status code.

Using codes for other errors like 404 or 500.

Both the decorator and the status code must be 400 to handle Bad Request errors properly.

5fill in blank

hard

Fill both blanks to create a custom error handler for 401 Unauthorized errors that returns a JSON message.

Flask

from flask import Flask, jsonify
app = Flask(__name__)

@app.errorhandler([1])
def unauthorized(error):
    response = jsonify({'message': 'Unauthorized access'})
    response.status_code = [2]
    return response

Drag options to blanks, or click blank then click option'

A401

B403

D500

Attempts:

3 left

💡 Hint

Common Mistakes

Using 403 Forbidden instead of 401.

Mismatching decorator and status code.

The error handler and status code must be 401 for Unauthorized errors. The JSON message explains the error.

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