Error handler decorators in Flask - Mini Project: Build & Apply

Error Handler Decorators in Flask

📖 Scenario: You are building a simple Flask web app that needs to handle errors gracefully. Instead of showing default error pages, you want to create custom error messages for common HTTP errors like 404 (Not Found) and 500 (Internal Server Error).

🎯 Goal: Build a Flask app that uses error handler decorators to catch 404 and 500 errors and return custom messages.

📋 What You'll Learn

Create a Flask app instance named app

Use the @app.errorhandler(404) decorator to handle 404 errors

Use the @app.errorhandler(500) decorator to handle 500 errors

Return a simple string message for each error handler function

💡 Why This Matters

🌍 Real World

Web apps need to handle errors gracefully to improve user experience and provide helpful feedback when something goes wrong.

💼 Career

Knowing how to use Flask error handler decorators is important for backend web developers to create robust and user-friendly applications.

1

Create the Flask app instance

Import Flask from flask and create a Flask app instance called app.

Flask

# Import Flask and create app instance
# Your code here

Hint

Use Flask(__name__) to create the app instance.

2

Add a 404 error handler decorator

Use the @app.errorhandler(404) decorator to create a function called handle_404 that returns the string 'Page not found'.

Flask

from flask import Flask
app = Flask(__name__)

# Add 404 error handler decorator and function
# Your code here

Hint

The function must accept one parameter, usually named error.

3

Add a 500 error handler decorator

Use the @app.errorhandler(500) decorator to create a function called handle_500 that returns the string 'Internal server error'.

Flask

from flask import Flask
app = Flask(__name__)

@app.errorhandler(404)
def handle_404(error):
    return 'Page not found'

# Add 500 error handler decorator and function
# Your code here

Hint

Like the 404 handler, the function must accept one parameter error.

4

Add a simple route and run the app

Add a route for '/' that returns 'Welcome to the homepage'. Then add the if __name__ == '__main__' block to run the app with app.run().

Flask

from flask import Flask
app = Flask(__name__)

@app.errorhandler(404)
def handle_404(error):
    return 'Page not found'

@app.errorhandler(500)
def handle_500(error):
    return 'Internal server error'

# Add a route for '/' and run the app
# Your code here

Hint

The route function should be named home and return the welcome string.

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