Exception handling in routes in Flask - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the Flask class.

Flask

from flask import [1]

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

Arequest

BFlask

Crender_template

Djsonify

Attempts:

3 left

💡 Hint

Common Mistakes

Importing request instead of Flask

Using jsonify which is for JSON responses

Importing render_template which is for HTML templates

The Flask class is imported from the flask module to create the app instance.

2fill in blank

medium

Complete the code to define a route that handles GET requests at '/' path.

Flask

@app.route('[1]', methods=['GET'])
def home():
    return 'Hello World!'

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

A/

B/home

C/index

D/main

Attempts:

3 left

💡 Hint

Common Mistakes

Using '/home' or '/index' which are valid but not root

Forgetting the leading slash

The root path '/' is used to define the main page route.

3fill in blank

hard

Fix the error in the code to catch exceptions in the route.

Flask

from flask import Flask
app = Flask(__name__)

@app.route('/data')
def data():
    try:
        result = 10 / [1]
        return str(result)
    except ZeroDivisionError:
        return 'Cannot divide by zero', 400

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

A''

B0

CNone

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 causes a ZeroDivisionError

Using None or empty string causes TypeError

Dividing by 5 is valid and won't raise an exception. Dividing by 0 causes ZeroDivisionError.

4fill in blank

hard

Fill both blanks to raise and catch a ValueError in the route.

Flask

@app.route('/check')
def check():
    try:
        if int('abc') [1] ValueError:
            raise ValueError('Invalid number')
    except [2] as e:
        return str(e), 400

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

A==

B!=

CValueError

DTypeError

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of '!=' in condition

Catching wrong exception type like TypeError

The code checks if conversion raises ValueError and catches it properly.

5fill in blank

hard

Fill all three blanks to handle a KeyError in a route and return JSON error message.

Flask

from flask import jsonify

@app.route('/item')
def item():
    data = {'name': 'apple'}
    try:
        value = data[[1]]
        return jsonify({{'value': value}})
    except [2] as e:
        return jsonify({{'error': str(e)}}), [3]

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

A'price'

BKeyError

C404

D'name'

Attempts:

3 left

💡 Hint

Common Mistakes

Using existing key 'name' so no error occurs

Catching wrong exception type

Returning wrong status code

Accessing missing key 'price' raises KeyError, caught and returns 404 with JSON error.

Practice

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

easy

A. To connect to a database automatically

B. To define a new route in the Flask app

C. To catch specific errors and return custom responses

D. To start the Flask application server

Solution

1. Step 1: Understand the role of @app.errorhandler

   This decorator is used to catch exceptions or errors that happen during request handling.

2. Step 2: Purpose in routes

   It allows you to return a custom message or page instead of a default error, improving user experience.

3. Final Answer:

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

4. Quick Check:

   Error handling = catch and respond [OK]

Hint: Errorhandler decorates functions to catch errors in routes [OK]

Common Mistakes:

• Confusing errorhandler with route decorator
• Thinking it starts the server
• Assuming it connects to databases

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

easy

A. @app.errorhandler(404)
def not_found(error):
return 'Page not found', 404

B. @app.route(404)
def not_found():
return 'Page not found'

C. @app.errorhandler('404')
def not_found(error):
return 'Page not found', 404

D. @app.errorhandler(404)
def not_found():
return 'Page not found'

Solution

1. Step 1: Check the decorator usage

   The correct decorator is @app.errorhandler(404) with the status code as an integer.

2. Step 2: Verify function signature and return

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

3. Final Answer:

   @app.errorhandler(404)\ndef not_found(error):\n return 'Page not found', 404 -> Option A

4. Quick Check:

   Correct decorator and function signature = A [OK]

Hint: Use integer status code and error parameter in handler [OK]

Common Mistakes:

• Using @app.route instead of @app.errorhandler
• Passing status code as string '404'
• Missing error parameter in handler function

3. Given this Flask code snippet, what will be the output when accessing /divide/10/0?

from flask import Flask
app = Flask(__name__)

@app.route('/divide/<int:a>/<int:b>')
def divide(a, b):
    try:
        result = a / b
        return str(result)
    except ZeroDivisionError:
        return 'Cannot divide by zero', 400

medium

A. ZeroDivisionError exception traceback

B. 0

C. 10

D. Cannot divide by zero with HTTP status 400

Solution

1. Step 1: Analyze the route and input

   The route divides a by b. Input is 10 and 0, so division by zero occurs.

2. Step 2: Check exception handling

   The ZeroDivisionError is caught, returning 'Cannot divide by zero' with status 400.

3. Final Answer:

   Cannot divide by zero with HTTP status 400 -> Option D

4. Quick Check:

   ZeroDivisionError caught returns message and 400 [OK]

Hint: Try-except catches zero division, returns error message [OK]

Common Mistakes:

• Expecting a crash or traceback
• Ignoring the exception handling block
• Assuming division returns 0

4. Identify the error in this Flask route error handling code:

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

medium

A. The handler function must accept an error parameter

B. The status code 500 is invalid for errorhandler

C. The return statement should not include status code

D. The decorator should be @app.route, not @app.errorhandler

Solution

1. Step 1: Check function signature for errorhandler

   Functions decorated with @app.errorhandler must accept one parameter for the error.

2. Step 2: Verify other parts

   Status code 500 is valid, return with status code is correct, and decorator usage is correct.

3. Final Answer:

   The handler function must accept an error parameter -> Option A

4. Quick Check:

   Errorhandler function needs error argument [OK]

Hint: Errorhandler functions always take an error argument [OK]

Common Mistakes:

• Omitting the error parameter in handler function
• Using wrong decorator for error handling
• Returning without status code

5. You want to handle both 404 and 500 errors in your Flask app with custom JSON responses. Which approach correctly implements this?

hard

A. Use one function with @app.errorhandler([404, 500]) to handle both errors

B. Define two separate functions with @app.errorhandler(404) and @app.errorhandler(500), returning JSON and status codes

C. Handle errors inside each route function with try-except blocks only

D. Set app.config['ERROR_404_JSON'] = True and Flask will auto-handle JSON errors

Solution

1. Step 1: Understand Flask errorhandler usage

   Flask requires separate functions for different error codes with their own decorators.

2. Step 2: Confirm JSON response and status code

   Each handler should return a JSON response and the correct HTTP status code for clarity.

3. Final Answer:

   Define two separate functions with @app.errorhandler(404) and @app.errorhandler(500), returning JSON and status codes -> Option B

4. Quick Check:

   Separate handlers per error code = A [OK]

Hint: Use separate errorhandler functions for each error code [OK]

Common Mistakes:

• Trying to handle multiple errors in one function decorator
• Relying only on try-except inside routes
• Assuming Flask auto-converts errors to JSON