CORS configuration with Flask-CORS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the Flask-CORS extension.

Flask

from flask_cors import [1]

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

ACrossOrigin

BCors

CCORS

DCorsConfig

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect class names like CrossOrigin or CorsConfig.

The Flask-CORS extension is imported using CORS.

2fill in blank

medium

Complete the code to initialize Flask-CORS for the Flask app.

Flask

app = Flask(__name__)
[1](app)

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

ACrossOrigin

BCORS

CFlaskCORS

Dcors_init

Attempts:

3 left

💡 Hint

Common Mistakes

Calling a non-existent function like cors_init or FlaskCORS.

To enable CORS on the Flask app, you call CORS(app).

3fill in blank

hard

Fix the error in the code to allow CORS only for the origin 'http://example.com'.

Flask

CORS(app, resources={r"/*": {"origins": [1])

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

A'*'

Bhttp://example.com

C'http://example.com'

D['http://example.com']

Attempts:

3 left

💡 Hint

Common Mistakes

Using a plain string without list when a list is expected.

Using '*' which allows all origins.

The origins value must be a list or string. To allow only 'http://example.com', use a list with that string.

4fill in blank

hard

Fill both blanks to configure CORS to allow only GET and POST methods from any origin.

Flask

CORS(app, resources={r"/*": {"origins": [1], "methods": [2])

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

A'*'

B['GET', 'POST']

C['PUT', 'DELETE']

D'GET, POST'

Attempts:

3 left

💡 Hint

Common Mistakes

Using a string with comma-separated methods instead of a list.

Restricting origins incorrectly.

Use '*' to allow any origin and a list ['GET', 'POST'] to specify allowed methods.

5fill in blank

hard

Fill all three blanks to configure CORS with Flask app to allow origins 'http://site1.com' and 'http://site2.com', allow credentials, and only allow PUT method.

Flask

CORS(app, resources={r"/*": {"origins": [1], "supports_credentials": [2], "methods": [3])

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

A['http://site1.com', 'http://site2.com']

BTrue

C['PUT']

DFalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using string instead of list for origins or methods.

Setting supports_credentials to string instead of boolean.

Origins must be a list of allowed URLs, supports_credentials is a boolean to allow cookies, and methods is a list of allowed HTTP methods.

Practice

1. What is the main purpose of using Flask-CORS in a Flask application?

easy

A. To allow or restrict cross-origin requests to the Flask app

B. To speed up the Flask app by caching responses

C. To handle database connections automatically

D. To manage user authentication and sessions

Solution

1. Step 1: Understand what CORS means

   CORS stands for Cross-Origin Resource Sharing, which controls which websites can access resources from your Flask app.

2. Step 2: Identify Flask-CORS role

   Flask-CORS is a tool to add headers that allow or restrict these cross-origin requests easily.

3. Final Answer:

   To allow or restrict cross-origin requests to the Flask app -> Option A

4. Quick Check:

   CORS controls access = A [OK]

Hint: Remember CORS controls cross-site access permissions [OK]

Common Mistakes:

• Confusing CORS with caching or authentication
• Thinking Flask-CORS speeds up the app
• Assuming Flask-CORS manages databases

2. Which of the following is the correct way to enable CORS for all routes in a Flask app using Flask-CORS?

easy

A. from flask_cors import CORS
app = Flask(__name__)
CORS()

B. from flask_cors import CORS
app = Flask(__name__)
app.enable_cors()

C. from flask_cors import CORS
app = Flask(__name__)
CORS(app)

D. from flask_cors import CORS
app = Flask(__name__)
CORS(app, origins='none')

Solution

1. Step 1: Import and create Flask app

   Import CORS from flask_cors and create the Flask app instance.

2. Step 2: Enable CORS on the app

   Call CORS(app) to enable CORS for all routes. This is the correct syntax.

3. Final Answer:

   from flask_cors import CORS\napp = Flask(__name__)\nCORS(app) -> Option C

4. Quick Check:

   Enable CORS by passing app to CORS() [OK]

Hint: Use CORS(app) to enable CORS globally [OK]

Common Mistakes:

• Calling CORS() without app argument
• Using non-existent app.enable_cors() method
• Setting origins='none' disables all CORS

3. Given this Flask app code snippet, what will be the CORS behavior for the route /data?

from flask import Flask, jsonify
from flask_cors import CORS

app = Flask(__name__)
CORS(app, origins=['https://example.com'])

@app.route('/data')
def data():
    return jsonify({'key': 'value'})

medium

A. Only localhost can access /data cross-origin

B. All origins can access /data without restriction

C. No origins can access /data due to missing CORS headers

D. Only requests from https://example.com will be allowed cross-origin access

Solution

1. Step 1: Check CORS configuration

   CORS is enabled with origins=['https://example.com'], so only that origin is allowed cross-origin access.

2. Step 2: Understand route behavior

   The /data route returns JSON and inherits the CORS policy set on the app.

3. Final Answer:

   Only requests from https://example.com will be allowed cross-origin access -> Option D

4. Quick Check:

   Origins list limits access = A [OK]

Hint: Origins list restricts which sites can access [OK]

Common Mistakes:

• Assuming all origins are allowed by default
• Thinking CORS disables all access without explicit allow
• Confusing localhost with example.com

4. Identify the error in this Flask app CORS setup:

from flask import Flask
from flask_cors import CORS

app = Flask(__name__)
CORS(origins='*')

@app.route('/')
def home():
    return 'Hello'

medium

A. CORS is called without passing the Flask app instance

B. The origins parameter '*' is invalid and causes an error

C. The route function home() must return JSON for CORS to work

D. Flask app instance must be created after calling CORS

Solution

1. Step 1: Check how CORS is called

   CORS() is called without passing the app instance, so it does not enable CORS on the app.

2. Step 2: Understand consequences

   Without passing app, no CORS headers are added, so cross-origin requests will fail.

3. Final Answer:

   CORS is called without passing the Flask app instance -> Option A

4. Quick Check:

   Pass app to CORS() to enable it [OK]

Hint: Always pass app instance to CORS() [OK]

Common Mistakes:

• Forgetting to pass app to CORS()
• Thinking '*' is invalid (it's valid)
• Believing CORS requires JSON response

5. You want to allow CORS only on the route /api/data for requests from https://trusted.com. Which Flask-CORS setup correctly achieves this?

hard

A. CORS(app, resources={r'/api/*': {'origins': '*'}})

B. CORS(app, resources={r'/api/data': {'origins': 'https://trusted.com'}})

C. CORS(app, origins='https://trusted.com')

D. CORS(app, resources={r'/api/data': {'origins': '*'}})

Solution

1. Step 1: Understand resource-specific CORS

   To restrict CORS to a specific route, use the resources parameter with a route pattern and origins.

2. Step 2: Match the route and origin

   CORS(app, resources={r'/api/data': {'origins': 'https://trusted.com'}}) correctly sets CORS only on '/api/data' with origins 'https://trusted.com'.

3. Final Answer:

   CORS(app, resources={r'/api/data': {'origins': 'https://trusted.com'}}) -> Option B

4. Quick Check:

   Use resources to limit routes and origins [OK]

Hint: Use resources dict to target specific routes [OK]

Common Mistakes:

• Setting origins globally instead of per route
• Using wildcard '*' when only one origin is needed
• Using wrong route pattern like '/api/*' for exact match