CORS configuration with Flask-CORS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does CORS stand for and why is it important in web development?

CORS stands for Cross-Origin Resource Sharing. It is important because it controls how resources on a web server can be requested from another domain, helping to keep web applications secure by restricting unwanted cross-origin requests.

beginner

How do you enable CORS for all routes in a Flask app using Flask-CORS?

You import CORS from flask_cors and then call CORS(app) after creating your Flask app. This allows all routes to accept cross-origin requests.

intermediate

What is the purpose of the 'resources' parameter in Flask-CORS configuration?

The 'resources' parameter lets you specify which routes or URL patterns should have CORS enabled. You can limit CORS to certain endpoints instead of enabling it globally.

intermediate

How can you restrict CORS to only allow requests from specific origins in Flask-CORS?

You use the 'origins' parameter in the CORS function to list allowed domains. For example, CORS(app, origins=['https://example.com']) only allows requests from that domain.

intermediate

What HTTP headers does Flask-CORS automatically handle to support CORS?

Flask-CORS automatically adds headers like 'Access-Control-Allow-Origin', 'Access-Control-Allow-Methods', and 'Access-Control-Allow-Headers' to responses to enable cross-origin requests.

Which Flask-CORS function call enables CORS for the entire Flask app?

A CORS(app)

B enable_cors(app)

C app.enable_cors()

D cors_enable(app)

The correct way is to import CORS from flask_cors and call CORS(app) to enable CORS globally.

How do you allow only 'https://mydomain.com' to access your Flask API using Flask-CORS?

A CORS(app, origins=['https://mydomain.com'])

B CORS(app, allow=['https://mydomain.com'])

C CORS(app, domains=['https://mydomain.com'])

D CORS(app, hosts=['https://mydomain.com'])

The 'origins' parameter is used to specify allowed domains.

What does the 'resources' parameter in Flask-CORS do?

A Defines the HTTP methods allowed

B Specifies which routes have CORS enabled

C Sets the allowed headers

D Configures the server port

'resources' lets you target specific URL patterns for CORS.

Which HTTP header is NOT automatically handled by Flask-CORS?

A Access-Control-Allow-Origin

B Access-Control-Allow-Methods

C Access-Control-Allow-Headers

D Content-Security-Policy

Content-Security-Policy is unrelated to CORS and not handled by Flask-CORS.

If you want to enable CORS only on the '/api/data' route, how would you configure Flask-CORS?

A CORS(app, only='/api/data')

B CORS(app, route='/api/data')

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

D CORS(app, path='/api/data')

The 'resources' parameter accepts a dictionary with route patterns as keys.

Explain how to set up Flask-CORS to allow cross-origin requests only from a specific domain for certain routes.

Think about how to combine 'resources' and 'origins' parameters.

Describe what CORS is and why Flask-CORS is useful in a Flask web application.

Focus on the problem CORS solves and how Flask-CORS helps.

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