CORS configuration with Flask-CORS - Mini Project: Build & Apply

CORS configuration with Flask-CORS

📖 Scenario: You are building a simple Flask web API that will be accessed by a frontend running on a different domain. To allow this cross-origin access safely, you need to configure CORS (Cross-Origin Resource Sharing) properly.

🎯 Goal: Set up a Flask app with Flask-CORS to allow cross-origin requests only from http://example.com. You will create the app, configure CORS, and add a simple route that returns JSON data.

📋 What You'll Learn

Create a Flask app instance named app

Import and use CORS from flask_cors

Configure CORS to allow only http://example.com as an origin

Add a route /data that returns JSON {"message": "Hello from Flask"}

💡 Why This Matters

🌍 Real World

Many web APIs need to allow frontend apps hosted on different domains to access their data safely. Configuring CORS correctly prevents security issues and enables smooth communication.

💼 Career

Backend developers often configure CORS in Flask or other frameworks to ensure their APIs are accessible only to trusted clients, which is a common task in web development jobs.

1

Create the Flask app instance

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

Flask

# Your code here

Hint

Use Flask(__name__) to create the app instance.

2

Import and configure Flask-CORS

Import CORS from flask_cors and configure it on the app to allow only http://example.com as an origin.

Flask

from flask import Flask

app = Flask(__name__)
# Your code here

Hint

Use CORS(app, origins=["http://example.com"]) to restrict access.

3

Add a route that returns JSON data

Add a route /data to the app that returns a JSON response with {"message": "Hello from Flask"}. Use @app.route and a function named data.

Flask

from flask import Flask
from flask_cors import CORS

app = Flask(__name__)

CORS(app, origins=["http://example.com"])

# Your code here

Hint

Use jsonify to return JSON data from the route.

4

Add the app run command

Add the code to run the Flask app only if the script is executed directly. Use if __name__ == '__main__': and run app.run().

Flask

from flask import Flask, jsonify
from flask_cors import CORS

app = Flask(__name__)

CORS(app, origins=["http://example.com"])

@app.route('/data')
def data():
    return jsonify({"message": "Hello from Flask"})

# Your code here

Hint

This code ensures the app runs only when you execute this script directly.

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