How to Enable CORS in Python

In today’s world, where websites are a crucial part of our daily lives, it is essential to be able to access data from different sources. However, sometimes we face issues called Cross-Origin Resource Sharing (CORS) when we try to access data from different sources. CORS restricts web applications from accessing resources from other sources, which can cause problems when trying to use APIs or other web services. In this article, we will discuss how to enable CORS in Python.

Table of Contents

Understanding CORS

Before we get started on enabling CORS in Python, let’s first understand what CORS is and why it is needed. As stated earlier, CORS is Cross-Origin Resource Sharing, which is a mechanism used by web browsers to enforce security policies. This policy restricts web applications from accessing resources from other sources than the one that served the web page.

For example, if you try to access data from a different origin(domain) than the one that served the web page, the web browser will block the request. This is done to prevent malicious websites from stealing data from trusted websites.

CORS Errors

When CORS is not enabled, you might encounter errors like the following:

  • No ‘Access-Control-Allow-Origin’ header is present on the requested resource
  • Response to preflight request doesn’t pass access control check: No ‘Access-Control-Allow-Origin’ header is present on the requested resource

To resolve these issues, you need to enable CORS in your web application.

Enabling CORS

Enabling CORS in a Python web application is quite easy. You can use the Flask-CORS library to enable CORS in your Flask application. Flask-CORS is a simple Flask extension that adds support for Cross-Origin Resource Sharing (CORS) to your Flask application.

To get started, you need to install Flask-CORS. You can install it using pip:

pip install flask-cors

After installing Flask-CORS, you can enable it in your Flask application like this:

from flask import Flask
from flask_cors import CORS, cross_origin

app = Flask(__name__)
cors = CORS(app)
app.config['CORS_HEADERS'] = 'Content-Type'

@app.route('/')
@cross_origin()
def index():
    return 'Hello, World!'

Here, we have imported the Flask and Flask-CORS libraries. We have initialized the Flask application and enabled CORS using the CORS class.

The @cross_origin() decorator is used to enable CORS for the specific route. In the above example, we have enabled CORS for the root route ("/").

Configuring CORS

Flask-CORS allows you to configure CORS for your application. You can set different configuration options to control the behavior of CORS. Here are some of the configuration options available:

  • CORS_ORIGINS: A list of origins that are allowed to access the resources. By default, all origins are allowed.
  • CORS_METHODS: A list of HTTP methods that are allowed for CORS requests. By default, all methods are allowed.
  • CORS_ALLOW_HEADERS: A list of HTTP headers that are allowed for CORS requests. By default, all headers are allowed
  • CORS_EXPOSE_HEADERS: A list of HTTP headers that are exposed to the client. By default, no headers are exposed.
  • CORS_SUPPORTS_CREDENTIALS: A flag that indicates whether the server supports user credentials in CORS requests. By default, this flag is set to False.

You can set these configuration options using the app.config object. Here’s an example:

from flask import Flask
from flask_cors import CORS, cross_origin

app = Flask(__name__)
cors = CORS(app, resources={r"/api/*": {"origins": "*"}})

app.config['CORS_HEADERS'] = 'Content-Type'
app.config['CORS_ORIGINS'] = ['http://localhost:8080', 'http://localhost:3000']
app.config['CORS_SUPPORTS_CREDENTIALS'] = True

@app.route('/api/data')
@cross_origin(supports_credentials=True)
def api_data():
    return {'data': 'Hello, World!'}

In the above example, we have set different configuration options for our Flask application. We have set the CORS_ORIGINS option to allow access from two different origins. We have also set the CORS_SUPPORTS_CREDENTIALS option to True to support user credentials in CORS requests.

Testing CORS

After enabling CORS in your Flask application, it’s important to test it to ensure that it’s working correctly. You can use the Postman tool to test your application.

Here are the steps to test your Flask application using Postman:

  1. Open the Postman tool and create a new request.
  2. Set the request method to GET.
  3. Set the URL to your Flask application URL (e.g., http://localhost:5000/).
  4. Click the Send button to send the request.

If CORS is enabled correctly, you should see a response with the "Access-Control-Allow-Origin" header set to the origin of the request.

Conclusion

In this article, we have discussed how to enable CORS in Python. We have explained what CORS is and why it is needed. We have also shown how to enable CORS in a Flask application using the Flask-CORS library. Additionally, we have discussed how to configure CORS and how to test it using Postman.

Enabling CORS in your web application is important to allow your application to access resources from different sources. By enabling CORS, you can ensure that your application is secure and protected from malicious attacks.

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