python-boto3/docs/source/guide/s3-example-configuring-buckets.rst

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#########################
Bucket CORS configuration
#########################

Cross Origin Resource Sharing (CORS) enables client web applications in one 
domain to access resources in another domain. An S3 bucket can be configured 
to enable cross-origin requests. The configuration defines rules that specify 
the allowed origins, HTTP methods (GET, PUT, etc.), and other elements.


Retrieve a bucket CORS configuration
====================================

Retrieve a bucket's CORS configuration by calling the AWS SDK for Python 
``get_bucket_cors`` method.

.. code-block:: python

    import logging
    import boto3
    from botocore.exceptions import ClientError


    def get_bucket_cors(bucket_name):
        """Retrieve the CORS configuration rules of an Amazon S3 bucket

        :param bucket_name: string
        :return: List of the bucket's CORS configuration rules. If no CORS
        configuration exists, return empty list. If error, return None.
        """

        # Retrieve the CORS configuration
        s3 = boto3.client('s3')
        try:
            response = s3.get_bucket_cors(Bucket=bucket_name)
        except ClientError as e:
            if e.response['Error']['Code'] == 'NoSuchCORSConfiguration':
                return []
            else:
                # AllAccessDisabled error == bucket not found
                logging.error(e)
                return None
        return response['CORSRules']


Set a bucket CORS configuration
===============================

A bucket's CORS configuration can be set by calling the ``put_bucket_cors`` 
method.

.. code-block:: python

    # Define the configuration rules
    cors_configuration = {
        'CORSRules': [{
            'AllowedHeaders': ['Authorization'],
            'AllowedMethods': ['GET', 'PUT'],
            'AllowedOrigins': ['*'],
            'ExposeHeaders': ['GET', 'PUT'],
            'MaxAgeSeconds': 3000
        }]
    }

    # Set the CORS configuration
    s3 = boto3.client('s3')
    s3.put_bucket_cors(Bucket='BUCKET_NAME',
                       CORSConfiguration=cors_configuration)