def METHOD_NAME(self):
    """
    Sets up a key for the demo. Either creates a new key or uses one supplied by
    the user.
    :return: The ARN or ID of the key to use for the demo.
    """
    answer = input("Do you want to create a new key for the demo (y/n)? ")
    if answer.lower() == 'y':
        try:
            key = self.kms_client.create_key(
                Description="Alias management demo key")['KeyMetadata']
            self.created_key = key
        except ClientError as err:
            logger.error(
                "Couldn't create key. Here's why: %s", err.response['Error']['Message'])
            raise
        else:
            key_id = key['KeyId']
    else:
        key_id = input("Enter a key ID or ARN to use for the demo: ")
        if key_id == '':
            key_id = None
    return key_id