Roll back an IAM policy version using an Amazon SDK

def rollback_policy_version(policy_arn):
    """
    Rolls back to the previous default policy, if it exists.

    1. Gets the list of policy versions in order by date.
    2. Finds the default.
    3. Makes the previous policy the default.
    4. Deletes the old default version.

    :param policy_arn: The ARN of the policy to roll back.
    :return: The default version of the policy after the rollback.
    """
    try:
        policy_versions = sorted(
            iam.Policy(policy_arn).versions.all(),
            key=operator.attrgetter('create_date'))
        logger.info("Got %s versions for %s.", len(policy_versions), policy_arn)
    except ClientError:
        logger.exception("Couldn't get versions for %s.", policy_arn)
        raise

    default_version = None
    rollback_version = None
    try:
        while default_version is None:
            ver = policy_versions.pop()
            if ver.is_default_version:
                default_version = ver
        rollback_version = policy_versions.pop()
        rollback_version.set_as_default()
        logger.info("Set %s as the default version.", rollback_version.version_id)
        default_version.delete()
        logger.info("Deleted original default version %s.", default_version.version_id)
    except IndexError:
        if default_version is None:
            logger.warning("No default version found for %s.", policy_arn)
        elif rollback_version is None:
            logger.warning(
                "Default version %s found for %s, but no previous version exists, so "
                "nothing to roll back to.", default_version.version_id, policy_arn)
    except ClientError:
        logger.exception("Couldn't roll back version for %s.", policy_arn)
        raise
    else:
        return rollback_version