def sfn_status(session, arn):
    """Get the status of a StepFunction execution

    Args:
        session (Session): Boto3 session
        arn (string): ARN of the execution to get the status of

    Returns:
        string: One of 'RUNNING', 'SUCCEEDED', 'FAILED', 'TIMED_OUT', 'ABORTED'
    """
    client = session.client('stepfunctions')
    resp = client.describe_execution(executionArn = arn)
    return resp['status']