def lambda_arn_lookup(session, lambda_name):
    """
    Returns the arn for a lambda given a lambda function name.
    Args:
        session (Session): boto3.session.Session object
        lambda_name (str): name of the lambda function

    Returns:
        (str):
    """
    if session is None:
        return None

    client = session.client("lambda")
    response = client.get_function(FunctionName=lambda_name)
    if response is None:
        return None
    else:
        return response['Configuration']['FunctionArn']