def tag_instances_on_cluster(cluster_name, project='cwc'):
    """Adds project tag to untagged instances in a given cluster.

    Parameters
    ----------
    cluster_name : str
        The name of the AWS ECS cluster in which running instances
        should be tagged.
    project : str
        The name of the project to tag instances with.
    """
    # Get the relevant instance ids from the ecs cluster
    ecs = boto3.client('ecs')
    task_arns = ecs.list_tasks(cluster=cluster_name)['taskArns']
    if not task_arns:
        return
    tasks = ecs.describe_tasks(cluster=cluster_name, tasks=task_arns)['tasks']
    container_instances = ecs.describe_container_instances(
        cluster=cluster_name,
        containerInstances=[task['containerInstanceArn'] for task in tasks]
        )['containerInstances']
    ec2_instance_ids = [ci['ec2InstanceId'] for ci in container_instances]

    # Instantiate each instance to tag as a resource and create project tag
    for instance_id in ec2_instance_ids:
        tag_instance(instance_id, project=project)
    return
Adds project tag to untagged instances in a given cluster.

    Parameters
    ----------
    cluster_name : str
        The name of the AWS ECS cluster in which running instances
        should be tagged.
    project : str
        The name of the project to tag instances with.