如何将服务器 ID 元组展平为字符串？

我正在尝试创建一个文件，其中包含使用以下count属性生成的多个服务器主机的 ID ：

resource "aws_instance" "workers" {
  count                  = "${var.worker_count}"
  ...
}

resource "local_file" "stop_instances" {
  filename = "${path.module}/generated/stop_instances.py"
  content =<<EOF
import boto3

# Boto Connection
ec2 = boto3.resource('ec2', '${var.region}')

def lambda_handler(event, context):
  # Retrieve instance IDs
  instance_ids = ["${aws_instance.controller.id}", "${aws_instance.gateway.id}", "${aws_instance.workers.*.id}"]

  # stopping instances
  stopping_instances = ec2.instances.filter(InstanceIds=instance_ids).stop()
EOF
}

但是，我收到以下错误：

 447:   instance_ids = ["${aws_instance.controller.id}", 
              "${aws_instance.gateway.id}", "${aws_instance.workers.*.id}"]
 449: 
 450: 
 451: 
    |----------------
    | aws_instance.workers is tuple with 3 elements

Cannot include the given value in a string template: string required.

有没有办法将元组展平为字符串？

我试过tostring()方法，但它只接受原始类型。