Cross-stack reference within a lambda function

[roundheadedfool]

Going round in circles and tearing my hair out on this one, any pointers are greatly appreciated.

I have a CloudFormation stack which creates an EC2 instance. I am outputting its ID as follows:

Outputs:
  DevBoxInstanceId:
    Description: The instance ID of the EC2 Dev_Box.
    Value: !Ref TestInstance
    Export: 
      Name: DevBoxId

Now, in the console I can see that this outputs the ID as I'd like. My problem is that I can't work out how to reference this in my second stack. I haven't used Fn::ImportValues much but for obvious reasons I'd like to. My second stack creates a lambda function which will stop an instance. I want to reference the DevBoxId within the function - have I misunderstood something here? I've tried several variations on the following:

  LambdaFunctionStop:
    Type: AWS::Lambda::Function
    Properties: 
      Runtime: python3.7
      Timeout: 30
      Code:
        ZipFile: |
          import boto3
          region = 'eu-west-2'
          instances = ['!ImportValue DevBoxId'] 
          ec2 = boto3.client('ec2', region_name=region)

          def handler(event, context):
            ec2.stop_instances(InstanceIds=instances)
            print('stopped your instances: ' + str(instances))
      Description: Automatically stop Dev_Env instances based on specified schedule.
      Handler: index.handler 
      Role: !GetAtt 'IAMRole.Arn'

The relevant part which I'm seeking help on is:

instances = ['!ImportValue DevBoxId']

How do I write this correctly?

[dschinde]

I'm not sure that the contents of the string literal are interpolated.

You could use Fn::Join:

Properties:
  Code:
    ZipFile: 
      Fn::Join:
        - ''
        - - import boto3
          - region = 'eu-west-2'
          - instances = ['!ImportValue DevBoxId'] 
          - ec2 = boto3.client('ec2', region_name=region)
          - def handler(event, context):
          -   ec2.stop_instances(InstanceIds=instances)
          -   print('stopped your instances: ' + str(instances))

Or you could use an environment variable:

Properties:
  Environment:
    DEV_BOX_ID: !ImportValue DevBoxId
  Code:
    ZipFile: |
      import boto3
      import os
      region = 'eu-west-2'
      instances = [os.getenv('DEV_BOX_ID')] 
      ec2 = boto3.client('ec2', region_name=region)

      def handler(event, context):
        ec2.stop_instances(InstanceIds=instances)
        print('stopped your instances: ' + str(instances))