How to reference single item from a list with the stack_output resolver?

[craighurley]

Given an output in template.yaml that is a list of resource IDs like this:

Outputs:
  ResourceList:
    Value: !Sub '${ResourceA},${ResourceB},${ResourceC}'
    Export:
      Name: !Sub '${AWS::StackName}-ResourceList'

Is it possible to select a single item from that list in a sceptre stack config when using the stack_output resolver?

parameters:
  ParameterList:
    # How to select the Nth item in this list?
    - !stack_output '{{ template.yaml::ResourceList }}'

I know I could output each resource ID individually, but I'm looking to reduce the number of outputs if possible.

Thanks.

[cornerman]

Afaik, selecting an element in a list does not work with stack_outputs.

One idea/workaround comes to my mind: Move the element select to the cloudformation template and select the right index in that template.

Example template.yaml:

Parameters:
  ParameterList:
    Type: CommaDelimitedList

  ParameterListIndex:
    Type: Number
    Default: 0

Resources:
  SomeResource:
    SomeProperty: !Select [ !Ref ParameterListIndex, !Ref ParameterList ] 

You could still use it with a single string (defaulting to using the first element in the list) or use it with a list and provide the index to select. It is a bit ugly, because it leaks details of the calling site into the template.