Custom CloudFormation Templates - Setting PhysicalResourceId's and checking against !Ref output

template.yaml

---
Resources:
  # lambda function to be used as a custom resource 
  MyLambda:
    Type: AWS::Lambda::Function
    Properties:
      Handler: index.handler
      Role: !GetAtt BasicExecutionRole.Arn
      Runtime: python3.7
      Timeout: 60
      Code:
        ZipFile: |
          import cfnresponse

          def handler(event, context):
            props = event["ResourceProperties"]
            logicalId = event["LogicalResourceId"]
            
            physicalId = logicalId
            if "CustomPhysicalId" in props:
              physicalId = props["CustomPhysicalId"]

            return cfnresponse.send(
              event,
              context,
              cfnresponse.SUCCESS,
              responseData={ "LogicalId": logicalId },
              physicalResourceId=physicalId
            )

  BasicExecutionRole:
    Type: AWS::IAM::Role
    Properties:
      AssumeRolePolicyDocument:
        Version: 2012-10-17
        Statement:
          - Effect: Allow
            Principal:
              Service:
                - lambda.amazonaws.com
            Action:
              - 'sts:AssumeRole'
      ManagedPolicyArns:
        - arn:aws:iam::aws:policy/AWSLambdaExecute

  MyCustomResource1:
    Type: Custom::MyCustomResource
    Properties:
      ServiceToken: !GetAtt MyLambda.Arn

  MyCustomResource2:
    Type: Custom::MyCustomResource
    Properties:
      ServiceToken: !GetAtt MyLambda.Arn
      CustomPhysicalId: HiMom

  MyCustomResource3:
    Type: Custom::MyCustomResource
    Properties:
      ServiceToken: !GetAtt MyLambda.Arn
      CustomPhysicalId: !GetAtt MyLambda.Arn

Outputs:
  MyCustomResource1:
    Description: Output of the Ref function for MyCustomResource1. Same as PhysicalResourceId
    Value: !Ref MyCustomResource1
  MyCustomResource2:
    Description: Output of the Ref function for MyCustomResource2. Same as PhysicalResourceId
    Value: !Ref MyCustomResource2
  MyCustomResource3:
    Description: Output of the Ref function for MyCustomResource3. Same as PhysicalResourceId
    Value: !Ref MyCustomResource3

So, the three tests show that the Physical ID corresponds to the output of Ref, right?

So, the three tests show that the Physical ID corresponds to the output of Ref, right?

Exactly :)
This is not documented anywhere, but I haven't seen a single case so far where this is not true - looks like the !Ref function always returns the PhysicalResourceId