python - CloudFormation !Ref Topic not giving ARN

Question

I'm having an issue getting the ARN of an SQS topic passed via an environment variable to a lambda using python runtime.

Here is the relevant part of the cloud formation yaml:

  HandlerFunction:
    Type: AWS::Serverless::Function # More info about Function Resource: https://github.com/awslabs/serverless-application-model/blob/master/versions/2016-10-31.md#awsserverlessfunction
    Properties:
      CodeUri: xxx/
      Handler: app.handler
      Runtime: python3.8
      Timeout: 60
      ... 
      Environment:
        Variables:
          SNS_TOPIC_ARN: !Ref EmailNotificationTopic
  EmailNotificationTopic:
    Type: AWS::SNS::Topic
    Properties:
      Subscription:
        - Endpoint: [email protected]
          Protocol: email

Inside my handler when I call sns_topic = os.environ['SNS_TOPIC_ARN'] I get EmailNotificationTopic instead of the full ARN, which is what I would expect from the docs.

This is creating a problem when publishing the topic. I know I can hardcode the ARN but other tutorials (and the official docs) seem to indicate the Ref function should be giving the ARN.

I'm pretty new to cloud formation, any advice is appreciated. Thank you

Edit: I added the !Ref EmailNotificationTopic to output in the sam template and confirmed it is displayed as the full ARN, I also confirmed in the topic on the AWS console the full ARN is there, so it seems like when the environment variable is being passed/read in python everything is stripped except for the last bit after the last :? Anyone know how to overcome this?

Answer 1

I am not sure why this is not working exactly, but you can use the long-form which should work always.

Environment:
    Variables:
      SNS_TOPIC_ARN:
         Ref: EmailNotificationTopic