AWS: how to use transform cloudformation function in YAML database name

Question

I'm a little stuck trying to combine two code functions that don't seem to be working. the aws cloudformation cli tool seems to be having problems and throws an error: 'list' object has no attribute 'get'.

I'm trying to specify a lambda function (Type AWS::Serverless::Function), which otherwise works, but I want to specify the Event Schedule object as lower case (required for database).

Code in question, the last 8 lines are the issue:

Resources:
  UpdatePartitionsLogTableFunction:
    Type: AWS::Serverless::Function
    Properties:
      Handler: updatePartitionsTable.handler
      Runtime: nodejs12.x
      CodeUri: src
      Description: Updates partitions Athena tables
      MemorySize: 128
      Timeout: 60
      Policies:
        - Version: "2012-10-17"
          Statement:
            - Effect: Allow
              Action:
                - "athena:*"
              Resource:
                - "*"
            - Effect: Allow
              Action:
                - "glue:*"
              Resource:
                - "*"
            - Effect: Allow
              Action:
                - "s3:*"
              Resource:
                - "*"
      Events:
        Timer:
          Type: Schedule
          Properties:
            Schedule: cron(0 * * * ? *)
            Input:
              'Fn::Transform':
              - Name: 'String'
                Parameters:
                  InputString: !Sub |
                      {"database": "${AWS::StackName}_logs",
                        "tableNames": "${PartitionedTables}"}
                  Operation: Lower

I get an error:

'list' object has no attribute 'get'

Simplified code:

            Input: !Sub |
                {"database": "${AWS::StackName}_logs",
                  "tableNames": "${PartitionedTables}"}

If I simplify the code, I get no error, but I need the lowercase for my usecase. Can anyone help? I'm kinda stuck.

Answer 1

I managed to solve this, thanks to help from @Marcin Unfortunately, I got into a rabbit hole of other deployment issues, so I wanted to post my final findings as the answer to this question. I hope this might help someone else.

In my original question, there was indeed a - that was causing issues, needed to remove this and resolved the first error.

Working code below: (Don't forget to similarly transform any references to DB within same template and others, the same Macro Transform code can be used)

            Input:
              'Fn::Transform':
                Name: 'StringTransform'
                Parameters:
                  InputString: !Sub |
                      {"database": "${AWS::StackName}_logs",
                        "tableNames": "${PartitionedTables}"}
                  Operation: Lower

Code of CloudFormation Macro Lambda required to make this work:

Resources:
  # Custom Transform Macro Function
  # Function is called to manipulate (transform) strings
  TransformFunction:
    Type: AWS::Lambda::Function
    Properties:
      Code:
        ZipFile: |
          import traceback


          def handler(event, context):
              response = {
                  "requestId": event["requestId"],
                  "status": "success"
              }
              try:
                  operation = event["params"]["Operation"]
                  input = event["params"]["InputString"]
                  no_param_string_funcs = ["Upper", "Lower", "Capitalize", "Title", "SwapCase"]
                  if operation in no_param_string_funcs:
                      response["fragment"] = getattr(input, operation.lower())()
                  elif operation == "Strip":
                      chars = None
                      if "Chars" in event["params"]:
                          chars = event["params"]["Chars"]
                      response["fragment"] = input.strip(chars)
                  elif operation == "Replace":
                      old = event["params"]["Old"]
                      new = event["params"]["New"]
                      response["fragment"] = input.replace(old, new)
                  elif operation == "MaxLength":
                      length = int(event["params"]["Length"])
                      if len(input) <= length:
                          response["fragment"] = input
                      elif "StripFrom" in event["params"]:
                          if event["params"]["StripFrom"] == "Left":
                              response["fragment"] = input[len(input)-length:]
                          elif event["params"]["StripFrom"] != "Right":
                              response["status"] = "failure"
                      else:
                          response["fragment"] = input[:length]
                  else:
                      response["status"] = "failure"
              except Exception as e:
                  traceback.print_exc()
                  response["status"] = "failure"
                  response["errorMessage"] = str(e)
              return response

      Handler: index.handler
      Runtime: python3.7
      Role: !GetAtt TransformExecutionRole.Arn
      FunctionName: !Sub ${AWS::StackName}-StringTransform

Other accompanying resources for Macro:

  # Role for TransformFunction
  TransformExecutionRole:
    Type: AWS::IAM::Role
    Properties:
      AssumeRolePolicyDocument:
        Version: 2012-10-17
        Statement:
          - Effect: Allow
            Principal:
              Service: [lambda.amazonaws.com]
            Action: ['sts:AssumeRole']
      Path: /
      Policies:
        - PolicyName: root
          PolicyDocument:
            Version: 2012-10-17
            Statement:
              - Effect: Allow
                Action: ['logs:*']
                Resource: 'arn:aws:logs:*:*:*'
  # Permission for TransformFunction
  TransformFunctionPermissions:
    Type: AWS::Lambda::Permission
    Properties:
      Action: 'lambda:InvokeFunction'
      FunctionName: !GetAtt TransformFunction.Arn
      Principal: 'cloudformation.amazonaws.com'
  # Actual CloudFormationMacro entrypoint for TransformFunction
  Transform:
    Type: AWS::CloudFormation::Macro
    Properties:
      Name: 'StringTransform'
      Description: Provides various string processing functions
      FunctionName: !GetAtt TransformFunction.Arn

Thanks again @Marcin