CloudFormation Templating Custom Resource

cf-templater.yaml

# NOTE: there is a 4k CustomResource response limit, so keep all templates well under that
AWSTemplateFormatVersion: 2010-09-09
Description: 'CloudFormation Templating Custom Resource'

Resources:
  TemplateFunctionRole:
    Type: AWS::IAM::Role
    Properties:
      AssumeRolePolicyDocument:
        Version: 2012-10-17
        Statement:
        - Action: ['sts:AssumeRole']
          Effect: Allow
          Principal:
            Service: [lambda.amazonaws.com]

  TemplateFunction:
    Type: AWS::Lambda::Function
    Properties:
      Code:
        ZipFile: |
          import cfnresponse
          from string import Template
          import hashlib

          def handler(event, context):
            try:
              props = event['ResourceProperties']
              vars = props['Vars']
              # TODO: replace with your templating language of choice
              templ = Template(props['Template'])
              rendered = templ.safe_substitute(vars)

              responseData = {}
              if len(templ.template) < 1900:
                responseData['Template'] = templ.template # 4K response limit
              responseData['Rendered'] = rendered
              resourceID = hashlib.sha256(rendered).hexdigest()
              cfnresponse.send(event, context, cfnresponse.SUCCESS, responseData, resourceID)
            except Exception as e:
              print e
              cfnresponse.send(event, context, cfnresponse.FAILED, {}, None)
      Description: Python string.Template rendering for CloudFormation 
      Handler: index.handler
      Role: !GetAtt TemplateFunctionRole.Arn
      Runtime: 'python2.7'

  Example:
    Type: Custom::Template
    Version: 1.0
    Properties:
      ServiceToken: !GetAtt TemplateFunction.Arn
      Template: |
        This is an example of string.Template '$value'
      Vars:
        value: '$deferred'

  ExampleRecursion:
    Type: Custom::Template
    Version: 1.0
    Properties:
      ServiceToken: !GetAtt TemplateFunction.Arn
      Template: !GetAtt Example.Rendered
      Vars:
        deferred: some other value

Outputs:
  FunctionArn:
    Description: CF Templating Function ARN
    Value: !GetAtt TemplateFunction.Arn
    Export:
      Name: CFTemplater
  ExampleIn:
    Value: !GetAtt Example.Template
  ExampleOut:
    Value: !GetAtt Example.Rendered
  ExampleRecursionIn:
    Value: !GetAtt ExampleRecursion.Template
  ExampleRecursionOut:
    Value: !GetAtt ExampleRecursion.Rendered

result.txt

ExampleIn           This is an example of string.Template '$value'
ExampleOut          This is an example of string.Template '$deferred'
ExampleRecursionIn  This is an example of string.Template '$deferred'
ExampleRecursionOut This is an example of string.Template 'some other value'