Asking for a friend - Cloudformation Parameters - Default : If condition

Question

Hello All,

I have a ci/cd with a multi account strategy and multi accounts - right now I am trying to pass parameters to a cloudformation template that will be choosing in between different parameters set depending if the branchname is test xxxx or dev xxxx

Please find sample below 
```
############ VPC used depending on the Branchname ########################
  VpcA:
    Type: String
    Default: "vpc-redacteda"

VpcB:
    Type: String
    Default: "vpc-redactedb"  
  
  VPCid:
    Type: String
    Description: VPC Used for the cluster position
    Default: !If [IsTest, !Ref VpcA, !Ref VpcB]

```
The error being returned is that every default member must be a string - is there a way around that ? perhaps using commadelimitedlist instead of string and passing the parameter in a single line.

There's no matching use case on github or other platform....

Help me awesome people

Accepted Answer

The right way to do that is to have VPCId being an SSM parameter : https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ssm-parameter.html

You will use !GetAtt Value to obtain its value: see final section of the above

Then you can drive its value with your !If statement to VpcA or VpcB by using the !If construct that you already have.

I am using this myself quite often

You may even go on a simpler path if you need VPCIOd only once.  You can insert  the condition directly at the place were you need id.

See https://blog.shikisoft.com/aws-cloudformation-no-value-pseudo-parameter/

```
SampleInstance:
    Type: AWS::EC2::Instance
    Properties:
      InstanceType: t2.micro
      SubnetId: !Ref WebServerSubnet
      ImageId: !Ref ImageId
      SecurityGroupIds:
        - !If [ EnvironmentIsProduction, !Ref SecurityGroup, !Ref 'AWS::NoValue' ]
```

Answer

Unfortunately decided against using the SSM Parameters route. However I have found you can get the effect I was looking for without.

So to Address that point I have added all the parameters bunch that I needed in Mappings. Those mappings can be addressed afterwards within the same cloudformation template 
```
          ExecutionRoleArn: 
            !If [BranchNameEqualsTest,
              !FindInMap [BranchParameters, test, 'ClusterTaskRole'], 
              !FindInMap [BranchParameters, dev, 'ClusterTaskRole']]         
```

Answer

You may be able to use the PyPlate example Macro - see https://github.com/awslabs/aws-cloudformation-templates/blob/master/aws/services/CloudFormation/MacrosExamples/PyPlate/README.md.

I haven't in any way tested this code, but once you make the PyPlate macro available, you could use something roughly like:
```
Transform: [PyPlate]
Parameters:
  VpcA:
    Type: String
    Default: "vpc-redacteda"

VpcB:
    Type: String
    Default: "vpc-redactedb"  
  
  VPCid:
    Type: String
    Description: VPC Used for the cluster position
    Default: |
      #!PyPlate
      isTest = ... (Get this from "params" dict or a condition in the "template" dict - not clear to me where you're getting this from)
      output = params[VPCid]
      if not output.startswith('vpc-'):
        output = params[VpcA] if isTest else params[VpcB]
```

Asking for a friend - Cloudformation Parameters - Default : If condition

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