TemplateGenerator: Cloudformation split function call wrapped inside list

[ilons]

I'm trying to parse a cloudformation template into a troposphere template using the TemplateGenerator class.

This works good for most parts, but I've ran into an issue where TemplateGenerator would wrap calls to Fn::Split inside a list, making the result invalid (as it becomes a list of lists when deployed by cloudformation).

Small snippet to reproduce this issue:

template = troposphere.template_generator.TemplateGenerator({
    "Resources": {
        "TestLB": {
            "Properties": {
                "IpAddressType": "ipv4",
                "Name": "MyTestLB",
                "Scheme": "internet-facing",
                "SecurityGroups": [
                    {
                        "Ref": "MySG"
                    }
                ],
                "Subnets": {
                    "Fn::Split": [
                        ",",
                        {
                            "Fn::ImportValue": "MyPublicSubnets"
                        }
                    ]
                },
                "Type": "application"
            },
            "Type": "AWS::ElasticLoadBalancingV2::LoadBalancer"
        }
    }
})
assert isinstance(template.resources["TestLB"].properties["Subnets"], troposphere.Split), "Expected troposphere.Split"

I'm getting this running:

Python 3.10.6

cfn-flip==1.3.0
click==8.1.3
PyYAML==6.0
six==1.16.0
troposphere==4.3.2

Is there any way around this?

[ilons]

Looking at this again 2 years later (seeing my local fix is still in place), I wonder if this is caused by the Subnets property of troposphere.elasticloadbalancingv2.LoadBalancer being defined as list[str]?

The CloudFormation Split function would always produce a list of strings, and should as such be valid as the value for any property of a converted definition which is defined as list[str], and should never further be wrapped in a list.

[ilons]

So it seems to me that this is indeed the case, as we are calling _create_instance with:
cls=[str], args={'Fn::Split': [',', {'Fn::ImportValue': 'MyPublicSubnets'}]}, ref='Subnets', which results in the args to be wrapped in a list.

Taking a step back and looking at LoadBalancer it defines the property as "Subnets": ([str], False),.
There is multiple solutions to this issue, two of which being:

1. Patch the TemplateGenerator._create_instance method to avoid wrapping if the args would generate a Split definition (which is a bit messy, as the wrapping is done before the split is generated)
2. Patch LoadBalancer to accept either [str] or Split, which I have no idea about the implications of

As I have not worked in this code-base before, I do not really know what the preferred way would be.

[markpeek]

Thank you for bringing this back up and the additional information. I'll have to think some on this when I get some cycles. For your #2 approach, there is an implied AWSHelperFn in the validation so I likely would not want to clutter the property definitions with Split. And also, those are autogenerated which would require another patch. I think your #1, detecting the Fn::Split (and other similar functions) would be a better approach. This could be expanded further in the future to ensure the receiver type matches the function output for all functions.

[ilons]

A very crude quick patch for this (wich I just used to test this) would be to replace _create_instance line 224

                return [self._create_instance(cls[0], v) for v in args]

with:

                result = [self._create_instance(cls[0], v) for v in args]
                if len(result) == 1 and isinstance(result[0], Split):
                    return result[0]
                return result