How to provision a bunch of resources using the pyplate macro - aws-cloudformation

Question

In this learning exercise I want to use a PyPlate script to provision the BucketA, BucketB and BucketC buckets in addition to the TestBucket.
Imagine that the BucketNames parameter could be set by a user of this template who would specify a hundred bucket names using UUIDs for example.
AWSTemplateFormatVersion: "2010-09-09"
Transform: [PyPlate]
Description: A stack that provisions a bunch of s3 buckets based on param names
Parameters:
BucketNames:
Type: CommaDelimitedList
Description: All bucket names that should be created
Default: BucketA,BucketB,BucketC
Resources:
TestBucket:
Type: "AWS::S3::Bucket"
#!PyPlate
output = []
bucket_names = params['BucketNames']
for name in bucket_names:
output.append('"' + name + '": {"Type": "AWS::S3::Bucket"}')
The above when deployed responds with a Template format error: YAML not well-formed. (line 15, column 3)

Answer 1

Although the accepted answer is functionally correct, there is a better way to approach this.
Essentially PyPlate code recursively reads through all the key-value pairs of the stack and replaces the values with their Python computed values (ie they match the #!PyPlate regex). So we need to have a corresponding key to the PyPlate code.
Here's how a combination of PyPlate and Explode would solve the above problem.
AWSTemplateFormatVersion: "2010-09-09"
Transform: [PyPlate, Explode]
Description: A stack that provisions a bunch of s3 buckets based on param names
Parameters:
BucketNames:
Type: CommaDelimitedList
Description: All bucket names that should be created
Default: BucketA,BucketB,BucketC
Mappings: {}
Resources:
MacroWrapper:
ExplodeMap: |
#!PyPlate
param = "BucketNames"
mapNamespace = param + "Map"
template["Mappings"][mapNamespace] = {}
for bucket in params[param]:
template["Mappings"][mapNamespace][bucket] = {
"ResourceName": bucket
}
output = mapNamespace
Type: "AWS::S3::Bucket"
TestBucket:
Type: "AWS::S3::Bucket"
This approach is powerful because:
You can append resources to an existing template, because you won't tamper with the whole Resources block
You don't need to rely on hardcoded Mappings, as required by Explode. You can drive dynamic logic in Cloudformation.
Most of the Cloudformation props/KV's remain in the YAML part, and there is minimal python part which augments to the CFT functionality.
Please be aware of the macro order through - PyPlate needs to be executed before Explode, which is why the order [PyPlate, Explode]. The execution is sequential.
If we walk through the source code of PyPlate, it gives us control of more template related variables to work with, namely
params (stack parameters)
template (the entire template)
account_id
region
I utilised the template variable in this case.
Hope this helps

Answer 2

This works for me:
AWSTemplateFormatVersion: "2010-09-09"
Transform: [PyPlate]
Description: A stack that provisions a bunch of s3 buckets based on param names
Parameters:
BucketNames:
Type: CommaDelimitedList
Description: All bucket names that should be created
Default: BucketA,BucketB,BucketC
Resources:
|
#!PyPlate
output = {}
bucket_names = params['BucketNames']
for name in bucket_names:
output[name] = {"Type": "AWS::S3::Bucket"}
Explanation:
The python code outputs a dict object where the key is the bucket name and the value is its configuration:
{'BucketA': {'Type': 'AWS::S3::Bucket'}, 'BucketB': {'Type': 'AWS::S3::Bucket'}}
Prior to Macro execution, the YAML template is transformed to JSON format, and because the above is valid JSON data, I can plug it in as value of Resources.
(Note that having the hardcoded TestBucket won't work with this and I had to remove it)