An example using `ytt`'s data annotations and a function in a separate source code file.

_ ytt starlark and lib-data example.md

This example uses the @ytt/data library to declare configuration data in yml.

The yml data is then processed in a pythonic script (starlark is a limited variant of python).

To see this example live, vist: https://get-ytt.io/#gist:https://gist.github.com/JollyWizard/7a735c3477fe7a5ec20d926088f59fd3

funcs.star

load("@ytt:struct", "struct")
def process(data):
  r = []
  
  #! data module returns struct.
  #! struct needs to be decoded to a collection.
  decoded = struct.decode(data)
  
  r.append('TYPE ' + type(decoded))
  
  #! decoded data is assumed to be list
  #! a.k.a `data.volumes`
  for d in decoded:
    #! data collected before inner keys modified
    r.append(d)
    
    #! Narrow context to the mappings array.
    mapping = d['mapping'] 
      
    #! Read map keys
    host = mapping['host']
    guest = mapping['guest']
      
    #! Create derived string.
    mapping['volume'] = host + ":" + guest
  end
  
  return r
end

main.yml

#! This example uses the `@ytt/data` library to declare configuration data in yml.
#! The yml data is then processed in a pythonic script.

#! Use data file mechanism to load data.
#@ load("@ytt:data", "data")

#! Use `starlark` to provide pythonic functions.
#@ load("funcs.star", "process")

output: #@ process(data.values.volumes)

volumes.yml

#@data/values
#! This file contains volume maping information to be transformed for injection into a docker compose file.
---
volumes:
 - name: host-dir
   description: Maps the host working directory inside the container.
   mapping:
     host: ./
     guest: /host/

 - name: mysql-data
   description: The data folder for the mysql service.
   mapping:
    host: ./.volumes/mysql/
    guest: /var/lib/mysql

 - name: wordpress-data
   description: The root folder of the wordpress application.
   mapping:
    host: ./.volumes/wp-data
    guest: /var/www/html

 - name: wordpress-cli-data
   description: The user folder where wp-cli packages are installed.
   mapping:
    host: ./.volumes/wp-data
    guest: /home/www-data/.wp-cli