add-locale.yaml

#@ load("@ytt:overlay", "overlay")
#@ load("@ytt:toml", "toml")

#@ def app_config_secret():
kind: Secret
metedata:
  name: app-config
#@ end

#@ def add_locale(conf_str):
#@   # parse to Starlark `dict`s and `list`s:
#@   conf = toml.decode(conf_str)
#@  
#@   conf["Global"]["locale"] = "Amsterdam"
#@
#@   # create a new section by adding an empty dictionary
#@   conf["Locales"] = {}
#@   conf["Locales"]["Amsterdam"] = {}
#@   conf["Locales"]["Amsterdam"]["max_instances"] = 15
#@   conf["Locales"]["Amsterdam"]["min_instances"] = 4
#@ 
#@   # encode to TOML-formatted string
#@   return toml.encode(conf)
#@ end

#@overlay/match by=overlay.subset(app_config_secret())
---
stringData:
  #@overlay/replace via=lambda old, new: add_locale(old)
  app.conf:

config.yaml

kind: Secret
metedata:
  name: app-config
stringData:
  app.conf: |
    [Global]
    user = "FOO\\bar"