lang-ref-def.md

Functions

Refer to Starlark function specification for details about various types of function arguments. Note that ytt's Starlark use requires functions to be closed with an end.

• function definition within YAML

Labels returns map with two keys: test1, and test2:

#@ def my_labels():
test1: 123
test2: 124
#@ end

Above is almost equivalent to (differnce is that return type in one case is a YAML fragment and in another it's a dict):

#@ def my_labels():
#@   return {"test1": 123, "test2": 124}
#@ end

• function definition within Starlark (.star files)

def my_labels():
  return {"test1": 123, "test2": 124}
end

• function arguments (positional and keyword arguments)

#@ def my_deployment(name, replicas=1, labels={}):
kind: Deployment
metadata:
  name: #@ name
  labels: #@ labels
spec:
  replicas: #@ replicas
#@ end

---
kind: List
items:
- #@ my_deployment("dep1", replicas=3)

• common function usage

To set labels key to return value of my_labels():

labels: #@ my_labels()
labels_as_array:
- #@ my_labels()

To merge return value of my_labels() into labels map:

#@ load("@ytt:template", "template")

labels:
  another-label: true
  _: #@ template.replace(my_labels())

Note that in most cases template.replace is not necessary since it's only helps replacing one item (array item, map item or document) with multiple items of that type.