YAML config as nested maps (Python + Groovy)

Loading a YAML file gives you nested dictionaries (Python) or maps (Groovy). The concepts are identical; only the syntax differs. Examples use a config shaped like the Fedora CoreOS pipeline:

ocp_node_builds:
  release:
    "4.22-9.8":
      source_config:
        url: https://github.com/openshift/rhel-coreos-config
        ref: release-4.22-9.8
      from: quay.io/coreos/rhel-coreos-base:9.8
      build_args_file: true
      arches: [x86_64, aarch64]

Python

import yaml
config = yaml.safe_load(open("config.yaml"))

config["ocp_node_builds"]["release"]["4.22-9.8"]   # bracket access only
release = "4.22-9.8"
info = config["ocp_node_builds"]["release"][release]  # key from a variable
info["source_config"]["url"]
info["arches"][0]                                  # lists → Python lists

Missing keys raise KeyError - use .get() for safe access:

info.get("skip_brew_upload")          # → None (no error)
info.get("skip_brew_upload", False)   # → explicit default
info.get("yumrepo", {}).get("url")    # → None if either level is missing

YAML	Python type	Access
mapping (key: val)	dict	d['key'] / d.get('key')
sequence (- item)	list	d[0] / for x in d / len(d)
scalar	str/int/bool	direct value
missing key	KeyError	.get(key) / .get(key, default)

Groovy

In a Jenkins pipeline, readYaml returns plain Groovy maps and lists. Dot and bracket notation are interchangeable; brackets are needed for variable keys.

def pipecfg = readYaml file: 'config.yaml'

pipecfg.ocp_node_builds.release."4.22-9.8"
// params.RELEASE is a string like "4.22-9.8" chosen at build time:
def info = pipecfg.ocp_node_builds.release[params.RELEASE]
info.source_config.url
info.from
info.arches[0]; info.arches.size()

Missing keys are null (not an error), which is why opt-in flags work without defaults. Use ?: for a default and ?. for safe navigation:

info.skip_brew_upload            // → null (not in YAML)
if (info.skip_brew_upload) { }   // null is falsy → safely skipped
def skip = info.skip_brew_upload ?: false   // default

info.yumrepo.url                 // → NullPointerException if yumrepo absent
info.yumrepo?.url                // → null (safe)

Python vs Groovy side-by-side

Operation	Python	Groovy
Load YAML	yaml.safe_load(open(f))	readYaml file: f
Access key	d['key']	d.key or d['key']
Missing key	KeyError (crash)	null (silent)
Safe with default	d.get('key', False)	d.key ?: false
Safe nested access	d.get('a', {}).get('b')	d.a?.b
Iterate / length	for x in lst / len(lst)	for (x in lst) / lst.size()
String interpolation	f"hello {name}"	"hello ${name}"