YAML has two data elements that serve as building blocks for complex data structures: sequences and mappings.

Sequence

This is a sequence:

---
- Emperor
- Gentoo
- Little Blue

Mapping

This is a mapping:

---
Penguin: Emperor

In this case, Penguin is a key and Emperor is a value. This is often called a "key and value pair", but in YAML it's just called a mapping.

Combining data blocks

You can embed these data types into one another. Here is a mapping that has a sequence as its value:

---
Penguin:
  - Emperor
  - Gentoo
  - Little Blue

Here is a sequence of mappings:

---
- Penguin: Emperor
- Penguin: Gentoo
- Penguin: Little Blue

yamllint

Use yamllint to detect errors in your YAML. To install:

$ pip install yamllint

Run it:

$ yamllint good.yaml
$ yamllint bad.yaml
bad.yaml
  1:1       warning  missing document start "---"  (document-start)
  4:14      error    no new line character at the end of file  (new-line-at-end-of-file)

yaml2json

Sometimes it's useful to convert your YAML to JSON so you can view the data structure in a different way. There are probably dozens of YAML-to-JSON converters out there, but here's the one I use: https://gitlab.com/slackermedia/yaml2json.git

Run it:

$ cat example.yaml
---
penguins:
  - Gentoo
  - Little Blue
  - Rockhopper
dragons:
  - black
  - white
  - red
$ ~/bin/yaml2json.py example.yaml
{"penguins": ["Gentoo", "Little Blue", "Rockhopper"], "dragons": ["black", "white", "red"]}

YAML police

There are no YAML police. As long as yamllint finds no errors, your YAML is valid and can be parsed by any one of the dozens of YAML libraries out there. However, these YAML libraries aren't magical, so you must understand the internal logic of your own YAML data. Keep that in mind when devising a scheme for the data you're recording.

YAML is a great method for creating configuration files, or storing simple data structures, and it's essential for Ansible playbooks.

Enjoy!