YAML

• YAML Syntax — Ansible Community Documentation
• The Official YAML Web Site
  • Python:
    • PyYAML # YAML 1.1, pure python and libyaml binding
    • ruamel.yaml # YAML 1.2, update of PyYAML; comments round-trip
    • PySyck # YAML 1.0, syck binding
    • strictyaml # Restricted YAML subset

Notes / Remember remember / Q & A

YAML: Do I need quotes for strings in YAML?

After a brief review of the YAML cookbook cited in the question and some testing, here’s my interpretation:

• In general, you don’t need quotes.
• Use quotes to force a string, e.g. if your key or value is 10 but you want it to return a String and not a Fixnum, write '10' or "10".
• Use quotes if your value includes special characters, (e.g. :, {, }, [, ], ,, &, *, #, ?, |, -, <, >, =, !, %, @, \).
• Single quotes let you put almost any character in your string, and won’t try to parse escape codes. '\n' would be returned as the string \n.
• Double quotes parse escape codes. "\n" would be returned as a line feed character.
• The exclamation mark introduces a method, e.g. !ruby/sym to return a Ruby symbol.

Seems to me that the best approach would be to not use quotes unless you have to, and then to use single quotes unless you specifically want to process escape codes.

Update

“Yes” and “No” should be enclosed in quotes (single or double) or else they will be interpreted as TrueClass and FalseClass values:

en:
  yesno:
    'yes': 'Yes'
    'no': 'No'

Source: YAML: Do I need quotes for strings in YAML?