YAML has such a comic-book reputation around here that I want to offer a defense of it. Many criticisms of YAML are either about adding non-YAML bits (like turing-complete templates) on top of it, or complaining that it's a bad JSON superset without considering things that can't be expressed in JSON.

You know what's really interesting about YAML? !Tags (https://yaml.org/spec/1.2/spec.html#id2805019), when used as type hints.

In YAML, you can write: (forgive the contrived example)

    !Person
    name: Joe
    login: joe

In Python, you can use "yaml.add_constructor" to automatically dispatch specific Python code when !Person tags are encountered in the YAML. You can transform text into an object tree (edit: graph, actually -- don't forget about anchors and aliases) automatically.

In the right places, this is extremely handy. It's a middle ground between the "duck typing" you normally get with un-validated markup, and the quasi-"static typing" you get with the same inputs after a schema validation. There's an analogy with type annotations in Python.

As always, there are right and wrong places to apply this kind of thing.