What is wrong with flow style YAML?

Flow style is essentially JSON embedded in YAML - making use of curly { } and square brackets to denote lists and mappings.

Example:

a: 1
b: {c: 3, d: 4}

This use of JSONesque { and } is also ugly and hampers readability - especially when { and } are used for other purposes (e.g. templating) and the human reader/writer of YAML has to give themselves a headache figuring out what kind of curly bracket it is.

The first question in the FAQ of pyyaml actually subtly indicates that this feature wasn’t a good idea - see “why does my YAML look wrong?”.

To take a real life example, use of flow style in this saltstack YAML definition which blurs the distinction between flow style and jinja2, confusing the reader.

Parsing ‘dirty’ YAML with flow style

To parse YAML with flow style, you can use dirty load.

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