What is YAML and How To Use It¶

The default renderer for SLS files is the YAML renderer.

What is YAML¶

What does YAML stand for? It's an acronym for YAML Ain't Markup Language.

The Official YAML Website defines YAML as:

...a human friendly data serialization standard for all programming languages.

However, Salt uses a small subset of YAML that maps over very commonly used data structures, like lists and dictionaries. It is the job of the YAML renderer to take the YAML data structure and compile it into a Python data structure for use by Salt.

Defining YAML¶

Though YAML syntax may seem daunting and terse at first, there are only three very simple rules to remember when writing YAML for SLS files.

Rule One: Indentation¶

YAML uses a fixed indentation scheme to represent relationships between data layers. Salt requires that the indentation for each level consists of exactly two spaces. Do not use tabs.

Rule Two: Colons¶

Python dictionaries are, of course, simply key-value pairs. Users from other languages may recognize this data type as hashes or associative arrays.

Dictionary keys are represented in YAML as strings terminated by a trailing colon. Values are represented by either a string following the colon, separated by a space:

my_key: my_value

In Python, the above maps to:

{"my_key": "my_value"}

Alternatively, a value can be associated with a key through indentation.

my_key:
  my_value

Note

The above syntax is valid YAML but is uncommon in SLS files because most often, the value for a key is not singular but instead is a list of values.

In Python, the above maps to:

{"my_key": "my_value"}

Dictionaries can be nested:

first_level_dict_key:
  second_level_dict_key: value_in_second_level_dict

And in Python:

{"first_level_dict_key": {"second_level_dict_key": "value_in_second_level_dict"}}

Rule Three: Dashes¶

To represent lists of items, a single dash followed by a space is used. Multiple items are a part of the same list as a function of their having the same level of indentation.

- list_value_one
- list_value_two
- list_value_three

Lists can be the value of a key-value pair. This is quite common in Salt:

my_dictionary:
  - list_value_one
  - list_value_two
  - list_value_three

In Python, the above maps to:

{"my_dictionary": ["list_value_one", "list_value_two", "list_value_three"]}

Learning more about YAML¶

One easy way to learn more about how YAML gets rendered into Python data structures is to use an online YAML parser to see the Python output.

Here are some excellent links for experimenting with and referencing YAML:

Online YAML Parser: Convert YAML to JSON or Python data structures.
The Official YAML Specification
The Wikipedia page for YAML

Templating¶

Jinja statements and expressions are allowed by default in SLS files. See Understanding Jinja.