YAML Format

YAML is a data serialization language that is often used for writing configuration files. Depending on whom you ask, YAML stands for yet another markup language or YAML ain’t markup language (a recursive acronym), which emphasizes that YAML is for data, not documents. 

YAML is a popular programming language because it is human-readable and easy to understand. It can also be used in conjunction with other programming languages.

YAML syntax

YAML has features that come from Perl, C, XML, HTML, and other programming languages. YAML is also a superset of JSON, so JSON files are valid in YAML.

YAML uses Python-style indentation to indicate nesting. Tab characters are not allowed, so whitespaces are used instead. There are no usual format symbols, such as braces, square brackets, closing tags, or quotation marks. YAML files use a .yml or .yaml extension. 

The structure of a YAML file is a map or a list.

Maps allow you to associate key-value pairs. Each key must be unique, and the order doesn’t matter. Think of a Python dictionary or a variable assignment in a Bash script.

A map in YAML needs to be resolved before it can be closed, and a new map is created. A new map can be created by either increasing the indentation level or by resolving the previous map and starting an adjacent map. 

A list includes values listed in a specific order and may contain any number of items needed. A list sequence starts with a dash (-) and a space, while indentation separates it from the parent. You can think of a sequence as a Python list or an array in Bash or Perl. A list can be embedded into a map. 

YAML also contains scalars, which are arbitrary data (encoded in Unicode) that can be used as values such as strings, integers, dates, numbers, or booleans.

When creating a YAML file, you’ll need to ensure that you follow these syntax rules and that your file is valid. A linter is an application that verifies the syntax of a file. The yamllint command can help to ensure you’ve created a valid YAML file before you hand it over to an application.

YAML syntax example

Here’s an example of a simple YAML file for an employee record that demonstrates the syntax rules.

—

# An employee record

name: Martin D’vloper

job: Developer

skill: Elite

employed: True

foods:

  – Apple

  – Orange

  – Strawberry

  – Mango

languages:

  perl: Elite

  python: Elite

  pascal: Lame

education: |

  4 GCSEs

  3 A-Levels

  BSc in the Internet of Things

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