Dictionaries and lists in Ansible

Today I had the chance to explain to a colleague how dictionaries and lists are defined in Ansible. The information in this post can be useful to understand how Python, JSON and YAML handle data structures, as some of the patterns shown here are used by these tools. I will use YAML for the examples as it is the most commonly used format for Configuration as Code

Let’s tackle initializating first. Dictionaries use {}, lists use [].

empty_dict: {}

empty_list: []

Dictionaries can be populated in two ways, both equivalent.

my_dict: {key: 'value', kee: 'bar'}

my_dict:
  key: 'value'
  kee: 'bar'

Same with lists:

my_list: ['first', 'second', 'third']

my_list:
  - first
  - second
  - third

Things get complicated when both data structures are nested, so I am leaving some examples below, which hopefully will help you identify easily these structures.

list_of_dictionaries: [first_key: 'first_value', second_key: 'second_value']

list_of_dictionaries:
  - { first_key: 'first_value' }
  - { second_key: 'second_value' }


dictionary_of_lists: {first_list: ['one', 'two', 'three'], second_list: ['four', 'five', 'six']}

dictionary_of_lists:
  first_list:
    - one
    - two
    - three
  second_list:
    - four
    - five
    - six

And the data above can be queried as follows:

# To retrieve 'second_value'
list_of_dictionaries[1]['second_key']

# To retrieve 'three'
dictionary_of_lists['first_list'][2]