Extract values from a list of dictionaries in Ansible

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05-10-2024

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Extracting Values from a List of Dictionaries in Ansible: A Comprehensive Guide

Ansible is a powerful automation tool often used to manage infrastructure and applications. One common task involves working with data structures like lists of dictionaries. This article will guide you through the process of extracting specific values from such lists, providing practical examples and insightful explanations.

The Problem:

Let's say you have a list of dictionaries containing information about servers in your infrastructure. You need to extract specific values like server names, IP addresses, or operating systems for further processing in your Ansible playbook.

Scenario:

Imagine you have a list of dictionaries called servers representing your server inventory:

servers:
  - name: "server1"
    ip: "192.168.1.10"
    os: "Ubuntu 20.04"
  - name: "server2"
    ip: "192.168.1.11"
    os: "CentOS 7"
  - name: "server3"
    ip: "192.168.1.12"
    os: "Windows Server 2019"

You want to retrieve the names of all servers and store them in a separate variable for later use.

Solution:

Ansible provides various methods to extract values from lists of dictionaries. We'll explore the most common techniques:

1. Using the map Filter

The map filter iterates through each element of a list and applies a specified function to each element. In this case, we'll use it to extract the name value from each server dictionary.

- set_fact:
    server_names: "{{ servers | map(attribute='name') }}"

This code will create a new variable server_names containing a list of server names:

server_names:
  - "server1"
  - "server2"
  - "server3"

2. Using a Loop with a Set Fact

Another approach is to use a loop to iterate through the list of dictionaries and set the desired values to a new variable within the loop.

- set_fact:
    server_names: []
  delegate_to: localhost
- debug:
    msg: "{{ item.name }}"
  loop: "{{ servers }}"
  loop_control:
    loop_var: item
  when: server_names is undefined
- set_fact:
    server_names: "{{ server_names + [item.name] }}"
  loop: "{{ servers }}"
  loop_control:
    loop_var: item

This code first initializes an empty list server_names. Then, it iterates through each server dictionary and appends the name value to the server_names list.

3. Using the json_query Filter

The json_query filter allows you to query data using JSONPath syntax, a powerful way to select specific elements from JSON-like structures.

- set_fact:
    server_names: "{{ servers | map(attribute='name') | json_query('[*]') }}"

This code first extracts the name value from each dictionary using the map filter and then uses the json_query filter to extract all elements from the resulting list.

Choosing the Right Method

The best method to extract values from a list of dictionaries depends on the specific task and your preference.

• The map filter is a concise and efficient solution when you need to extract a single value from each dictionary.
• The loop method provides more flexibility when you need to perform more complex operations on each element.
• The json_query filter allows you to select elements based on complex criteria using JSONPath syntax.

Additional Considerations:

• Remember to use the delegate_to: localhost when working with set_fact inside a loop, as the loop itself does not perform the set_fact.
• Ansible provides numerous filters and modules for working with data structures. Explore the Ansible documentation for more advanced techniques.

Conclusion:

Extracting values from lists of dictionaries is a common task in Ansible playbooks. By using the appropriate methods, you can easily retrieve the data you need for further processing. This article provides a comprehensive guide with practical examples and insights to help you master this essential technique.