Ansible lookup or conditionals

There are multiple ways to how Ansible deals with differences in distributions. Take these 2 examples:

Conditional

Here is an example that uses when to differentiate for different situations:

- name: Install packages
  ansible.builtin.package:
    - name: python3-docker
  when:
    - ansible_os_family == "Debian"

The example above installs python3-docker if the managed node is “Debian”. (ansible_os_family includes Debian, Ubuntu and others.

Lookup

Another method is to make a map of distributions and define the required packages:

_my_packages:
  Debian:
    - python3-docker

my_packages: "{{ _my_packages[ansible_os_family] | default([]) }}"

Above you see the map _my_packages. It’s basically a variable that is a dictionary or map. In this case the intent is to map different operating systems to a list of packages.

The my_packages variable looks up a value from the _my_packages map. The results in:

• python3-docker when the ansible_os_family is Debian.
• [] (an empty list) for any other ansible_os_family.

And just to complete the example: I prefer this method:

# vars/main.yml
_my_packages:
  default: []
  Debian:
    - python3-docker

my_packages: "{{ _my_packages[ansible_os_family] | default('default') }}"

# tasks/main.yml
- name: Install packages
  ansible.builtin.package:
    - name: "{{ my_packages }}"

As a maintainer of the above code, you can focus on the _my_packages map/dict. In my experience, this is simpler; Think of the logic once (tasks/main.yml or playbook.yml) and focus on “data” later. (vars/main.yml or anywhere else you’d put variables.)

By the way, I’m not sure how to call this mechanism, I guess “lookup” is pretty close.

Conclusion

You can use either of the above examples; I prefer the lookup method, but I see others using the conditional method more often.

The best argument for me is ease of maintentance or readability.