How to split an ansible role's `defaults/main.yml` file into multiple files?

The feature I'm describing below has been available since Ansible 2.6, but got a bugfix in v2.6.2 and another (minor) one in v2.7.
To see a solution for older versions, see Paul's answer.

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defaults/main/

Instead of creating defaults/main.yml, create a directory — defaults/main/ — and place all YAML files in there.

• defaults/main.yml → defaults/main/*.yml

Ansible will load any *.yml file inside that directory, so you can name your files like roles/my-role/defaults/main/{1,2}.yml.

Note, the old file — defaults/main.yml — must not exist. See this Github comment.

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vars/main/

By the way, the above solution also works for vars/:

• vars/main.yml → vars/main/*.yml

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further details

The feature has been introduced in v2.6 — git commit, Pull Request, main Github issue.

There have been two bugfixes:

• v2.7 fix: git commit, Pull Request — backported to v2.6.2: commit, Pull Request
• v2.7 fix: git commit, Pull Request, bug discussion

If you aren't using 2.6 (which you probably should, but I understand that isn't always an option), then you might find include_vars useful.

- name: Include vars of stuff.yaml into the 'stuff' variable (2.2).
  include_vars:
    file: stuff.yaml
    name: stuff

- name: Conditionally decide to load in variables into 'plans' when x is 0, otherwise do not. (2.2)
  include_vars:
    file: contingency_plan.yaml
    name: plans
  when: x == 0

- name: Load a variable file based on the OS type, or a default if not found. Using free-form to specify the file.
  include_vars: "{{ item }}"
  with_first_found:
    - "{{ ansible_distribution }}.yaml"
    - "{{ ansible_os_family }}.yaml"
    - default.yaml

- name: Bare include (free-form)
  include_vars: myvars.yaml

- name: Include all .json and .jsn files in vars/all and all nested directories (2.3)
  include_vars:
    dir: vars/all
    extensions:
        - json
        - jsn

- name: Include all default extension files in vars/all and all nested directories and save the output in test. (2.2)
  include_vars:
    dir: vars/all
    name: test

- name: Include default extension files in vars/services (2.2)
  include_vars:
    dir: vars/services
    depth: 1

- name: Include only files matching bastion.yaml (2.2)
  include_vars:
    dir: vars
    files_matching: bastion.yaml

Note that this is a task directive, though. It isn't as neat as just being able to include it into the defaults file itself.