Ansible: Update Existing Variable List

I can define a variable/list in my main playbook (main.yml) or in my group variables (group_vars/all.yml) but the main.yml variables have precedence over the group variables.

group_vars/all.yml

my_list:
 - "item 1"
 - "item 2"

main.yml

  vars:
    my_list:
      - "item 3"
      - "item 4"

I would like to combine the two but am not sure how to do it. Desired:

my_list: ['item 1', 'item 2', 'item 3', 'item 4']

I have tried adding to the list. For example if main.yml could see group_vars before overwriting (but this example throws an error).

    my_list:
     - "{{ my_list }}"
     - "item 3"
     - "item 4"

I am doing this because I am using code from another repository. I don't want to make changes to their code, but I do want to update (not overwrite) their variable list.

Solution

You can't do exactly what you want: you can only have one variable with a given name; there's no way to merge my_list in one place with my_list in another place. You can replace my_list, by doing something like:

- hosts: localhost
  gather_facts: false
  vars:
    my_new_list_content:
      - item 3
      - item 4
    my_new_list: "{{ my_list + my_new_list_content }}"

  tasks:
    - set_fact:
        my_list: "{{ my_new_list }}"

    - debug:
        var: my_list

Assuming that group_vars/all.yaml contains:

my_list:
  - item 1
  - item 2

Then running the above playbook will produce as output:

TASK [debug] ********************************************************************************************************************************************************************************************************************************************************************************
ok: [localhost] => {
    "my_list": [
        "item 1",
        "item 2",
        "item 3",
        "item 4"
    ]
}