0-inventories-test_inventory-hosts.ini

[test_group]
localhost test_var=88

# This is the group definition used for the second run (remove previous)
# [test_group]
# localhost

[test_group:vars]
test_var=99

1-roles-test_role-tasks-defaults.yml

---
test_var: 77

2-roles-test_role-tasks-main.yml

---
- name: Debug value of test_var
  debug:
    var: test_var

3-playbook.yml

---
- hosts: test_group
  gather_facts: false

  roles:
    - role: test_role

4-session.txt

$ # Please note that gists do not accept subdirs in file names, so I replaced / with - above for the paths.
$ # I also prepended each file with a number that is not part of the path to keep a coherent order
$ # Commands below are using the resulting file tree and are run from the playbook root.

$ # Run playbook with initial inventory
$ ansible-playbook -i inventories/test_inventory/hosts.ini playbook.yml 

PLAY [test_group] **********************************************************************************************************************************************************************************************************************

TASK [test_role : Debug value of test_var] *********************************************************************************************************************************************************************************************
ok: [localhost] => {
    "test_var": 88
}

PLAY RECAP *****************************************************************************************************************************************************************************************************************************
localhost                  : ok=1    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0  



$ # Now edit the inventory file and change for the group def that is commented out in my example
$ ansible-playbook -i inventories/test_inventory/hosts.ini playbook.yml 

PLAY [test_group] **********************************************************************************************************************************************************************************************************************

TASK [test_role : Debug value of test_var] *********************************************************************************************************************************************************************************************
ok: [localhost] => {
    "test_var": 99
}

PLAY RECAP *****************************************************************************************************************************************************************************************************************************
localhost                  : ok=1    changed=0    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0