- name: Set default value to db_type variable
  set_fact:
    db_type: "{{ lookup('env', 'ACRAEEPG_DB_TYPE') | default(db_type_default) }}"

- name: Define DB variables
  set_fact:
    db_port: "{{ {'postgresql':'5432', 'mysql':'3306'}[db_type] | default('') }}"
    db_image: "{{ {'postgresql':'postgres:11', 'mysql':'mariadb:10'}[db_type] | default('') }}"

TASK [variables : Set default value to db_type variable] ***********************
ok: [17***********]

TASK [variables : Define DB variables] *****************************************
ok: [17***********]

TASK [variables : fail] ********************************************************
fatal: [17*********]: FAILED! => {"changed": false, "msg": "db_type is incorrect"}

ansible-playbook 2.9.6
  config file = /etc/ansible/ansible.cfg
  configured module search path = ['/home/shmel/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /usr/lib/python3/dist-packages/ansible
  executable location = /usr/bin/ansible-playbook
  python version = 3.8.5 (default, Jul 28 2020, 12:59:40) [GCC 9.3.0]

TASK [variables : Set default value to db_type variable] **
task path: /home/shmel/git/project/ansible/roles/variables/tasks/main.yaml:18
ok: [178.*********] => {
    "ansible_facts": {
        "db_type": ""
    },
    "changed": false
}

TASK [variables : Define DB variables] **
task path: /home/shmel/git/project/ansible/roles/variables/tasks/main.yaml:22
ok: [178.*********] => {
    "ansible_facts": {
        "db_image": "",
        "db_port": ""
    },
    "changed": false
}

TASK [variables : fail] **
task path: /home/shmel/git/project/ansible/roles/variables/tasks/main.yaml:27
fatal: [178.*********]: FAILED! => {
    "changed": false,
    "msg": "db_type is incorrect"
}

db_type_default: 'postgresql'