How to assign an empty value to a variable in Ansible?

Question

If firewall_allowed_ports in:

- name: port {{ item }} allowed in firewall
  ufw:
    rule: allow
    port: "{{ item }}"
    proto: tcp
  with_items: 
    - 22
    - "{{ firewall_allowed_ports }}"

is undefined, then this error occurs:

fatal: [host.example.com]: FAILED! => {"failed": true, "msg": "the field 'args' 
has an invalid value, which appears to include a variable that is undefined.

Attempt to solve the issue

"{{ firewall_allowed_ports | }}"

results in:

fatal: [host.example.com]: FAILED! => {"failed": true, "msg": "template error
while templating string: expected token 'name', got 'end of print statement'. 
String: {{ firewall_allowed_ports | }}"}

Answer 1

Use default([]) to create an empty list if firewall_allowed_ports is undefined. The with_items will skip it when empty.

- name: port {{ item }} allowed in firewall
  ufw:
    rule: allow
    port: "{{ item }}"
    proto: tcp
  with_items: 
    - 22
    - "{{ firewall_allowed_ports | default([]) }}"