Example of using the `default()` jinja filter with a variable

default-w-var-sub.yml

# 1.                                                                                                                                                                                                                                           
#    $ ansible-playbook default-w-var-sub.yml                                                                                                                                                                                                  
#                                                                                                                                                                                                                                              
#    PLAY [Test default variable using substitution] ********************************                                                                                                                                                          
#                                                                                                                                                                                                                                              
#    TASK [setup] *******************************************************************                                                                                                                                                          
#    ok: [localhost]                                                                                                                                                                                                                           
#                                                                                                                                                                                                                                              
#    TASK [debug] *******************************************************************                                                                                                                                                          
#    ok: [localhost] => {                                                                                                                                                                                                                      
#        "override_variable": "I am a default"                                                                                                                                                                                                 
#        }                                                                                                                                                                                                                                     
#                                                                                                                                                                                                                                              
#        PLAY RECAP *********************************************************************                                                                                                                                                      
#        localhost                  : ok=2    changed=0    unreachable=0    failed=0                                                                                                                                                           
#                                                                                                                                                                                                                                              
# 2.                                                                                                                                                                                                                                           
#    $ ansible-playbook default-w-var-sub.yml -e 'user_provided_var="what"'                                                                                                                                                                    
#                                                                                                                                                                                                                                              
#    PLAY [Test default variable using substitution] ********************************                                                                                                                                                          
#                                                                                                                                                                                                                                              
#    TASK [setup] *******************************************************************                                                                                                                                                          
#    ok: [localhost]                                                                                                                                                                                                                           
#                                                                                                                                                                                                                                              
#    TASK [debug] *******************************************************************                                                                                                                                                          
#    ok: [localhost] => {                                                                                                                                                                                                                      
#        "override_variable": "what"                                                                                                                                                                                                           
#        }                                                                                                                                                                                                                                     
#                                                                                                                                                                                                                                              
#        PLAY RECAP *********************************************************************                                                                                                                                                      
#        localhost                  : ok=2    changed=0    unreachable=0    failed=0                                                                                                                                                           
---
- name: Test default variable using substitution
  hosts: localhost
  vars:
      default_variable: "I am a default"
      override_variable: "{{ user_provided_var | default(default_variable) }}"
  tasks:
      - debug: var=override_variable