Ansible: Set variable only if undefined

- set_fact: namespace="default_namespace"
  when: namespace is undefined

In case you need to set your variable by priority order, likewise:

1. Set variable as var_priority_1.
   But if it is not defined, then:
2. Set variable as var_priority_2.
   But if it is not defined either, then:
3. Set variable to a default value.

I had to use such condition while trying to get value from inventory hosts variables, that sometimes need to be overridden by role vars:

shell: |
    ...
    shell_var={{ role_var if role_var is defined else hostvars[item].host_var | default('defalut_value') }}
    ...
  with_items: "{{ groups[hosts_group_iterator] }}"

It seems like you are taking a wrong approach.

Take a look at the Ansible documentation concerning variable precedence. It is a built-in feature of Ansible to use the default variable if the variable is not defined.

In Ansible 2.x the variable precedence starts like this:

role defaults

inventory vars

So if you want to define a default value for a variable you should set it in role/defaults/main.yml. Ansible will use that value only if the variable is not defined somewhere else.

Another option is to use a Jinja2 filter. With a Jinja filter you can set a default value for a variable like this:

{{ some_variable | default(5) }}