Ansible how to ignore undefined item in a loop - ansible

Question

I'm trying to have list of paths.
The variables folder (web_folder, app_folder or db_folder ) may be undefined and is expected to be undefined.
In this case I just don't want the undefined value in the the list.
- set_fact: root_paths="{{ root_paths | default([]) + [ item ] }}"
loop:
- "{{ web_folder }}"
- "{{ app_folder }}"
- "{{ db_folder }}"
when: item is defined
When I do this I get an error message 'The task includes an option with an undefined variable.'
I can make this work if I define default values e.g. '-' and replace the when condition with
when: item != '-'
I don't like this solution.
I tried a few things like
'when: vars[item] is undefined' from this post, but it didn't work for me.
I also replaces 'loop' with 'with_items' didn't work either
Any suggestions?

Answer 1

The variables folder (web_folder, app_folder or db_folder ) may be undefined and is expected to be undefined.
you could use the default filter to initialize them to an empty array, in case they are not defined:
- set_fact: root_paths="{{ root_paths | default([]) + [ item ] }}"
loop:
- "{{ web_folder | default([]) }}"
- "{{ app_folder | default([]) }}"
- "{{ db_folder | default([]) }}"
when: item is defined
hope it helps.

Answer 2

Q: I get an error message 'The task includes an option with an undefined variable.' I can make this work if I define default values e.g. '-' and replace the when condition with when: item != '-'. I don't like this solution.
A: The expansion of an undefined variable causes the error. Either the variables are tested explicitly or default filter is used.
Testing each variable is cumbersome
tasks:
- set_fact:
folders_defined: []
- set_fact:
folders_defined: "{{ folders_defined + [web_folder] }}"
when: web_folder is defined
- set_fact:
folders_defined: "{{ folders_defined + [app_folder] }}"
when: app_folder is defined
- set_fact:
folders_defined: "{{ folders_defined + [db_folder] }}"
when: db_folder is defined
- debug:
var: folders_defined
The next option (which is very similar to the one you don't like) is to test a default value assigned to undefined variables. For example, the playbook below tests default empty string. If necessary fit this condition to your needs.
- hosts: localhost
vars:
web_folder: /my/web/folder
app_folder: /my/app/folder
folders: [web_folder, app_folder, db_folder]
tasks:
- set_fact:
folders_defined: "{{ folders_defined|default([]) +
[lookup('vars', item)] }}"
loop: "{{ folders }}"
when: lookup('vars', item, default='')|length > 0
- debug:
var: folders_defined
gives
ok: [localhost] => {
"folders_defined": [
"/my/web/folder",
"/my/app/folder"
]
}