Ansible with_items if item is defined

Question

Ansible 1.9.4.

The script should execute some task only on hosts where some variable is defined. It works fine normally, but it doesn't work with the with_items statement.

- debug: var=symlinks
  when: symlinks is defined

- name: Create other symlinks
  file: src={{ item.src }} dest={{ item.dest }} state=link
  with_items: "{{ symlinks }}"
  when: symlinks is defined

But I get:

TASK: [app/symlinks | debug var=symlinks] *********************
skipping: [another-host-yet]

TASK: [app/symlinks | Create other symlinks] ******************
fatal: [another-host-yet] => with_items expects a list or a set

Maybe I am doing something wrong?

Answer 1

with_items: "{{ symlinks | default([]) }}"

Answer 2

The reason for this behavior is conditions work differently inside loops. If a loop was defined the condition is evaluated for every item while iterating over the items. But the loop itself requires a valid list.

This is also mentioned in the docs:

Note that when combining when with with_items (see Loops), be aware that the when statement is processed separately for each item. This is by design:

tasks:
  - command: echo {{ item }}
    with_items: [ 0, 2, 4, 6, 8, 10 ]
    when: item > 5

_{I think this is a bad design choice and for this functionality they better should have introduced something like with_when.}

As you have already figured out yourself, you can default to an empty list.

with_items: "{{ symlinks  | default([]) }}"

Finally if the list is dynamically loaded from a var, say x, use:

with_items: "{{ symlinks[x|default('')] | default([])}}" 

This will default to an empty list when 'x' is undefined

Accordingly, fall back to an empty dict with default({}):

# service_facts skips, then dict2items fails?
with_dict: "{{ ansible_facts.services|default({})|dict2items|selectattr('key', 'match', '[^@]+@.+\\.service')|list|items2dict }}"