How to prepend every string in a list with a prefix in Ansible

Let’s say you have an external dict (hostvars) where you take one element by a key (f.e. hostvars[host].foobar). This element is a list. You need to keep it as list and prepend (add at front) a string. Moreover, you need to join those lists together.

Here my answer. To simplify our task we will have: hv as imitation of hostvars, and, a list of keys for hv, imitation of groups.groupname. We want to join all foobars from hv with keys from gr, prepending every element with a fixed prefix prefix.

My solution:

---
- hosts: localhost
  gather_facts: false
  tasks:
   -set_fact:
     x: '{{ x|d([]) + [p]|product(hv[item].foo)|map("join")|list }}'
    with_items: '{{ gr }}'
   - debug: var=x
  vars:
    hv:
      host1:
        foobar:
         - element one
         - element two
        junk:
          - to annoy you
      host2:
        foobar:
          - element 42
      host3:
        foobar: []
        junk: to annoy you
      host4:
        - should be ignored
    gr:
      - host1
      - host2
      - host3
    p: 'The '

Output is:

ok: [localhost] => {
    "output": [
        "The element one",
        "The element two",
        "The element 42"
    ]
}

Explanation

1. hv[item].foo is an imitation of a normal hostvars[item].foo
2. [p]|product(any_list) produces pairs of p and the next each next element of any_list (Cartesian product). (I use ‘p’ for prefix to fit the expression on the one line).
3. map(“join”)|list apply join function to each pair, producing a string “{p}{element}” (it is applied to each pair from step 2). Additionally, list converts from generator to list.
4. x|d([]) + everything else uses d (default) filter to assign x to empty list if x is undefined and adds every next expression into it.

Word of caution

You should be really careful with such tricks. Putting too many filters in one line makes code unreadable.