How to loop over Ansible variable names

Question

I'd like to find all variables with the myvar_ prefix and make a shell script that uses their names and contents.

---
- set_fact:
   myvar_a: "a"
   myvar_b: "b"
   myvar_c: "c"

- name: Format Django webapp parameters
  shell: |
    truncate -s0 out.txt
    {% for param in lookup('ansible.builtin.varnames', '^myvar_.+') %}
    echo {{ param }}  >> out.txt
    {% endfor %}

The output is:

m
y
v
a
r
_
a
,
m
y
v
a
r
_
b
,
m
y
v
a
r
_
c

It iterates a string character by character for some reason.

How to get variable names into out.txt?
How to get the contents of these variables to out.txt? (ok, it'll be {{ lookup("vars", param) }})

Answer 1

This happens because the lookup return you a comma separated list of variable names, not a list.

As demonstrated by the task:

- debug:
    var: lookup('ansible.builtin.varnames', '^myvar_.+')
  vars:
    myvar_a: a
    myvar_b: b
    myvar_c: c

Which yields:

ok: [localhost] => 
  lookup('ansible.builtin.varnames', '^myvar_.+'): myvar_a,myvar_b,myvar_c

There are three fixes possible:

1. use the wantlist=true named argument of the lookup:

   {% for param in lookup(
        'ansible.builtin.varnames', 
        '^myvar_.+', 
        wantlist=true
      ) 
   %}
   

2. use a query instead of a lookup, as query always returns a list:

   {% for param in query(
        'ansible.builtin.varnames', 
        '^myvar_.+'
      ) 
   %}
   

3. split the string:

   {% for param in lookup(
        'ansible.builtin.varnames', 
        '^myvar_.+'
      ).split(',') 
   %}