[09:50] myuser@machine:~/$ cat test.yml
---
- hosts: localhost
  gather_facts: true
  become: true
  tasks:
    - set_fact:
        my_user: myuser
    - debug: msg="{{ lookup('pipe', 'id -u ' + my_user) }}"
[09:50] myuser@machine:~/$ ansible-playbook test.yml
SUDO password:

PLAY ***************************************************************************

TASK [setup] *******************************************************************
ok: [localhost]

TASK [set_fact] ****************************************************************
ok: [localhost]

TASK [debug] *******************************************************************
ok: [localhost] => {
    "msg": "5670"
}

PLAY RECAP *********************************************************************
localhost                  : ok=3    changed=0    unreachable=0    failed=0