Ansible - Use a variable as a slice value [0:var_name]

Is there a way to dynamically assign a variable as the slice amount when working with lists?

My goal is to grab a random 10% of hosts from an inventory file. Something along the lines of this:

tasks: 
- name: Get 10% of the servers 
  set_fact:
    win_amt: "{{ ((groups.windows_servers | length) * 0.1) | round(0, 'floor') | int }}"
    lin_amt: "{{ ((groups.linux_servers | length) * 0.1) | round(0, 'floor') | int }}"

- name: Get random hosts from windows and linux 
  set_fact:
    random_win: "{{ (groups.windows_servers | shuffle) [0:win_amt] }}"
    random_lin: "{{ (groups.linux_servers | shuffle) [0:lin_amt] }}"

This code produces the following error:

fatal: [localhost]: FAILED! => {"msg": "Unexpected templating type error occurred on ({{ (groups.windows_servers | shuffle) [0:win_amt] }}): slice indices must be integers or None or have an __index__ method"}

Has anyone done something similar to this or can point me to a better solution? This needs to be dynamic due to company policy requiring 10% of servers to be scanned periodically, and the amount of servers is subject to change as new resources are added/deleted.

Solution

Explicitly convert the slice limit to an integer. For example, given the inventory of 30 Windows and 70 Linux servers

shell> cat hosts
[win]
node[1:30]

[lin]
node[31:100]

Declare the 10% of the servers

  win_amt: "{{ (groups.win|length * 0.1)|round(0, 'floor') }}"
  lin_amt: "{{ (groups.lin|length * 0.1)|round(0, 'floor') }}"

gives

  win_amt: '3.0'
  lin_amt: '7.0'

Get the random nodes

  win_rnd: "{{ (groups.win|shuffle)[0:win_amt|int] }}"
  lin_rnd: "{{ (groups.lin|shuffle)[0:lin_amt|int] }}"

gives

  win_rnd: [node8, node4, node17]
  lin_rnd: [node71, node69, node61, node42, node40, node73, node99]

^{Example of a complete playbook for testing
- hosts: all

vars:

win_amt: "{{ (groups.win|length * 0.1)|round(0, 'floor') }}"
lin_amt: "{{ (groups.lin|length * 0.1)|round(0, 'floor') }}"
win_rnd: "{{ (groups.win|shuffle)[0:win_amt|int] }}"
lin_rnd: "{{ (groups.lin|shuffle)[0:lin_amt|int] }}"

tasks:

- block:

- debug:
var: win_amt
- debug:
var: lin_amt

- debug:
var: win_rnd|to_yaml
- debug:
var: lin_rnd|to_yaml

run_once: true}