How to block overriding variables by extra_vars in Ansible?

Question

I am using Ansible Tower to expose play. User stars job calling REST API and she/he provides some extra_vars. I have to validate provided variable against some other variables. For example: user provides hostname and I have in the inventory variable: allowed_hostnames. Problem is extra_vars trumps everything, so user can always override variable for list of allowed values, and test does not make sense. In Tower there is a Survey feature that can be used to limit variables allowed to change by user, but enabling Survey will block dict variables and I need it.

Answer 1

Q: "Problem is extra_vars trumps everything"

A: Avoid variable. For example the task

- debug:
    msg: "{{ my_host|default('') }} is allowed to ..."
  when: "my_host|default('') in  lookup('file', 'allowed_hosts.yml')|from_yaml"

with the data

$ cat allowed_hosts.yml
  - host1
  - host2
  - host3
  - host9

gives

$ ansible-playbook play.yml -e 'my_host=host2'

    "msg": "host2 is allowed to ..."

---

Next options might be pipe, redis, modgodb ... lookup plugins, custom filter, or custom lookup plugin.

Answer 2

Based on @Vladimir answer I have done this like that:

- name: Check variables
  fail:
  when: "{{ {'restricted_variables':restricted_variables} != lookup('file', 'restricted_variables.yml')|from_yaml }}"

where restricted_variables.yml:

restricted_variables:
  variable1: somevalue
  variable2:
    var1: 1
    var2: 2