How to validate list of host names from extra variables via `assert`?

I have an Ansible playbook which requires --extra-vars to be passed and would like to know if there is any way that validate the hostnames from the --extra-vars values is within the variables.

Sample playbook test.yml

- hosts: localhost
  gather_facts: false
  vars:
    prg:
      prod_mgmt_network: "{{ ['prodsrv00']|product(range(prod_start,prod_end))|map('join')|list }}"
      test_mgmt_network: "{{ ['testsrv00']|product(range(test_start,test_end))|map('join')|list }}"
    prod_start: 4281
    prod_end: 4283
    test_start: 8281
    test_end: 8283


  tasks:
    - debug:
        var: prg.prod_mgmt_network, prg.test_mgmt_network

Sample command

ansible-playbook --extra-vars hostname=['prodsrv004281', 'prodsrv004282', 'testsrv008281', 'testsrv008282', 'testsrv008283', 'testsrv008284'] test.yml

I would like to fail the task if a hostname is not within the variable. for example, testsrv008283 and testsrv008284 should fail the task.

Solution

The first problem is the format of the extra variables. See Defining variables at runtime. The key=value (INI) syntax are always interpreted as strings. You might want to test it. For example, the below play

shell> cat pb.yml
- hosts: localhost

  tasks:

    - debug:
        var: hostname
    - debug:
        var: hostname|type_debug

gives (abridged)

shell> ansible-playbook pb.yml -e hostname=['host1','host2']

  hostname: '[host1,host2]'

  hostname|type_debug: str

There are more options for how to get a list:

Convert the string to a list

  hostname_list: "{{ hostname|from_yaml }}"

Then, you'll get

  hostname_list:
  - host1
  - host2

  hostname_list|type_debug: list

Use JSON format

shell> ansible-playbook pb.yml -e '{"hostname": ["host1", "host2"]}'

  hostname:
  - host1
  - host2

  hostname_list|type_debug: list

Put the extra variable(s) into a file. For example,

shell> cat hostname.yml 
hostname:
  - host1
  - host2

Then, the below play gives the same result

shell> ansible-playbook pb.yml -e @hostname.yml

For testing your case, the simplest option is to put the list into a file

shell> cat hostname.yml
hostname:
  - prodsrv004281
  - prodsrv004282
  - testsrv008281
  - testsrv008282
  - testsrv008283
  - testsrv008284

Q: "Fail if hostname is not within the variable. For example, testsrv008283 and testsrv008284 should fail."

A: Select the lists of hosts and compare the differences. For example,

  prod_hosts: "{{ hostname|select('match', 'prodsrv') }}"
  test_hosts: "{{ hostname|select('match', 'testsrv') }}"

gives

  prod_hosts:
  - prodsrv004281
  - prodsrv004282

  test_hosts:
  - testsrv008281
  - testsrv008282
  - testsrv008283
  - testsrv008284

Evaluation of the networks gives

  prg:
    prod_mgmt_network:
    - prodsrv004281
    - prodsrv004282
    test_mgmt_network:
    - testsrv008281
    - testsrv008282

The below assert will pass

    - assert:
        that: prod_out|length == 0
        fail_msg: "Hosts: {{ prod_out }} out of range."
      vars:
        prod_out: "{{ prod_hosts|difference(prg.prod_mgmt_network) }}"

ok: [localhost] => changed=false 
  msg: All assertions passed

but, this one will fail

    - assert:
        that: test_out|length == 0
        fail_msg: "Hosts: {{ test_out }} out of range."
      vars:
        test_out: "{{ test_hosts|difference(prg.test_mgmt_network) }}"

fatal: [localhost]: FAILED! => changed=false 
  assertion: test_out|length == 0
  evaluated_to: false
  msg: 'Hosts: [''testsrv008283'', ''testsrv008284''] out of range.'

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

vars:

prg:
prod_mgmt_network: "{{ ['prodsrv00']|
product(range(prod_start,prod_end))|map('join') }}"
test_mgmt_network: "{{ ['testsrv00']|
product(range(test_start,test_end))|map('join') }}"
prod_start: 4281
prod_end: 4283
test_start: 8281
test_end: 8283

prod_hosts: "{{ hostname|select('match', 'prodsrv') }}"
test_hosts: "{{ hostname|select('match', 'testsrv') }}"

tasks:

- debug:
var: prg.prod_mgmt_network
- debug:
var: prg.test_mgmt_network
- debug:
var: hostname

- debug:
var: prod_hosts
- debug:
var: test_hosts

- assert:
that: prod_out|length == 0
fail_msg: "Hosts: {{ prod_out }} out of range."
vars:
prod_out: "{{ prod_hosts|difference(prg.prod_mgmt_network) }}"

- assert:
that: test_out|length == 0
fail_msg: "Hosts: {{ test_out }} out of range."
vars:
test_out: "{{ test_hosts|difference(prg.test_mgmt_network) }}"}