Ansible cannot set hosts: value as previously defined variable with set_fact - ansible

Question

I have checked this subject on SO, but still cannot fix my issue.
I set variable in play 1, to use in hosts: value in play 2, But Ansible 2.9 says, undefined Variable.
Can someone help pls?
Below is the example :
---
############ Play 1 ###############
- name: Test Play 1
hosts: localhost
gather_facts: no
tasks:
- name: Set facts
set_fact:
action_host: localhost
############ Play 2 ###############
- name: Test Play 2
hosts: "{{ action_host }}"
#hosts: localhost
gather_facts: no
tasks:
- name: Test Play 2
shell: |
echo toto
Output:
PLAY [Test Play 1] ***********************************************************************************************
TASK [Set facts] *************************************************************************************************
ok: [localhost]
ERROR! The field 'hosts' has an invalid value, which includes an undefined variable. The error was: 'action_host' is undefined
Tried This, but Still Error:
############ Play 2 ###############
- name: Test Play 2
#hosts: localhost
hosts: hostvars.localhost.action_host
gather_facts: no
tasks:
- name: Test Play 2
shell: |
echo toto
Output :
[WARNING]: Could not match supplied host pattern, ignoring: hostvars.localhost.action_host
PLAY [Test Play 2] ***********************************************************************************************
skipping: no hosts matched
Update:
So I've done half a way with the solution provided by #Zeitounator .
It seems set_facts works between the plays, as long as It's on the same host. (here localhost).
It's a pity I cannot change dynamically hosts: between plays.
My idea is to find the "correct host" where the VM has specific type of disks, and then run the play 2 only on this host. And this host is already in the inventory.
Need to find a different workaround.
Final yaml is here, if someone has an idea.
---
############ Play 1 ###############
- name: Test Play 1
hosts: localhost
gather_facts: yes
tasks:
- name: Set facts
set_fact:
action_host: localhost # OK with Play 2, only when localhost
#action_host: host0008.net.intra # NOT OK with play 2, even this host is in inventory
test_string: "Test string from Play 1" # OK with Play 2, only when action_host=localhost
run_once: yes # No effect
############ Play 2 ###############
- name: Test Play 2
#hosts: localhost # OK With play 2
# hosts: "{{ action_host }}" # Not OK, never
hosts: "{{ hostvars.localhost.action_host }}" # OK with Play 2, only when action_host=localhost
#hosts: "{{ hostvars[action_host].inventory_hostname }}" # Not OK
tasks:
- name: Test Play 2
shell: |
echo toto
- name: Display Play 1 variables
debug:
msg: "{{ test_string }}"
Output :
PLAY [Test Play 1] ***********************************************************************************************
TASK [Gathering Facts] *******************************************************************************************
ok: [localhost]
TASK [Set facts] *************************************************************************************************
ok: [localhost]
PLAY [Test Play 2] ***********************************************************************************************
TASK [Test Play 2] ***********************************************************************************************
changed: [localhost]
TASK [Display Play 1 variables] **********************************************************************************
ok: [localhost] => {
"msg": "Test string from Play 1"
}
PLAY RECAP *******************************************************************************************************
localhost : ok=4 changed=1 unreachable=0 failed=0 skipped=0 rescued=0 ignored=0
$ ansible-inventory --graph
#all:
|--#all_pure:
| |--#pure_dr:
| | |--host0003.net.intra
| | |--host0008.net.intra
| |--#pure_prod:
| | |--host60003.net.intra
| | |--host60008.net.intra
|--#local:
| |--localhost
|--#ungrouped: