Ansible set_facts based on OS version not working - ansible

Question

I want to set the patch based on the OS version. came up with this playbook in Ansible version 2.8. but its giving The task includes an option with an undefined variable. error message in the debug line.
---
- hosts: all
gather_facts: yes
vars:
patch_name_8: 'centos8-updates'
patch_name_7: 'centos7-updates'
tasks:
- name: Set fact for CentOS 7
set_fact:
install_patch_name: "{{ patch_name_7 }}"
when: ansible_distribution_major_version == 7
- name: Set fact for CentOS 8
set_fact:
install_patch_name: "{{ patch_name_8 }}"
when: ansible_distribution_major_version == 8
- name: patch name display
debug:
msg: "install {{ install_patch_name }}"
How to set the install_patch_name variable value based on the OS version?
adding the error message:
TASK [patch name display] ************************************************************************************************************
fatal: [host01]: FAILED! => {"msg": "The task includes an option with an undefined variable. The error was: 'install_patch_name' is undefined\n\nThe error appears to be in 't.yaml': line 23, column 7, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n\n
- name: patch name display\n ^ here\n"}
thanks

Answer 1

TLDR;
Use
when: ansible_distribution_major_version == "8"
or
when: ansible_distribution_major_version | int == 8
Explanation
Note: all following examples have been played against a centos:8 docker image.
The fact your are looking after is returned as a string :
[root#f6408271fc8c ~]# ansible localhost -m setup -a filter=ansible_distribution_major_version
localhost | SUCCESS => {
"ansible_facts": {
"ansible_distribution_major_version": "8"
},
"changed": false
}
Variables keep their types when using comparison and need to be correctly casted if need be, as demonstrated by the following playbook.
---
- hosts: localhost
tasks:
- name: default compare
debug:
msg: Comparison is true
when: ansible_distribution_major_version == 8
- name: compare as strings
debug:
msg: Comparison is true
when: ansible_distribution_major_version == "8"
- name: compare as ints
debug:
msg: Comparison is true
when: ansible_distribution_major_version | int == 8
Which gives
[root#f6408271fc8c ~]# ansible-playbook play.yml
PLAY [localhost] **************************************************************************************************************************************************************************************************
TASK [Gathering Facts] ********************************************************************************************************************************************************************************************
ok: [localhost]
TASK [default compare] ********************************************************************************************************************************************************************************************
skipping: [localhost]
TASK [compare as strings] *****************************************************************************************************************************************************************************************
ok: [localhost] => {
"msg": "Comparison is true"
}
TASK [compare as ints] ********************************************************************************************************************************************************************************************
ok: [localhost] => {
"msg": "Comparison is true"
}
PLAY RECAP ********************************************************************************************************************************************************************************************************
localhost : ok=3 changed=0 unreachable=0 failed=0 skipped=1 rescued=0 ignored=0