Ansible variable 'null' vs. 'None' - ansible

Question

From what I understood reading about the topic, null and "{{ None }}" are basically the same thing in Ansible. The difference is that the former is language agnostic YAML syntax and the latter is Python specific (I do not speak that language so I do not know how correct that is). However, as the following Ansible playbook shows, they are different.
- hosts: localhost
vars:
a: null
b: "{{ None }}"
tasks:
- name: a always
debug:
var: a
- name: a if none
debug:
var: a
when: a==None
- name: a if not none
debug:
var: a
when: a!=None
- name: b always
debug:
var: b
- name: b if none
debug:
var: b
when: b==None
- name: b if not none
debug:
var: b
when: b!=None
The output from the above is this:
PLAY [localhost] *******************************************************************************************************************************************************************************
TASK [Gathering Facts] *************************************************************************************************************************************************************************
ok: [localhost]
TASK [a always] ********************************************************************************************************************************************************************************
ok: [localhost] => {
"a": null
}
TASK [a if none] *******************************************************************************************************************************************************************************
ok: [localhost] => {
"a": null
}
TASK [a if not none] ***************************************************************************************************************************************************************************
skipping: [localhost]
TASK [b always] ********************************************************************************************************************************************************************************
ok: [localhost] => {
"b": ""
}
TASK [b if none] *******************************************************************************************************************************************************************************
skipping: [localhost]
TASK [b if not none] ***************************************************************************************************************************************************************************
ok: [localhost] => {
"b": ""
}
PLAY RECAP *************************************************************************************************************************************************************************************
localhost : ok=5 changed=0 unreachable=0 failed=0 skipped=2 rescued=0 ignored=0
I am not so much concerned about the output null vs. "", but what really bothers me is the different test results a==None compared to b==None.
Can somebody please help me understand what I am missing here?

Answer 1

Regarding
From what I understood reading about the topic, null and "{{ None }}" are basically the same thing in Ansible.
there was a longer conversation under Ansible Issue #7984 "Ansible substitutes YAML null value with "None" in templates".
The difference is that the former is language agnostic YAML syntax and the latter is Python specific
Right, such is discussed under the above mentioned issue.
From the discussion there I understand also that such test is meant for testing defined. And since
To check if it is defined and truthy ... The string "None" evaluates as truthy and defined, the actual python None evaluates as "falsey" and defined.
a test might look like
---
- hosts: localhost
become: false
gather_facts: false
vars:
a: null
b: "{{ None }}"
tasks:
- name: a if none
debug:
var: a
when: a == None
- name: b if none
debug:
var: b
when: b is defined and not b
Further Discussions and Documentation
Ansible Issue #37441 "Ansible incorrectly evaluates 'null' as a variable name in when statements"
Ansible: How to check if a variable is not null?
How to create a null default in Ansible?
Making variables optional
If you are interested more in they types of the variables you may have a look into
---
- hosts: localhost
become: false
gather_facts: false
vars:
a: null
b: "{{ None }}"
c: !!null
tasks:
- name: a if none
debug:
msg: 'a: "{{ a }}" is of {{ a | type_debug }}'
when: a == None
- name: b if none
debug:
msg: 'b: "{{ b }}" is of {{ b | type_debug }}'
when: b is defined and not b
- name: c if none
debug:
msg: "{{ c }}"
when: c == None
resulting into an output of
TASK [a if none] ************
ok: [localhost] =>
msg: 'a: "" is of NoneType'
TASK [b if none] ************
ok: [localhost] =>
msg: 'b: "" is of unicode'
TASK [c if none] ************
ok: [localhost] =>
msg: null