How to skip null values in ansible loop - loops

Question

I need to skip the value null from a list in ansible loop. I am using the when condition, still the null value gets printed.
Below is my playbook:
- hosts: localhost
vars:
show:
- read
- write
- null
- test
val: []
tasks:
- name: Fact
set_fact:
val: "{{val+[item]}}"
loop: "{{show}}"
when: item != "null"
- name: Print
debug:
msg: "{{val}}"
Output:
TASK [Print] ***
ok: [localhost] => {
"msg": [
"read",
"write",
null,
"test"
]
}
Please advise.

Answer 1

Quoting from YAML 10.2.1.1. Null
"Represents the lack of a value. This is typically bound to a native null-like value (e.g., undef in Perl, None in Python). ..."
There are more options on how to test null.
Compare to Python None
when: item != None
Use Jinja test none
when: item is not none
If you for whatever reason have to compare to a string the Jinja filter string converts null to string 'None'
when: item|string != 'None'
The most efficient way is to remove null values from the list before the iteration
loop: "{{ show|reject('none')|list }}"

Answer 2

I don't know if you want to check if the variable is "null" as a text or if the variable doesn't have a value and it's nullified so I will write both examples :), make null value a string and compare as a string:
- hosts: localhost
vars:
show:
- read
- write
- "null"
- test
val: []
tasks:
- name: Fact
set_fact:
val: "{{val+[item]}}"
loop: "{{show}}"
when: "'{{item}}' != 'null'"
- name: Print
debug:
msg: "{{val}}"
Test if the value is nullified and it really doesn't have any value we will use, is none for that example:
- hosts: localhost
vars:
show:
- read
- write
- null
- test
val: []
tasks:
- name: Fact
set_fact:
val: "{{val+[item]}}"
loop: "{{show}}"
when: item is not none
- name: Print
debug:
msg: "{{val}}"