ansible - example of appending to list one or more vars and handling '' or undefined.

01_play.yml

---
- hosts: localhost
  connection: local
  gather_facts: false
  vars:
    mylist:
      - one
      - two
      - three
    myvar1: foo
    myvar2: bar
    myvar3: ''
    # myvar4:
    listofvars:
      - myvar1
      - myvar2
      - myvar3
      - myvar4
  tasks:

    # these are all equivalent

    # select('ne', '') is equivalent to reject('eq', '')
    - name: using default per var and filtering out '' with select or reject filter
      set_fact:
        newlist1: "{{ (mylist + [myvar1|default('', true)] + [myvar2|default('', true)] + [myvar3|default('', true)] + [myvar4|default('', true)])|select('ne', '')|list }}"

    # |default('', true) is equivalent to  (var is defined)|ternary([var],[''])
    - name: using ternary per var (instead of default) and filtering out ''
      set_fact:
        newlist2: "{{ (mylist + (myvar1 is defined)|ternary([myvar1],[]) + (myvar2 is defined)|ternary([myvar2],[]) + (myvar3 is defined)|ternary([myvar3],[]) + (myvar4 is defined)|ternary([myvar4],[]))|select('ne', '')|list }}"

    - name: using loop against static list of vars
      set_fact:
        newlist3: "{{ newlist3|default(mylist) + [item] }}"
      when: item != ''
      loop: "{{ [myvar1, myvar2, myvar3, myvar4]|select('defined')|list }}"

    # select('defined')|select('ne', '') is equivalent to reject('undefined')|reject('eq', '')
    - name: using select or reject filters only
      set_fact:
        newlist4: "{{ newlist4|default(mylist) + [myvar1, myvar2, myvar3, myvar4]|select('defined')|select('ne', '')|list }}"

    - name: using loop against dynamic list of vars
      set_fact:
        newlist5: "{{ newlist5|default(mylist) + [lookup('vars', item, default='')]|select('ne', '')|list }}"
      loop: "{{ listofvars }}"

    - debug:
        var: newlist5

02_output

PLAY [localhost] ***********************************************************************************************************************************************************************************************************************************************************************************************************************************************************

TASK [using default per var and filtering out '' with select or reject filter] *********************************************************************************************************************************************************************************************************************************************************************************************
ok: [localhost]

TASK [using ternary per var (instead of default) and filtering out ''] *****************************************************************************************************************************************************************************************************************************************************************************************************
ok: [localhost]

TASK [using loop against static list of vars] ******************************************************************************************************************************************************************************************************************************************************************************************************************************
ok: [localhost] => (item=foo)
ok: [localhost] => (item=bar)
skipping: [localhost] => (item=)

TASK [using select or reject filters only] *********************************************************************************************************************************************************************************************************************************************************************************************************************************
ok: [localhost]

TASK [using loop against dynamic list of vars] *****************************************************************************************************************************************************************************************************************************************************************************************************************************
ok: [localhost] => (item=myvar1)
ok: [localhost] => (item=myvar2)
ok: [localhost] => (item=myvar3)
ok: [localhost] => (item=myvar4)

TASK [debug] ***************************************************************************************************************************************************************************************************************************************************************************************************************************************************************
ok: [localhost] => {
    "newlist5": [
        "one",
        "two",
        "three",
        "foo",
        "bar"
    ]
}

PLAY RECAP *****************************************************************************************************************************************************************************************************************************************************************************************************************************************************************
localhost                  : ok=6    changed=0    unreachable=0    failed=0