ansible-playbook with_flattened migration to loop - ansible

Question

I am trying to migrate my old playbooks that uses with_flattened to loop.
I tried to follow ansible user guide but failed to do so.
This is my host_var:
- hosts: example.com
vars:
- configureddisks:
- xvdb
- xvdc
- btrfsdisks:
- xvdf
- xvdg
My original task and its output is as follow:
# Task
- debug:
msg: "{{ item }}"
with_flattened:
- "{{ configureddisks | select('defined') | list }}"
- "{{ btrfsdisks | select('defined') | list }}"
# ansible-playbook output
TASK [devices : debug] **********************************************************************************************************************************************************************************************************************
ok: [example.com] => (item=xvdb) => {
"msg": "xvdb"
}
ok: [example.com] => (item=xvdc) => {
"msg": "xvdc"
}
ok: [example.com] => (item=xvdf) => {
"msg": "xvdf"
}
ok: [example.com] => (item=xvdg) => {
"msg": "xvdg"
}
My new task and its output is as follow:
# Task
- debug:
msg: "{{ item }}"
loop:
- "{{ configureddisks | select('defined') | list | flatten }}"
- "{{ btrfsdisks | select('defined') | list | flatten }}"
# ansible-playbook output
ok: [example.com] => (item=[u'xvdb', u'xvdc']) => {
"msg": [
"xvdb",
"xvdc"
]
}
ok: [example.com] => (item=[u'xvdf', u'xvdg']) => {
"msg": [
"xvdf",
"xvdg"
]
}
How should I write the new task using loop so that it has the same outputs as the old task?

Answer 1

You are misuing the flatten filter. When you write this:
loop:
- "{{ configureddisks | select('defined') | list | flatten }}"
- "{{ btrfsdisks | select('defined') | list | flatten }}"
The flatten filter has no effect: you are providing as input (twice) a list that is already flattened. You would need to apply the filter to the generated list, but rather than doing that, you can rewrite your expression so that no flattening is necessary:
- debug:
msg: "{{ item }}"
loop: "{{ (configureddisks + btrfsdisks) | select('defined') | list }}"
If you really wanted to go the "build a list of lists and flatten it" route, that might look something like:
- debug:
msg: "{{ item }}"
loop: >-
{{
(
(configureddisks | select('defined') | list) +
(btrfsdisks | select('defined') | list)
)|flatten
}}
Note that I've spread that across multiple lines for legibility, but you could just as easily write it all on one line:
loop: "{{ ((configureddisks | select('defined') | list) + (btrfsdisks | select('defined') | list))|flatten }}"