How to run an ansible playbook only if a group exists? - ansible

Question

I wonder if anyone found a solution that would avoid displaying any warnings if an inventory group is undefined or empty.
I just want to make a section of the playbook run if a group exists and is not empty, skipping without warnings if not.
Please read https://github.com/ansible/ansible/issues/35255#issuecomment-388455001 and test alternatives because I spend a good amount of time trying to find a workaround for this issue.
So far I was not able to find any way to avoid the warnings when group is not defined.

Answer 1

I'm slightly unsure if I'm answering the right question, but here goes. I'm interpreting "if a group exists and is not empty" to mean "the currently executing host belongs to a certain group".
If you meant to ask something like "can I find out from the current host if any other hosts belong to a group that the current host does not belong to," or "can I run a playbook without errors when the hosts defined for some groups are unreachable," then I'm afraid this doesn't answer your question :)
But running a task based on whether or not the current host belongs to a group can be done with one of Ansible's default vars, groups group_names.
The following playbook contains two tasks, one to run a debug task when the current host belongs to the group existent, and one to run a debug task when the current host belongs to the group nonexistent. As the output shows, the first task runs and the second does not.
hosts.yml
[existent]
localhost ansible_connection=local
playbook.yml
- hosts: all
gather_facts: true
tasks:
- name: This command will run.
debug:
msg: "The group `existent_1` exists!"
when:
- "'existent_1' in groups"
- name: This command will not run.
debug:
msg: "The group `existent_1` exists and this host is in it!"
when:
- "'existent_1' in groups"
- "'existent_1' in group_names"
- name: This command will run.
debug:
msg: "The group `existent_2` exists and this host is in it!"
when:
- "'existent_2' in groups"
- "'existent_2' in group_names"
- name: This command will not run.
debug:
msg: "The group `nonexistent` exists!"
when:
- "'nonexistent' in groups"
- "'nonexistent' in group_names"
Output
➜ ansible-playbook -i hosts.yml playbook.yml
PLAY [all] *********************************************************************
TASK [Gathering Facts] *********************************************************
ok: [localhost]
TASK [This command will run.] **************************************************
ok: [localhost] =>
msg: The group `existent_1` exists!
TASK [This command will not run.] **********************************************
skipping: [localhost]
TASK [This command will run.] **************************************************
ok: [localhost] =>
msg: The group `existent_2` exists and this host is in it!
TASK [This command will not run.] **********************************************
skipping: [localhost]
PLAY RECAP *********************************************************************
localhost : ok=3 changed=0 unreachable=0 failed=0

Answer 2

Not sure if this will suppress the warnings or not, but it should accomplish the first part ("make a section of the playbook run if a group exists and is not empty"):
- hosts: all
gather_facts: true
tasks:
- name: "This command will only run if {{ group_to_test }} is a non-empty group that exists"
debug:
msg: The group 'existent' exists and contains hosts!
when: group_to_test in groups and groups[group_to_test]
Give 'er a test and let me know if she works to suppress the warnings!
Obviously group_to_test must be replaced with a constant string or set as a variable/fact/default.