ansible - example of dynamic even/odd groups

play.yml

# requirement
# given 2 groups (A and B), target 50% of each for code deploy
- hosts: localhost
  tasks:
  
    # this might need to be group_by or add_host
    - set_fact: 
        even: "{{ groups['groupA'][::2] | union(groups['groupB'][::2] }}"
        odd: "{{ groups['groupA'][1::2] | union(groups['groupB'][1::2] }}"


- hosts: even
  roles:
    - deploy something

- hosts: odd
  roles:
    - deploy something
    
# it may also be possible to do
#- hosts: even;odd
#  serial: 50%

I can make sure I assigned a list of hosts to the fact even in the first play, but in the second play I still got the no hosts matched error.
Can anyone give some hints?

Thanks for this trick.

I think it's not necessary to do set_fact (except to increase readibility):

- name: work on odd servers
  hosts: "{{ groups['web'][::2] | union(groups['database'][::2]) }}"
  tasks:
    - name: reboot odd servers
      debug:
        msg: "I will reboot odd server: {{ ansible_hostname}}"

- name: work on even servers
  hosts: "{{ groups['web'][1::2]  | union(groups['database'][1::2]) }}"
  tasks:
    - name: reboot even servers
      debug:
        msg: "I will reboot even server: {{ ansible_hostname}}"