profhase

- hosts: webservers
  serial: "{{ num_serial }}"
  tasks:
    - debug: inventory_hostname
    
    
# ansible-playbook serial_role.yml -e num_serial=3

profhase

class MyMeta(type):
    def __new__(mcs, name, bases, clsdict):

        clsdict['fields'] = {k: v
                             for k, v in clsdict.items()
                             if not k.startswith("_")}
        return super(MyMeta, mcs).__new__(mcs, name, bases, clsdict)

class MyBaseClass(metaclass=MyMeta):
    fields = {}