- etc_hosts tasks

## tasks:
- name: Build hosts file
  lineinfile: dest=/etc/hosts regexp='.*{{ item }}$' line="{{ hostvars[item].ansible_default_ipv4.address }} {{ item }}" state=present
  when: hostvars[item].ansible_default_ipv4 is defined
  with_items: "{{ groups['all'] }}"

- name: Add aliases for hosts excluded in playbook to hosts file
  lineinfile: dest=/etc/hosts regexp='.*{{ item }}$' line="{{ hostvars[hostvars[item].ansible_host].ansible_default_ipv4.address }} {{ item }}" state=present
  when: hostvars[item].ansible_default_ipv4 is not defined
  with_items: "{{ groups['all'] }}"
  
## inventory:
[jenkins]
jenkins
[logstash]
logstash1 ansible_host=web1
logstash2 ansible_host=web2
logstash3 ansible_host=web3
[web]
web1
web2
web3 

## tasks:
- name: Build hosts file
  lineinfile: 
    dest: /etc/hosts 
    regexp: '.*{{ item }}$' 
    line: "{{ hostvars[item].ansible_default_ipv4.address | default(hostvars[item]['ansible_host']) }} {{ item }}" 
    state: present
  with_items: "{{ groups['all'] }}"

cool. this is an improvement over the current task. its actually the second giving us problems. thanks for the help!

fatal: [loststash1: FAILED! => {"failed": true, "msg": "the field 'args' has an invalid value, which appears to include a variable that is undefined. The error was: 'dict object' has no attribute 'ansible_default_ipv4'\n\nThe error appears to have been in 'ansible/roles/hostname/tasks/main.yml': line 34, column 3, but may\nbe elsewhere in the file depending on the exact syntax problem.\n\nThe offending line appears to be:\n\n\n- name: Build hosts file\n ^ here\n"}

ah, sorry, should have realised - I always forget that address is a sub key of ansible_default_ipv4. You need an interim default. I can't think of the ansible filter way at present - here it is in the python way.

## tasks:
- name: Build hosts file
  lineinfile: 
    dest: /etc/hosts 
    regexp: '.*{{ item }}$' 
    line: "{{ hostvars[item].get('ansible_default_ipv4', {}).get('address', hostvars[item]['ansible_host']) }} {{ item }}" 
    state: present
  with_items: "{{ groups['all'] }}"

in summary, try and get the key (if not defined use empty dict) then get address key (if not defined use ansible_host).