Ansible: Get all the IP addresses of a group

Question

Let's imagine an inventory file like this:

node-01 ansible_ssh_host=192.168.100.101
node-02 ansible_ssh_host=192.168.100.102
node-03 ansible_ssh_host=192.168.100.103
node-04 ansible_ssh_host=192.168.100.104
node-05 ansible_ssh_host=192.168.100.105

[mainnodes]
node-[01:04]

In my playbook I now want to create some variables containing the IP addresses of the group mainnodes:

vars:
  main_nodes_ips: "192.168.100.101,192.168.100.102,192.168.100.103,192.168.100.104"
  main_nodes_ips_with_port: "192.168.100.101:3000,192.168.100.102:3000,192.168.100.103:3000,192.168.100.104:3000"

This is what I got so far:

vars:
  main_nodes_ips: "{{groups['mainnodes']|join(',')}}"
  main_nodes_ips_with_port: "{{groups['mainnodes']|join(':3000,')}}"

but that would use the host names instead of the IP addresses.

Any ideas how this could be done?

Update:

looking at the docs for a while, I think this would allow me to loop through all the ip adresses:

{% for host in groups['mainnodes'] %}
    {{hostvars[host]['ansible_ssh_host']}}
{% endfor %}

But I just can't figure out how to create an array that holds all these IPs. So that I can use the |join() command on them.

Update2:
I just thought I had figured it out... but it turns out that you cannot use the {% %} syntax in the playbook... or can I? Well in the vars section it didn't. :/

vars:
  {% set main_nodes_ip_arr=[] %}
  {% for host in groups['mesos-slave'] %}
     {% if main_nodes_ip_arr.insert(loop.index,hostvars[host]['ansible_ssh_host']) %} {% endif %}
  {% endfor %}
  main_nodes_ips: "{{main_nodes_ip_arr|join(',')}}"
  main_nodes_ips_with_port: "{{main_nodes_ip_arr|join(':3000,')}}"

Answer 1

I find the magic map extract here.

main_nodes_ips: "{{ groups['mainnodes'] | map('extract', hostvars, ['ansible_host']) | join(',') }}"
main_nodes_ips_with_port: "{{ groups['mainnodes'] | map('extract', hostvars, ['ansible_host']) | join(':3000,') }}:3000"

An alternative(idea comes from here):

main_nodes_ips: "{{ groups['mainnodes'] | map('extract', hostvars, ['ansible_eth0', 'ipv4', 'address']) | join(',') }}"

(Suppose the interface is eth0)