Generate dictionary by template in inventory - ansible

Question

At the moment I have the following inventory (simplified):
children:
child-1111:
param: some-data-1111
child-1112:
param: some-data-1112
child-1113:
param: some-data-1113
Of course, it's not good when you need a thousand hosts instead of just three and many parameters with substitutions. I would like to write something like this (which doesn't work):
children:
"child-{{item}}":
param: "some-data-{{item}}"
loop:
- 1111
- 1112
- 1113
All the examples I see always produce lists, not dictionaries.
Also, loop seems to work only for tasks, not inventories.
How do I achieve this?

Answer 1

Those kind of requirements tend to be managed with the add_host module.
Given the playbook:
- hosts: localhost
gather_facts: no
tasks:
- add_host:
name: "child-{{ item.name }}"
param: "some-data-{{ item.param }}"
group: dynamic_hosts
loop: "{{ _hosts }}"
vars:
_hosts:
- name: 1111
param: 1111-bis
- name: 1112
param: 1112-bis
- name: 1113
param: 1113-bis
- hosts: dynamic_hosts
gather_facts: no
tasks:
- debug:
msg: "On host `{{ item }}` the value of `param` is `{{ hostvars[item].param }}`"
loop: "{{ ansible_play_hosts }}"
run_once: true
We are constructing a dynamic inventory group named dynamic_hosts, which we can then use in the same playbook.
This would actually yields:
PLAY [localhost] ****************************************************************
TASK [add_host] *****************************************************************
ok: [localhost] => (item={'name': 1111, 'param': '1111-bis'})
ok: [localhost] => (item={'name': 1112, 'param': '1112-bis'})
ok: [localhost] => (item={'name': 1113, 'param': '1113-bis'})
PLAY [dynamic_hosts] ************************************************************
TASK [debug] ********************************************************************
ok: [child-1111] => (item=child-1111) =>
msg: On host `child-1111` the value of `param` is `some-data-1111-bis`
ok: [child-1111] => (item=child-1112) =>
msg: On host `child-1112` the value of `param` is `some-data-1112-bis`
ok: [child-1111] => (item=child-1113) =>
msg: On host `child-1113` the value of `param` is `some-data-1113-bis`

Answer 2

Some forenotes:
Your question is not very precise. My example will only give a general approach which is a bit more scalable than yours but will hit limits very soon.
Your example is not a valid yaml inventory. Moreover it is not clear if you use the name children to declare children groups for the default all or if you want to use this as a group name (which I strongly suggest you don't do).
Having to declare host specific variables depending on the host name inside a static inventory for loads of hosts is generally a sign of either a bad design or that you should switch to a fully dynamic inventory or by managing the hosts in your playbook dynamically through add_host (see #β.εηοιτ.βε's answer)
My example below declares a range of hosts in a group named my_children and takes advantage of the host naming convention to capture their number and reuse it in a variable declared for the group
inv.yml
---
my_children:
vars:
param: "some-data-{{ inventory_hostname.split('-')[1] }}"
hosts:
child-[1111:1113]:
You can see the hosts are parsed correctly:
$ ansible-inventory -i inv.yml --list
{
"_meta": {
"hostvars": {
"child-1111": {
"param": "some-data-{{ inventory_hostname.split('-')[1] }}"
},
"child-1112": {
"param": "some-data-{{ inventory_hostname.split('-')[1] }}"
},
"child-1113": {
"param": "some-data-{{ inventory_hostname.split('-')[1] }}"
}
}
},
"all": {
"children": [
"my_children",
"ungrouped"
]
},
"my_children": {
"hosts": [
"child-1111",
"child-1112",
"child-1113"
]
}
}
And the vars are interpreted correctly:
$ ansible my_children -i inv.yml -m debug -a "msg={{ param }}"
child-1111 | SUCCESS => {
"msg": "some-data-1111"
}
child-1112 | SUCCESS => {
"msg": "some-data-1112"
}
child-1113 | SUCCESS => {
"msg": "some-data-1113"
}

Answer 3

Q: "Need a thousand hosts (.e.g. 'child-1111' param: 'some-data-1111')"
A: Use ranges and constructed plugin. For example
shell> cat inventory/01-hosts
child-[1111:1113]
shell> cat inventory/02-constructed.yml
plugin: constructed
strict: True
groups:
group_children: inventory_hostname.startswith('child-')
compose:
param: "'some-data-' ~ inventory_hostname|split('-')|last"
Test it
shell> ansible-inventory -i inventory --list --yaml
all:
children:
group_children:
hosts:
child-1111:
param: some-data-1111
child-1112:
param: some-data-1112
child-1113:
param: some-data-1113
ungrouped: {}
and use it in a playbook
- hosts: all
tasks:
- debug:
var: param
gives
ok: [child-1111] =>
param: some-data-1111
ok: [child-1112] =>
param: some-data-1112
ok: [child-1113] =>
param: some-data-1113