execution

ansible-playbook -i inventory -c local play.yml

PLAY [api] *********************************************************************

TASK [setup] *******************************************************************
Thursday 07 July 2016  11:10:52 +0600 (0:00:00.050)       0:00:00.050 *********
ok: [localhost]

TASK [debug] *******************************************************************
Thursday 07 July 2016  11:10:53 +0600 (0:00:01.559)       0:00:01.609 *********
ok: [localhost] => {
    "foo": "bar"
}

PLAY [web] *********************************************************************

TASK [debug] *******************************************************************
Thursday 07 July 2016  11:10:54 +0600 (0:00:00.072)       0:00:01.681 *********
ok: [localhost] => {
    "foo": "bar"
}

PLAY RECAP *********************************************************************
localhost                  : ok=3    changed=0    unreachable=0    failed=0

Thursday 07 July 2016  11:10:54 +0600 (0:00:00.051)       0:00:01.733 *********
===============================================================================
setup ------------------------------------------------------------------- 1.56s
debug ------------------------------------------------------------------- 0.07s
debug ------------------------------------------------------------------- 0.05s

inventory

[api]
localhost

[web:children]
api

[api:vars]
foo=bar

[web:vars]
foo=baz

play.yml

---
- hosts: api
  tasks:
      - debug: var=foo
- hosts: web
  tasks:
      - debug: var=foo