testing add_host across multiple plays in playbook

_output

$ ansible-playbook test-add_host.yml 
 [WARNING]: provided hosts list is empty, only localhost is available


PLAY [Play 1] ******************************************************************

TASK [Show inventory - play 1a] ************************************************
ok: [localhost] => {
    "groups": {
        "all": [], 
        "ungrouped": [
            "localhost"
        ]
    }
}

TASK [add_host] ****************************************************************
changed: [localhost]

TASK [add_host] ****************************************************************
changed: [localhost]

TASK [Show inventory - play 1b] ************************************************
ok: [localhost] => {
    "groups": {
        "all": [
            "box1", 
            "box2"
        ], 
        "boxes": [
            "box1", 
            "box2"
        ], 
        "ungrouped": [
            "localhost"
        ]
    }
}

PLAY [Play 2] ******************************************************************

TASK [Show inventory - play 2a] ************************************************
ok: [box1] => {
    "groups": {
        "all": [
            "box1", 
            "box2"
        ], 
        "boxes": [
            "box1", 
            "box2"
        ], 
        "ungrouped": [
            "localhost"
        ]
    }
}
ok: [box2] => {
    "groups": {
        "all": [
            "box1", 
            "box2"
        ], 
        "boxes": [
            "box1", 
            "box2"
        ], 
        "ungrouped": [
            "localhost"
        ]
    }
}

TASK [add_host] ****************************************************************
changed: [box1]

TASK [add_host] ****************************************************************
changed: [box1]

TASK [Show inventory - play 2b] ************************************************
ok: [box2] => {
    "groups": {
        "all": [
            "box1", 
            "box2", 
            "box3", 
            "box4"
        ], 
        "boxes": [
            "box1", 
            "box2", 
            "box3", 
            "box4"
        ], 
        "ungrouped": [
            "localhost"
        ]
    }
}
ok: [box1] => {
    "groups": {
        "all": [
            "box1", 
            "box2", 
            "box3", 
            "box4"
        ], 
        "boxes": [
            "box1", 
            "box2", 
            "box3", 
            "box4"
        ], 
        "ungrouped": [
            "localhost"
        ]
    }
}

PLAY RECAP *********************************************************************
box1                       : ok=4    changed=2    unreachable=0    failed=0   
box2                       : ok=2    changed=0    unreachable=0    failed=0   
localhost                  : ok=4    changed=2    unreachable=0    failed=0   

Show inventory - play 1b ------------------------------------------------ 0.04s
Show inventory - play 2b ------------------------------------------------ 0.04s
Show inventory - play 2a ------------------------------------------------ 0.03s
Show inventory - play 1a ------------------------------------------------ 0.03s
 ------------------------------------------------------------------------ 0.01s

Playbook finished: Sun Jan 29 13:02:17 2017, 5 total tasks.  0:00:00 elapsed.

test-add_host.yml

---

- name: "Play 1"

  hosts: localhost
  gather_facts: no 
  
  tasks:

    - name: "Show inventory - play 1a"
      debug: var=groups

    - add_host:
        name: "box1"
        group: "boxes"

    - add_host:
        name: "box2"
        group: "boxes"

    - name: "Show inventory - play 1b"
      debug: var=groups

- name: "Play 2"

  hosts: boxes
  gather_facts: no 
 
  tasks:

    - name: "Show inventory - play 2a"
      debug: var=groups

    - add_host:
        name: "box3"
        group: "boxes"

    - add_host:
        name: "box4"
        group: "boxes"

    - name: "Show inventory - play 2b"
      debug: var=groups