ansible/docs/docsite/rst/inventory/implicit_localhost.rst

Implicit 'localhost'

When you try to reference a localhost and you don't have it defined in inventory, Ansible will create an implicit one for you.:

- hosts: all
  tasks:
    - name: check that i have log file for all hosts on my local machine
      stat: path=/var/log/hosts/{{inventory_hostname}}.log
      delegate_to: localhost

In a case like this (or local_action) when Ansible needs to contact a 'localhost' but you did not supply one, we create one for you. This host is defined with specific connection variables equivalent to this in an inventory:

...

hosts:
  localhost:
   vars:
     ansible_connection: local
     ansible_python_interpreter: "{{ansible_playbook_python}}"

This ensures that the proper connection and Python are used to execute your tasks locally. You can override the built-in implicit version by creating a localhost host entry in your inventory. At that point, all implicit behaviors are ignored; the localhost in inventory is treated just like any other host. Group and host vars will apply, including connection vars, which includes the ansible_python_interpreter setting. This will also affect delegate_to: localhost and local_action, the latter being an alias to the former.

Note

- This host is not targetable via any group, however it will use vars from host_vars and from the 'all' group. - This implicit host also gets triggered by using 127.0.0.1 or ::1 as they are the IPv4 and IPv6 representations of 'localhost'. - Even though there are many ways to create it, there will only ever be ONE implicit localhost, using the name first used to create it. - Having connection: local does NOT trigger an implicit localhost, you are just changing the connection for the inventory_hostname.