draft implicit localhost docs (#31840)

* draft implicit localhost docs

* updated per feedback

* corrected docs to follow desired behaviour

* Edits

1 files changed, 31 insertions, 0 deletions

diff --git a/docs/docsite/rst/inventory/implicit_localhost.rst b/docs/docsite/rst/inventory/implicit_localhost.rst
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+.. _implicit_localhost:
+
+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 appy, 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``.