Difference between hosts: localhost and connection: local

Question

Running the task on the localhost can be achieved in different ways.

I do not understand if there is a difference between the way the two tasks are executed below or if they are exactly the same thing.

If so, fakehost in the second task is just a placeholder, right?

- hosts: localhost
  gather_facts: no

  tasks:
  - name: localhost without explicit connection
    boto3_facts:

- hosts: fakehost
  connection: local
  gather_facts: no

  tasks:
  - name: runner host using local connection
    boto3_facts:

Answer 1

Q: "fakehost in the second task is just a placeholder, right?"

A: Yes. It's just an alias (placeholder).

Run the playbook with -vvvv to enable connection debugging. You'll see

verbosity: 4
connection: smart

Connection plugins says nothing about smart. It's necessary to see the source.

ansible/lib/ansible/config/base.yml says

DEFAULT_TRANSPORT:
  name: Connection plugin
  default: smart
  description: "Default connection plugin to use, the 'smart' option will toggle
                between 'ssh' and 'paramiko' depending on controller OS and ssh
                versions"

The next step would be to dig the source and find out how ssh and paramiko handle localhost. (Wild guess, do nothing and use connection local).

---

Connection plugin local says

"The remote user is ignored, the user with which the ansible CLI was executed is used instead."

Let's add a remote_user to the playbook

shell> cat pb.yml
- hosts: localhost
  gather_facts: no
  remote_user: admin
  tasks:
    - name: localhost without explicit connection
      debug:
        msg:
          - "{{ inventory_hostname }}"
          - "{{ ansible_user }}"

- hosts: fakehost
  gather_facts: no
  connection: local
  remote_user: admin
  tasks:
    - name: runner host using local connection
      debug:
        msg:
          - "{{ inventory_hostname }}"
          - "{{ ansible_user }}"

gives

shell> whoami
vlado
shell> ansible-playbook -i hosts pb.yml

ok: [localhost] => 
  msg:
  - localhost
  - vlado

ok: [fakehost] => 
  msg:
  - fakehost
  - vlado

The inventory was

shell> cat hosts
fakehost

I'd conclude that Ansible is "smart" enough to use the local connection plugin in both cases.