Discrepancy between no inventory and the localhost entry in an inventory #56510

ansible 2.7.10
  config file = /etc/ansible/ansible.cfg
  configured module search path = ['/Users/nmasse/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /usr/local/Cellar/ansible/2.7.10/libexec/lib/python3.7/site-packages/ansible
  executable location = /usr/local/bin/ansible
  python version = 3.7.3 (default, Mar 27 2019, 09:23:39) [Clang 10.0.0 (clang-1000.11.45.5)]
CONFIGURATION
OS / ENVIRONMENT
MacOS, ansible installed using brew.sh.
STEPS TO REPRODUCE
Install Ansible with:
brew install ansible
/usr/local/Cellar/ansible/2.7.10/libexec/bin/pip install openshift 
Playbook and inventory:
cat <<EOF > playbook.yml
- hosts: localhost
  gather_facts: no
  pre_tasks:
  - python_requirements_facts:
      dependencies:
        - openshift
  tasks:
  - k8s:
      state: absent
      definition:
        apiVersion: v1
        kind: ConfigMap
        metadata:
          name: test
          namespace: default
        data:
cat <<EOF > inventory
localhost ansible_connection=local
ACTUAL RESULTS
With the default inventory, the playbook succeeds:
$ ansible-playbook playbook.yml 
 [WARNING]: Unable to parse /etc/ansible/hosts as an inventory source
 [WARNING]: No inventory was parsed, only implicit localhost is available
 [WARNING]: provided hosts list is empty, only localhost is available. Note that the implicit localhost does not match 'all'
PLAY [localhost] *******************************************************************************************************************************************************************************************
TASK [python_requirements_facts] ***************************************************************************************************************************************************************************
ok: [localhost]
TASK [k8s] *************************************************************************************************************************************************************************************************
ok: [localhost]
PLAY RECAP *************************************************************************************************************************************************************************************************
localhost                  : ok=2    changed=0    unreachable=0    failed=0   
With an explicit inventory, the playbook fails:
$ ansible-playbook playbook.yml -i inventory 
PLAY [localhost] *******************************************************************************************************************************************************************************************
TASK [python_requirements_facts] ***************************************************************************************************************************************************************************
ok: [localhost]
TASK [k8s] *************************************************************************************************************************************************************************************************
fatal: [localhost]: FAILED! => {"changed": false, "msg": "This module requires the OpenShift Python client. Try `pip install openshift`"}
	to retry, use: --limit @/Users/nmasse/tmp/test2-ansible/playbook.retry
PLAY RECAP *************************************************************************************************************************************************************************************************
localhost                  : ok=1    changed=0    unreachable=0    failed=1   
Full output with -vvv for both commands are here:

https://gist.github.com/nmasse-itix/77575fe8498471a10a59509dbee2fce3
As you can see:
with the default inventory, the tasks are run with the same python interpreter that runs the playbook
with the explicit inventory, the tasks are run with the default system python interpreter
EXPECTED RESULTS
I would have expected the same behavior with the default inventory as with an explicit one.
If I called the ansible-playbook command directly from the command line, I could set the ansible_python_interpreter variable in the inventory and that would be fixed. But those playbooks are run by ansible-runner from the operator-sdk command, so I cannot change the inventory or add switches to the ansible command line.
PROPOSED CHANGES
For all hosts that have ansible_connection=local, the python interpreter is set to the interpreter running the playbook, unless ansible_python_interpreter is set explicitely for this host.