Ansible uses version 2.x.x of Python to perform a number of different tasks and expects the Python command line tool to be located at /usr/bin/python. If a system has both version 2.x.x (e.g. /usr/bin/python) and version 3.x.x of Python (e.g. /usr/bin/python3), the ansible_python_interpreter variable can be used to tell Ansible to use /usr/bin/python or /usr/bin/python3.

There are many different ways to define a variable - refer to Getting Started with Variables.

The -e or --extra-vars command line option can be used, with both the ad-hoc ansible command

ansible localhost -m ping --extra-vars 'ansible_python_interpreter=/usr/bin/python3'

And with the ansible-playbook command.

ansible-playbook foo.yml --extra-vars 'ansible_python_interpreter=/usr/bin/python3'

Or the vars module could be used to define the ansible_python_interpreter variable.

NOTE - The gather_facts module must be set to true, which is the default setting of the gather_facts module. In other words, the gather_facts module must not be set to false.

---
- hosts: all
  vars:
    ansible_python_interpreter: /usr/bin/python
  tasks:
    - name: Return Python interpreter
      debug:
        msg: "Python interpreter = {{ ansible_python_interpreter }}"

Running this playbook should return the following.

TASK [debug]
ok: [server1.example.com] => {
    "msg: "Python interpreter = /usr/bin/python"
}