1. The most basic command to execute a Python program is as follows:

      $ python <script_name>.py

2. The following examples show how to launch specific versions of Python, as needed:

      $ python2 some_script.py # Use the latest version of Python 2
      $ python2.7 ... # Specifically use Python 2.7
      $ python3 ... # Use the latest version of Python 3
      $ python3.5.2 ... # Specifically use Python 3.5.2