What this command tells to operating system ” #! /usr/bin/env python3 “ ?
This is not technically a part of the program. It is there to tell the operating system what version of Python to choose, since the older Python 2 is incompatible with the newer Python 3. We will mostly run programs from inside the Idle programming environment, where this line is not needed. To run just by clicking on the program in an operating system window, however, the line is important if your computer also has Python 2.