set default python version when run the program as executable ./xxx.py - on linux

Question

I would like to run the python directly as executable file like ./xxx.py

However, when I run it like this, it will automatically use python 2.7 instead of python 3.

How could I use python 3 to run the executable python file?

In the case:

./magane.py shell

It will give me

Python 2.7.9 (default, Apr  2 2015, 15:33:21) 
[GCC 4.9.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> 

But I would like to have this:

python3 manage.py shell
/home/cliu/.local/lib/python3.4/site-packages/django/db/backends/sqlite3/base.py:57: RuntimeWarning: SQLite received a naive datetime (2015-05-31 15:24:46.785297) while time zone support is active.
  RuntimeWarning)

Python 3.4.3 (default, Mar 26 2015, 22:03:40) 
Type "copyright", "credits" or "license" for more information.

IPython 2.3.0 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object', use 'object??' for extra details.

In [1]: 

where python3 uses ipython.

Answer 1

To tell python what version to use and to make it executable on Linux you have to do the following steps:

Add #! to the python script

#!/usr/bin/env python3

Add permissions to run it

chmod +x xxx.py

Answer 2

As the first line of our script you may use :

#!/usr/bin/python3

In this way you can add this line to the scripts which are written to be ran on Python3 and ignore otherwise.