[python] How to switch between python 2.7 to python 3 from command line?

I'm trying to find the best way to switch between the two python compilers, 2.7 to 3.3. I ran the python script from the cmd like this:

python ex1.py

Where do I set the "python" environment in the window's environment variable to point to either python 3.3 or 2.7?

I am wondering if there is an easy way to switch between the two versions from the cmd line?

They are 3 ways you can achieve this using the py command (py-launcher) in python 3, virtual environment or configuring your default python system path. For illustration purpose, you may see tutorial https://www.youtube.com/watch?v=ynDlb0n27cw&t=38s

You can try to rename the python executable in the python3 folder to python3, that is if it was named python formally... it worked for me

There is an easier way than all of the above; You can use the PY_PYTHON environment variable. From inside the cmd.exe shell;

For the latest version of Python 2

set PY_PYTHON=2

For the latest version of Python 3

set PY_PYTHON=3

If you want it to be permanent, set it in the control panel. Or use setx instead of set in the cmd.exe shell.

No need for "tricks". Python 3.3 comes with PyLauncher "py.exe", installs it in the path, and registers it as the ".py" extension handler. With it, a special comment at the top of a script tells the launcher which version of Python to run:

#!python2
print "hello"

Or

#!python3
print("hello")

From the command line:

py -3 hello.py

Or

py -2 hello.py

py hello.py by itself will choose the latest Python installed, or consult the PY_PYTHON environment variable, e.g. set PY_PYTHON=3.6.

See Python Launcher for Windows

In case you have both python 2 and 3 in your path, you can move up the Python27 folder in your path, so it search and executes python 2 first.