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

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.

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

For Windows 7, I just rename the python.exe from the Python 3 folder to python3.exe and add the path into the environment variables. Using that, I can execute python test_script.py and the script runs with Python 2.7 and when I do python3 test_script.py, it runs the script in Python 3.

To add Python 3 to the environment variables, follow these steps -

1. Right Click on My Computer and go to Properties.
2. Go to Advanced System Settings.
3. Click on Environment Variables and edit PATH and add the path to your Python 3 installation directory.

For example,