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