I just got the answer. By typing "py -h" or "py --help" I got the help message:

C:\Users\admin>py -h
Python Launcher for Windows Version 3.7.1150.1013

usage:
py [launcher-args] [python-args] script [script-args]

Launcher arguments:

-2     : Launch the latest Python 2.x version
-3     : Launch the latest Python 3.x version
-X.Y   : Launch the specified Python version
     The above all default to 64 bit if a matching 64 bit python is present.
-X.Y-32: Launch the specified 32bit Python version
-X-32  : Launch the latest 32bit Python X version
-X.Y-64: Launch the specified 64bit Python version
-X-64  : Launch the latest 64bit Python X version
-0  --list       : List the available pythons
-0p --list-paths : List with paths

Which tells me that "-0" (zero, not letter "O") lists the available pythons:

C:\Users\admin>py -0
Installed Pythons found by py Launcher for Windows
 -3.7-64 *
 -3.7-32
 -2.7-64
 -2.7-32

While "-0p" lists not only the versions, but also the paths:

C:\Users\admin>py -0p
Installed Pythons found by py Launcher for Windows
 -3.7-64        C:\Users\admin\AppData\Local\Programs\Python\Python37\python.exe *
 -3.7-32        C:\Users\admin\AppData\Local\Programs\Python\Python37-32\python.exe
 -2.7-64        C:\Python27_64\python.exe
 -2.7-32        C:\Python27_32\python.exe

To install a Python version that is not listed there run py install followed by the version number, e.g. py install 3.14

Use py -0 to find all installed versions of python on your PC.

Wallbloggerbeing explained how to change your default.

I'm writing a Python IDE and I want to let user to choose the interpreter for executing the program.

Just do it like other IDEs then and simply supply a dialog where users can add interpreters they want to be able to run the code with.

Eclipse does this for example for Java Runtimes, and it’s perfectly fine to have it like that. Especially for languages like Python where virtual environments are an important thing which each have their own exectutable.

You certainly can come up with a one-time detection that checks some common locations. For Windows, this would obviously be the registry, as the py.exe launcher requires the interpreters to be registered there—at least the system-wide ones. On Unix machines, you could check the common bin/ folders, most prominently /usr/local/bin/ which is the standard location where Python installs itself. You could also check the PATH for Python executables. But all those things should be considered carefully and only offer an initial setup. There are always edge cases where a user didn’t do the “standard thing” where your detection will fail. For example I don’t have my Python interpreters in my path, and a linux server I access I have installed Python into a non-standard folder in my home directory. And finally, just because it looks like Python doesn’t mean it is Python.

Yes, you can do some guesswork to come up with an initial set of interpreters, but really don’t spend too much time on it. In the end, you won’t be able to detect everything perfectly anyway. And you will miss virtual environments—which might be very crucial to the project the user is working on in your IDE.

So instead of wasting time on bad detection, spend more time on creating a manual dialog to register interpreters. You will need that anyway, and a good interface can make it very easy—even for beginners—to use it.

To check the version of one's Python's Software version, one should use the following code in command prompt:

python -V

Reference: http://docs.python.org/using/cmdline.html#generic-options