sys.executable has the path to the current executable.  But you don't even need that.  When you import pip, it gets that itself to find site-packages.

You are making this way too complicated and are trying to do what pip can do. I don't see that we need to know the system.  Pip does that.  You don't need to know all installed pythons, certainly not to begin.  The first thing is to install to the current version. Expand on that later.  Don't worry about virtual environments.  Beginners don't know what they are, and this is aimed at beginners.

3.6.0a2 will be tagged on Sunday, and I can look at this more after that.