Yes you right, I already read a lot of stuff about this problem before posting on forums. It's pretty crazy that no where I can't find a clear answer about this point.
IMHO, XBMC HOW TO for beginners forgets simply the beginning which is really specific here.
Yesterday I found a forum where I was able to download librairies xbmc and xbmcgui allowing to script with the standard python shell if you add them as source to your project... but it's a work around not the solution of the enigma.
My mind doesn't leave me in peace: it asks the answer...
But now I quite convinced that the fact is that there no access to an interpreter with a shell but you have to execute script from inside the XBMC app via menus as add-on, ect.
I would like that an XBMC script/add-on developper give me indications about the IDE I could use. Really it's not obvious for me. I'm not familiar with python nor XBMC and this situation where you are looking for the interpreter is not usual for me.
But XBMC is a wonderfull framework for artistic video projects based on raspberry pi (HD video h264 runs perfectly). So I continue my investigations