Since Python hasn't done that until now, it won't help much with the
transition from 2to3.  The earliest versions that could add that would
be 3.1 and 2.7, it would seem, as it is a new fetaure.  Perhaps it would
be worth doing, in planning for Python 4...


Perl has done that for many versions.  The idea is useful for running
scripts that were designed and tested with a given version of Perl, on
that same version, to avoid the need to retest working, production,
code. Even when compatibility is supposed to be retained, and every
effort is made to do so in a maintenance release, sometimes little
things sneak in, that no one ever thought to test, and didn't to have
broken... so continuing to use the version with which a production
script was last extensively tested with, is "extra safe".

The technique you suggest isn't very good for testing a particular
script with various versions of python because you'd have to edit the
script to select the version.  It is good for being "extra safe" and
ensuring that a script continues to run with a particular version of the
interpreter.

I see this issue addressing the ability, in a testing environment, to
swap what version of implicit python is invoked for scripts invoked from
either the command line or from batch files, without the need to edit
the script or the batch file.  From the command line, it is relatively
easy just to invoke the right python via "c:\pythonNN\python script",
although that gets old if you are testing a large number of scripts.  So
I see it as primarily a convenience item for a testing environment.