Follow recommendation on Python commands

[anddam (Andrea D'Amore)]

With respect to ​The "python" Command on Unix-Like Systems PEP I'm proposing a patch to follow the guidelines. It introduces python2 and python3 binaries in python ports' selection files.

The PEP suggests that for now python command point to the same target as python2.

This leads to an issue with current port selection feature: if python34 has a "-" value for the corresponding "$prefix/bin/python2" target in its select files entry it means that the "$prefix/bin/python2" symlink gets removed when the port is selected.

What we would like instead is that the "$prefix/bin/python2" would be left linking to an active python2X port, if there's any available, or should it rather be left at system's python executable?

There's a more complicated case in which ports python26 and python27 are both active and the system has to pick one. It could be handled by selecting the most recent version but again this is could be against user's specific needs.

Splitting the select groups in python2 and python3 wouldn't work well since there would be an overlap of the common entries. There's somehow need to mix the selections.

Any thought on this is welcome.

[ned-deily (Ned Deily)]

"Splitting the select groups in python2 and python3 wouldn't work well since there would be an overlap of the common entries."

There shouldn't be any common entries between python2 and python3 installations and you could take this opportunity to fix that. I think the only overlap in current releases is the unversioned 2to3 link and that could/should be resolved by deferring to the python2 version. (2to3 is the same program for both py2 and py3, it's just a matter of which is the newest version.)

[larryv (Lawrence Velázquez)]

There are already a bunch of tickets open on this topic. Let’s consolidate them into #34326.