I'm wondering how and where PYTHON_TARGETS and PYTHON_SINGLE_TARGET are defined.
Since upgrading to 1.3 and lately to 1.4, I've been getting more and more emerge errors, complaining about these two settings. I even started a fresh install, thinking that one of the upgrades might have broken something, but already on the first (still empty world file) world update, I ran into the same kind of errors again.
I read the forums and found out, that the way to go should be to override the PYTHON_SINGLE_TARGET setting via USE flags (this thread), but I'm still getting errors: PYTHON_TARGETS either lists versions of python that are not installed or demands versions that are already installed, as if they weren't.
A few examples:
Currently installed python versions are:
[I] dev-lang/python 
eselect python list shows:
Available Python interpreters, in order of preference:
emerge -avuDN world gives:
!!! The ebuild selected to satisfy "dev-vcs/git" has unmet requirements.
- dev-vcs/git-2.22.0::core-kit USE="blksha1 curl gpg iconv nls pcre pcre-jit perl python threads webdav -cgi -cvs -doc -emacs -gnome-keyring -highlight -libressl -mediawiki -mediawiki-experimental (-ppcsha1) -subversion -test -tk -xinetd"
PYTHON_SINGLE_TARGET="python3_7 -python2_7 -python3_5 -python3_6" PYTHON_TARGETS="python3_6 -python2_7 -python3_5 -python3_7"
The following REQUIRED_USE flag constraints are unsatisfied:
python? ( python_single_target_python3_7? ( python_targets_python3_7 ) )
note, that I didn't set a python use flag for this package. The PYTHON_TARGETS should be pyhton3_7, python3_6 and pyhton2_7 as far as I understand.
There are no mentions of PYTHON_TARGETS in my make.conf. I even tried to change the python version via eselect to another version and back, hoping that would resolve the issue, but it didn't help.
Where does my system get those PYTHON_TARGETS settings from? I thought they should be defined via eselect python?
Is there any guide or wiki entry about these settings, where to define them, what overrides them and how to correctly override them per package via USE flags?
Thanks for your help.