Python preference to follow PYTHON_TARGETS

Title: Python preference to follow PYTHON_TARGETS
Author: Michał Górny <mgorny@gentoo.org>
Posted: 2021-01-30
Revision: 1
News-Item-Format: 2.0

On 2021-02-01 stable users will switch to a new method of updating
the preferred Python versions that employs the configuration update
mechanism in order to follow PYTHON_TARGETS.  We will also deprecate
app-eselect/eselect-python, and it will stop being installed by default
after 2021-07-01.  If you wish to use the newest Python version present
in your PYTHON_TARGETS, you only have to accept configuration changes.
If you wish to customize the behavior, read on.

Since 2017, /usr/bin/python and the related non-versioned symlinks
are wrapped through dev-lang/python-exec.  The list of preferred Python
implementations is stored in /etc/python-exec/python-exec.conf and/or
per-program /etc/python-exec/<basename>.conf configuration files.
To preserve backwards compatibility, app-eselect/eselect-python remained
a wrapper that updated this file.

However, this mechanism alone has proven inconvenient to end users who
had to update python-exec.conf whenever the default PYTHON_TARGETS
changed.  Thanks to the fallback logic, this was not a major problem
for software installed via Gentoo packages that always ensure that
a supported implementation is used.  However, users have reported that
whenever the preference for /usr/bin/python mismatched their
PYTHON_TARGETS, their custom scripts would break due to unsatisfied
dependencies.  This does not follow the principle of least surprise.

For this reason, we have decided to change the default python-exec
configuration to match PYTHON_TARGETS by default, in the eclass
preference order, that is from the newest CPython version to oldest,
with alternative Python implementations coming afterwards.  This change
will be propagated via the configuration protection mechanism whenever
dev-lang/python-exec-conf is installed or rebuilt due to PYTHON_TARGETS
changes.  This will permit the users to interactively confirm
the updates.

If the new default is not correct for you, please use your preferred
configuration update tool to discard or edit the new configuration file.

Furthermore, dev-lang/python will no longer attempt to automatically
update the Python interpreter preference, or pull in eselect-python
automatically.  If you wish to continue using it, please install/record
it explicitly to ensure that it is not unmerged, e.g.:

    emerge -n app-eselect/eselect-python