Title: Python 3.7 to become the default target
Author: Michał Górny <mgorny@gentoo.org>
Posted: 2020-04-22
Revision: 1
News-Item-Format: 2.0
Display-If-Installed: dev-lang/python:3.6
Display-If-Installed: dev-lang/python:3.7

On 2020-05-06 (or later), Python 3.7 will replace Python 3.6 as one
of the default Python targets for Gentoo systems.  The new default
values will be:

    PYTHON_TARGETS="python2_7 python3_7"
    PYTHON_SINGLE_TARGET="python3_7"

If you have not overriden these variables on your system, then your
package manager will switch to the new targets immediately.  In order
to avoid dependency conflicts, please clean stray packages up
and rebuild/upgrade all packages with USE flag changes after the change,
e.g.:

    emerge --depclean
    emerge -1vUD @world
    emerge --depclean

Please note that during the system upgrade some of the package
dependencies may temporarily become missing.  While this should not
affect programs that are already fully loaded, it may cause ImportErrors
when starting Python scripts or loading additional modules (only scripts
running Python 3.6 are affected).

In order to improve stability of the upgrade, you may choose to
temporarily enable both targets, i.e. set in /etc/portage/package.use
or its equivalent:

    */* PYTHON_TARGETS: python3_6 python3_7
    */* PYTHON_SINGLE_TARGET: -* python3_6

This will cause the dependencies to include both Python 3.6 and 3.7
support whenever possible, throughout the next system upgrade.  Once all
packages are updated, you can restart your scripts, remove the setting
and start another upgrade in order to cleanly remove Python 3.6.

There are still some Gentoo packages that do not support Python 3.7.
We will be pushing updates to these packages as time permits.  However,
some of them will probably be removed instead.

If you would like to postpone the switch to Python 3.7, you can copy
the current value of PYTHON_TARGETS and/or PYTHON_SINGLE_TARGET
to /etc/portage/package.use or its equivalent:

    */* PYTHON_TARGETS: -python3_7 python3_6
    */* PYTHON_SINGLE_TARGET: -* python3_6

If you would like to migrate your systems earlier, you can do
the opposite.  Note that if you are running ~arch, you may want to go
straight for Python 3.8 at this point.

Please note that this switch is long overdue.  Python 3.6 is no longer
receiving bug fixes.  Its planned end-of-life is 2021-12-23 but we will
probably remove it earlier than that.  [1]

All above snippets assume using package.use to control USE flags.
If you still have make.conf entries for these targets, you need
to remove them.  While using make.conf is possible, it is strongly
discouraged as it does not respect package defaults.  The latter
can help you keep some of Python 3.6 packages without the need to
explicitly toggle flags per-package.

[1] https://devguide.python.org/#status-of-python-branches