[gentoo-dev] [PATCH news] Add Python 3.9 news item

[Michał Górny]

Signed-off-by: Michał Górny <mgorny@g.o>
---
 .../2021-04-29-python3-9.en.txt               | 92 +++++++++++++++++++
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+Title: Python 3.9 to become the default target on 2021-06-01
+Author: Michał Górny <mgorny@g.o>
+Posted: 2021-04-29
+Revision: 1
+News-Item-Format: 2.0
+Display-If-Installed: dev-lang/python:3.7
+Display-If-Installed: dev-lang/python:3.8
+
+We are planning to switch the default Python target of Gentoo systems
+on 2021-06-01, from Python 3.8 to Python 3.9.  If you have not changed
+the values of PYTHON_TARGETS or PYTHON_SINGLE_TARGET, the change will
+have immediate effect on your system and the package manager will try
+to switch automatically on the next upgrade following the change.
+If you did change the values, prefer a safer approach or have problems
+with the update, read on.
+
+Please note that the default upgrade method switches packages to the new
+Python versions as they are rebuilt.  This means that all interdependent
+packages have to support the new version for the upgrade to proceed,
+and that some programs may temporarily fail to find their dependencies
+throughout the upgrade (although programs that are already started
+are unlikely to be affected).
+
+
+If you wish to avoid changing Python targets at this moment, you can
+force the old targets by setting your /etc/portage/package.use to e.g.:
+
+    */* PYTHON_TARGETS: -* python3_8
+    */* PYTHON_SINGLE_TARGET: -* python3_8
+
+This will enforce Python 3.8 as the current target choice and block
+any future updates.  However, please note that this solution will only
+be suitable for a few more months and you will eventually need to
+perform the migration.
+
+
+If you wish to use a safer approach to the migration and temporarily
+preserve the support for Python 3.7 and Python 3.8 simultaneously,
+set /etc/portage/package.use to:
+
+    */* PYTHON_TARGETS: -* python3_7 python3_8
+    */* PYTHON_SINGLE_TARGET: -* python3_7
+
+Afterwards, rebuild your system with emerge's --changed-use option or
+equivalent.  This will cause your packages to gain Python 3.8 support
+while preserving Python 3.7 support whenever possible.  Then, change
+the second line to:
+
+    */* PYTHON_SINGLE_TARGET: -* python3_8
+
+This will switch packages that can not support two Python versions
+simultaneously, to use Python 3.8.  Rebuild again.  Finally, switch
+the first line to the final form:
+
+    */* PYTHON_TARGETS: -* python3_8
+
+The next --changed-use rebuild will remove Python 3.7 support from your
+packages.
+
+
+You can also switch to Python 3.8 earlier by setting:
+
+    */* PYTHON_TARGETS: -* python3_8
+    */* PYTHON_SINGLE_TARGET: -* python3_8
+
+If you choose to follow this or the previous approach, you may want to
+remove the package.use overrides after the switch or just leave them
+in place to protect your system from the next automatic upgrade
+of Python.
+
+
+The Python 3.7 cleanup requires that Python 3.7 is removed from complete
+dependency trees in batch.  If some of the installed packages using
+an older Python version are not triaged for the upgrade, the package
+manager will throw dependency conflicts.  This makes it important that
+the upgrade is carried via a --deep --changed-use @world upgrade,
+as well as that any stray packages are removed prior to it, e.g.:
+
+    emerge --depclean
+    emerge -1vUD @world
+    emerge --depclean
+
+
+By the time of the switch, it is quite probable that Python 3.10 will
+already be available in ~arch Gentoo.  Users wishing a more bleeding
+edge experience may wish to switch to python3_10 target instead.
+
+
+If you have PYTHON_TARGETS or PYTHON_SINGLE_TARGET declared
+in make.conf, it is strongly recommended to remove the declarations
+and use package.use as presented above.  Use of make.conf to set flags
+is strongly discouraged as it does not respect package defaults.
-- 
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