Re: [gentoo-dev] [PATCH news v2] Add Python 3.9 news item

[Sam James]

> On 29 Apr 2021, at 22:01, Michał Górny <mgorny@g.o> wrote:
>
> Signed-off-by: Michał Górny <mgorny@g.o>
> ---
> .../2021-04-29-python3-9.en.txt               | 93 +++++++++++++++++++
> 1 file changed, 93 insertions(+)
> create mode 100644 2021-04-29-python3-9/2021-04-29-python3-9.en.txt
>
> diff --git a/2021-04-29-python3-9/2021-04-29-python3-9.en.txt b/2021-04-29-python3-9/2021-04-29-python3-9.en.txt
> new file mode 100644
> index 0000000..3075d72
> --- /dev/null
> +++ b/2021-04-29-python3-9/2021-04-29-python3-9.en.txt
> @@ -0,0 +1,93 @@
> +Title: Python 3.9 to become the default on 2021-06-01
>

Thanks for working on this. I kept meaning to do one for 3.8 and then 3.9 got so close
that I felt a bit daft doing it.

> +
> +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.

(Add a new line here).

> +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).
> +
> +

I’d consider numbering or marking with a bullet point or ‘-‘ each of
the possible options, in order of likely usefulness. So, we’d do e.g.

1) If you’ve not set anything, just upgrade as normal and then depclean.

2) In order to have both Python implementations enabled temporarily for safety ,…

[…]

But this isn’t required. It’s just about making it easier for people to see
what they need to do, and not misread it and then do silly things because
they just copied and pasted the stuff which looked right (it happens).

Let’s put all of the key information at the beginning, then the
configurability/tweaking can be later on.

> +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 3.8 and Python 3.9 simultaneously,
> +set /etc/portage/package.use to:
> +
> +    */* PYTHON_TARGETS: -* python3_8 python3_9
> +    */* PYTHON_SINGLE_TARGET: -* python3_8
> +
> +Afterwards, rebuild your system with emerge's --changed-use option or
> +equivalent.  This will cause your packages to gain Python 3.9 support
> +while preserving Python 3.8 support whenever possible.  Then, change
> +the second line to:
> +
> +    */* PYTHON_SINGLE_TARGET: -* python3_9
> +
> +This will switch packages that can not support two Python versions
> +simultaneously, to use Python 3.9.  Rebuild again.  Finally, switch
> +the first line to the final version:
> +
> +    */* PYTHON_TARGETS: -* python3_9
> +
> +The next --changed-use rebuild will remove Python 3.8 support from your
> +packages.
> +
> +
> +You can also switch to Python 3.9 earlier by setting:
> +
> +    */* PYTHON_TARGETS: -* python3_9
> +    */* PYTHON_SINGLE_TARGET: -* python3_9
> +
> +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.
> +

“It is especially important you do not forget IF you choose to add this,
because it interferes with the natural rolling-with-the-defaults."

> +
> +The Python 3.8 cleanup requires that Python 3.8 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
> +

Let’s put this right at the beginning.

> +
> +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.
> +However, this is currently discouraged on stable as it will require
> +unmasking multiple ~arch packages as  they gain Python 3.10 support.

I was in two minds about keeping this, but it’s useful to give people an idea
of what’s coming in the future.

> +
> +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 discouraged as it does not respect package defaults.

Let’s put this towards the beginning and also note that people should, in general,
not really need to set any PYTHON_TARGETS at all, other than for where packages
are lagging for having their support updated.