Re: Python 2 support in GNOME build tools

[Christoph Reiter <reiter christoph gmail com>]

> Slow-releasing/stable/"enterprise" distributions like RHEL, Debian,
> Ubuntu LTS and SLED are the usual sticking point for dependency versions.
>
> My understanding is that the main blocker for using Python 3 is
> that RHEL/CentOS 7 doesn't have it built-in, only as part of a secondary
> "software collection"?

Yeah, that's also what I heard when the topic came up on IRC, but similarly
vague re RHEL.. :)

> For what it's worth, requiring Python 3 would be no problem from Debian's
> perspective, as long as it isn't assumed to be /usr/bin/python: for
> compatibility with historical scripts, if /usr/bin/python exists then
> it is always Python 2, while Python 3 is available at /usr/bin/python3
> if installed. Using Python 3 for all programs that can work in either
> version is recommended, and in particular we've used Python 3 for the GLib
> and GObject-Introspection build tools since Debian 9 'stretch' (2017).
>
> We don't normally backport the latest GNOME versions to stable releases
> anyway; but if we do, the latest stable release (Debian 9 'stretch')
> has Python 3.5 as its supported Python 3 version, and the one before that
> (Debian 8 'jessie', 2015) had 3.4.
>
> Ubuntu is in about the same situation as Debian, with new LTS releases
> every 2 years, although a year out of phase with Debian (the most recent
> LTS releases were in 2018 and 2016 and have contemporary Python 3 versions).

That's good to know, thanks!

I'll try to summarize the remaining cases:

Windows:

I'm confident that on Windows Python 3 is enough in all cases. MSYS2 switched
some months ago and all looks good. gvsbuild also uses Python 3.

macOS:

There still isn't any system Python 3 in sight, and could be that it never
will happen. Homebrew works.

SLED/SLES:

They use old versions [0] [1], so unlikely that they will try to backport
things now imo. I couldn't find a working package index for SLED, I assume it
is similar to SLES.

[0] 
https://www.suse.com/LinuxPackages/packageRouter.jsp?product=server&version=12&service_pack=&architecture=x86_64&package_name=libglib-2_0-0
[1] 
https://www.suse.com/LinuxPackages/packageRouter.jsp?product=server&version=11&service_pack=&architecture=i386&package_name=glib2

RHEL7:

RHEL got a GNOME 3.26 update for 7.5 and there is a 3.28 update in the works
for 7.6 [3]. Full support for 7.X ends with "~Q4 of 2019" [4] and based on the
previous update intervals [5] this should mean RHEL 7.7 is the last feature
release. GNOME 3.32 comes out some months before that, so it theoretically
could make it into the last release.

This means we can drop Python 2 definitely after GNOME 3.32, or earlier if
someone from RH can rule out that 3.32 will make it into RHEL 7 (anyone?).

[3] https://bugzilla.redhat.com/show_bug.cgi?id=1567375
[4] https://access.redhat.com/support/policy/updates/errata
[5] https://en.wikipedia.org/wiki/Red_Hat_Enterprise_Linux#Version_history_and_timeline