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You can currently point the python interpreter to a different install say via
 export PYTHONHOME=<Prefix>
 export PYTHONPATH=<Prefix>/lib/python3.9
 python3
With the newly added platlibdir [1], if python was configured with platlibdir=lib64, this will break because i.e. the site-packages dir as returned by `sysconfig.get_paths()` will use lib64 and not lib as the other install may be using.
This PR adds the possibility to override the platlibdir via environment variable.
Full rationale: [2].
[1] https://github.com/python/cpython/pull/8068
[2] https://src.fedoraproject.org/rpms/python3.9/pull-request/10 

Fedora and OpenSUSE Linux distributions were using a downstream patch which replaced /lib/ with /lib64/ for like 15 years.
Is this issue a new issue? Or did Fedora always had the issue for example?
Which Linux distribution are you using?

I'm on Fedora. My use case is for the mingw-python package I maintain there, see [1] for the full details. I believe (though I haven't investigated) that the previous downstream lib64 patch behaved slightly differently, or that something else between python-3.8 and earlier and python-3.9 changed, in that I didn't have lib64 appear in the site packages path previously when invoking python as detailed in [1], whereas it now does.
[1] https://src.fedoraproject.org/rpms/python3.9/pull-request/10#comment-0 

New changeset 8f023a2f664f902a3d0b7a6f64d63afc0d1c15ae by Sandro Mani in branch 'master':
bpo-40854: Allow overriding sys.platlibdir via PYTHONPLATLIBDIR env-var (GH-20605)
https://github.com/python/cpython/commit/8f023a2f664f902a3d0b7a6f64d63afc0d1c15ae

I merged the patch in master, future Python 3.10. I would prefer to leave Python 3.9 unchanged, if possible. Can we close this issue?

Many thanks! - I'll ask downstream to carry the patch for the 3.9 cycle, so I'm ok with closing.

bpo-1294959 added sys.platlibdir and configure --with-platlibdir to use /usr/lib64 on Fedora and SuSE to Python 3.9.
The problem is that msg370655 use case seems to be a regression compared to Python 3.9. I now consider to backport the commit 8f023a2f664f902a3d0b7a6f64d63afc0d1c15ae to 3.9 since it looks like a fix part of bpo-1294959 rather than a bug fix.
I created PR 20725 to backport PyConfig.platlibdir and PYTHONPLATLIBDIR env var to Python 3.9.

New changeset 817506432dd1908cd154500ef18dc276b8dd7071 by Victor Stinner in branch '3.9':
bpo-40854: Allow overriding sys.platlibdir via PYTHONPLATLIBDIR env-var (GH-20605) (GH-20725)
https://github.com/python/cpython/commit/817506432dd1908cd154500ef18dc276b8dd7071

New changeset 5edb83241f2ff899917e895092aca0216faf42d3 by Victor Stinner in branch 'master':
bpo-40854: PYTHONPLATLIBDIR env var added to 3.9 (GH-20735)
https://github.com/python/cpython/commit/5edb83241f2ff899917e895092aca0216faf42d3

Thanks Sandro Mani. The new env var should be part of the incoming Python 3.9.0 beta3.

Follow-up: bpo-40947 "Replace PLATLIBDIR macro with config->platlibdir".

New changeset d72b9644a3e6eec83be48b1ebc2ec6ca776134d3 by Victor Stinner in branch 'master':
bpo-40947: getpath.c uses PyConfig.platlibdir (GH-20807)
https://github.com/python/cpython/commit/d72b9644a3e6eec83be48b1ebc2ec6ca776134d3

New changeset dc40105c88b968a50c3458e10e1d732e957ef0a3 by Victor Stinner in branch '3.9':
bpo-40947: getpath.c uses PyConfig.platlibdir (GH-20807) (GH-20813)
https://github.com/python/cpython/commit/dc40105c88b968a50c3458e10e1d732e957ef0a3