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sysconfig default scheme change in Python 3.10

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I realise we made a consequential changes in Python 3.10, that was never
announced to the team. At the time, I didn't realise how consequential
it would be, but it has forced a few packages to change in response.

The change was in python3.10 3.10.2-3 (25 Feb 2022).

The default sysconfig change has changed from posix_prefix to
posix_local. This matches the long-existing behaviour in Debian's
distutils.sysconfig. distutils is being removed from Python's stdlib in
3.12 [PEP-632], so setuptools etc. are being migrated to be configured
by sysconfig instead.

The practical reason for this is to make "python3 setup.py install"
install to /usr/local/lib/... rather than into the generally
package-managed /usr/lib/python3/.

$ python3.10 -m sysconfig
...
Current installation scheme: "posix_local"

Paths:
        data = "/usr/local"
        include = "/usr/include/python3.10"
        platinclude = "/usr/include/python3.10"
        platlib = "/usr/local/lib/python3.10/dist-packages"
        platstdlib = "/usr/lib/python3.10"
        purelib = "/usr/local/lib/python3.10/dist-packages"
        scripts = "/usr/local/bin"
        stdlib = "/usr/lib/python3.10"
...

This has caused some changes for some Debian packages. Generally it can
be fixed at a build-tool level, and shouldn't break other Python
packages, but for non-standard tools that haven't been fixed yet, you
may notice that modules install into /usr/local instead of /usr.
dh_python3 will move modules from /usr/local back to /usr, but it's
called after dh_install, so dh_install configuration can get broken by
this change.

The solution in build tools is to explicitly select the posix_prefix
scheme, and specify the appropriate prefix (/usr or /usr/local). For
build tools that default to /usr/local, this should be all you need to
do.

We've fixed this issue in pybind11 and automake-1.16, so far.
If you find any more tools that need work, feel free to ask me to help.

There have been some tweaks to this, since 3.10.2-3, to reduce breakage.
One of the significant wants was to not prefix the include paths with
/local/, as they are defined as referencing the location of cPython's
headers, not the install location for headers [bpo-44445]. But some
build tools may have their own interpretations of this...

Related to this change is something else worth noting:

Because distutils is being removed, modern setuptools now carries its
own distutils fork, internally. This doesn't have Debian's patches
implementing these scheme differences. Instead, it's configured by the
presence of a _distutils_system_mod module, which monkey-patches
distutils.

[PEP-632]: https://peps.python.org/pep-0632/
[bpo-44445]: https://bugs.python.org/issue44445

SR

-- 
Stefano Rivera
  http://tumbleweed.org.za/
  +1 415 683 3272

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Thread

sysconfig default scheme change in Python 3.10 Stefano Rivera <stefanor@debian.org> - 2022-03-28 23:20 +0200
  Re: sysconfig default scheme change in Python 3.10 Simon McVittie <smcv@debian.org> - 2022-03-28 23:50 +0200
    Re: sysconfig default scheme change in Python 3.10 Stefano Rivera <stefanor@debian.org> - 2022-03-29 01:30 +0200

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