sysconfig default scheme change in Python 3.10
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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