[New-bugs-announce] [issue43976] Introduce mechanism to allow Python distributors to add custom site install schemes

[Filipe Laíns]

New submission from Filipe Laíns <lains at riseup.net>:

As part of the distutils migration we plan to add a mechanism to let Python distributors to add site install schemes.

Currently, Python distributors are patching distutils to add custom install schemes for their packages. I think most of the reasoning boils down to them wanting to stop Python installers, such as pip, to modify/interfere with their packages.

With the distutils deprecation, and it becoming a 3rd party module, Python distributors can no longer patch it. Because of this, we made distutils use the sysconfig module instead, which fixes the issue at the moment -- Python distributors can now patch sysconfig itself -- but is not a long term solution.
To prevent Python distributors from having to patch implementation details, and have things break unexpectedly, we aim to introduce a system that distributors can use for this purpose.

The idea is that they have a config file, which they can pass to configure, and in that config file they can specify some extra install schemes. These install schemes will get added in sysconfig, and will be loaded in the site module initialization.

In practice, it will look something like this:

config.py
```
EXTRA_SITE_INSTALL_SCHEMES = {
    'posix_prefix': {
        'stdlib': '{installed_base}/{platlibdir}/python{py_version_short}',
        'platstdlib': '{platbase}/{platlibdir}/python{py_version_short}',
        'purelib': '{base}/lib/python{py_version_short}/vendor-packages',
        'platlib': '{platbase}/{platlibdir}/python{py_version_short}/vendor-packages',
        'include':
            '{installed_base}/include/python{py_version_short}{abiflags}',
        'platinclude':
            '{installed_platbase}/include/python{py_version_short}{abiflags}',
        'scripts': '{base}/bin',
        'data': '{base}',
    },
}
```

./configure --with-vendor-config=config.py

----------
components: Library (Lib)
messages: 392326
nosy: FFY00, jaraco
priority: normal
pull_requests: 24414
severity: normal
status: open
title: Introduce mechanism to allow Python distributors to add custom site install schemes
type: enhancement
versions: Python 3.10

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue43976>
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