Bug report

Bug description:

Even when disabling the site initialization (passing -S), sysconfig.get_paths() still returns site-specific paths, as there's no way in the current API to express the unavailability of such paths. A lot of code does not take this use-case into account, and assumes these are paths that are currently in-use for the active environment.

$ python -S
Python 3.14.0a1+ experimental free-threading build (heads/main:de0d5c6e2e1, Oct 23 2024, 15:37:46) [GCC 14.2.1 20240910] on linux
>>> import sys
>>> sys.path
['', '/usr/local/lib/python314t.zip', '/usr/local/lib/python3.14t', '/usr/local/lib/python3.14t/lib-dynload']
>>> import sysconfig
>>> sysconfig.get_paths()
{'stdlib': '/usr/local/lib/python3.14t', 'platstdlib': '/usr/local/lib/python3.14t', 'purelib': '/usr/local/lib/python3.14t/site-packages', 'platlib': '/usr/local/lib/python3.14t/site-packages', 'include': '/usr/local/include/python3.14td', 'platinclude': '/usr/local/include/python3.14td', 'scripts': '/usr/local/bin', 'data': '/usr/local'}

An example of this is ensurepip, which will still proceed and install pip to the site directories, despite the site module being disabled.

$ python -S -m ensurepip
Looking in links: /tmp/tmpqcwxcy4k
Requirement already satisfied: pip in /home/anubis/.virtualenvs/test-sysconfig-paths/lib/python3.14t/site-packages (24.3.1)

IMO, it should refuse to install when the site module is disabled, as there are no valid paths to install pip to under the current environment.

To make things worse, the result of sysconfig.get_paths(), currently, is inconsistent, and does not always return the same result (see #126789).

Newer APIs (#103481), should handle this scenario directly, by expressing directly the unavailability of purelib and platlib, for example.

CPython versions tested on:

3.9, 3.10, 3.11, 3.12, 3.13, 3.14, CPython main branch

Operating systems tested on:

Linux