`venv` on Windows preserves the `\\?\` extended-length path prefix in `sys.prefix` on 3.14, unlike 3.10 - breaks tools that join sysconfig paths with relative (`..`) components

[AlReiter]

Bug report

Bug description:

Repro

python -m venv "\\?\C:\path\to\venv" --system-site-packages --without-pip
"\?\C:\path\to\venv\Scripts\python.exe" -c "import sys; print(sys.prefix)"

• Python 3.10.0: prints C:\path\to\venv
• Python 3.14.0: prints \\?\C:\path\to\venv

Real-world consequence: installing any package with a scripts= file that has a shebang fails on 3.14, even on a fresh install. Minimal setup.py:

from setuptools import setup
setup(name="testpkg", version="1.0.0", py_modules=["testpkg"], scripts=["bottle.py"])

bottle.py:

#!/usr/bin/env python
print("hi")

pip install of the built wheel into the \\?\-prefixed venv above fails with:

ERROR: Could not install packages due to an OSError: [Errno 22] Invalid argument:
'\\?\C:\path\to\venv\Lib\site-packages\../../Scripts\bottle.py'

(Without the shebang line, the same package installs fine — the shebang is what makes pip rewrite the file and recompute its hash, which is what hits the broken path.)

(pip joins lib_dir with a RECORD-relative path like ../../Scripts/bottle.py via plain os.path.join — harmless on a normal path, but Windows does not resolve ..// for \\?\ paths, so open() rejects it. Reproduced identically with pip 24.3.1 and pip 26.2.1 — not pip-version-dependent.)

Root cause

• 3.10 (PC/getpathp.c): the executable path used to derive sys.prefix always comes from GetModuleFileNameW(NULL, ...) — a WinAPI call that reports the OS's own canonical module path and never includes \\?\, regardless of how the process was invoked.
• 3.14 (Modules/getpath.py/getpath.c): executable is taken from config.get('executable') (populated from the literal argv[0] used to invoke the process) and only falls back to the GetModuleFileNameW-derived real_executable when that's empty. Since argv[0] was \\?\C:\...\python.exe, it's used verbatim, \\?\ and all.

Suggested fix

In Modules/getpath.py, prefer real_executable (or strip a leading \\?\ / \\?\UNC\) over the literal configured executable on Windows when computing venv_prefix/prefix, restoring 3.10's behavior of always resolving through the OS-reported canonical path rather than trusting the invocation string.

CPython versions tested on:

3.14

Operating systems tested on:

Windows