Allow to decide whether to use distutils or sysconfig with sysconfig._PIP_USE_SYSCONFIG

Fixes https://github.com/pypa/pip/issues/10647

news/10647.feature.rst

@@ -0,0 +1,3 @@
+Allow Python distributors to opt-out from or opt-in to the ``sysconfig``
+installation scheme backend by setting ``sysconfig._PIP_USE_SYSCONFIG``
+to ``True`` or ``False``.

src/pip/_internal/locations/__init__.py

@@ -45,7 +45,22 @@ else:
 
 _PLATLIBDIR: str = getattr(sys, "platlibdir", "lib")
 
-_USE_SYSCONFIG = sys.version_info >= (3, 10)
+
+def _should_use_sysconfig() -> bool:
+    """
+    This function determines the value of _USE_SYSCONFIG.
+    By default, pip uses sysconfig on Python 3.10+.
+    But Python distributors can override this decision by setting:
+        sysconfig._PIP_USE_SYSCONFIG = True / False
+    Rationale in https://github.com/pypa/pip/issues/10647
+    """
+    if hasattr(sysconfig, "_PIP_USE_SYSCONFIG"):
+        return bool(sysconfig._PIP_USE_SYSCONFIG)  # type: ignore [attr-defined]
+    return sys.version_info >= (3, 10)
+
+
+# This is a function for testability, but should be constant during any one run.
+_USE_SYSCONFIG = _should_use_sysconfig()
 
 
 def _looks_like_bpo_44860() -> bool:

tests/unit/test_locations.py

@@ -6,13 +6,14 @@ import getpass
 import os
 import shutil
 import sys
+import sysconfig
 import tempfile
 from typing import Any, Dict
 from unittest.mock import Mock
 
 import pytest
 
-from pip._internal.locations import SCHEME_KEYS, get_scheme
+from pip._internal.locations import SCHEME_KEYS, _should_use_sysconfig, get_scheme
 from tests.lib.path import Path
 
 if sys.platform == "win32":
@@ -82,6 +83,24 @@ class TestLocations:
         result.pw_name = self.username
         return result
 
+    def test_default_should_use_sysconfig(
+        self, monkeypatch: pytest.MonkeyPatch
+    ) -> None:
+        monkeypatch.delattr(sysconfig, "_PIP_USE_SYSCONFIG", raising=False)
+        if sys.version_info[:2] >= (3, 10):
+            assert _should_use_sysconfig() is True
+        else:
+            assert _should_use_sysconfig() is False
+
+    @pytest.mark.parametrize("vendor_value", [True, False, None, "", 0, 1])
+    def test_vendor_overriden_should_use_sysconfig(
+        self, monkeypatch: pytest.MonkeyPatch, vendor_value: Any
+    ) -> None:
+        monkeypatch.setattr(
+            sysconfig, "_PIP_USE_SYSCONFIG", vendor_value, raising=False
+        )
+        assert _should_use_sysconfig() is bool(vendor_value)
+
 
 class TestDistutilsScheme:
     def test_root_modifies_appropriately(self) -> None: