From 738e6005067618c339e637ad78a839c97ad771ca Mon Sep 17 00:00:00 2001
From: Tzu-ping Chung <uranusjr@gmail.com>
Date: Thu, 18 Feb 2021 19:27:30 +0800
Subject: [PATCH 01/17] Move distuitls location logic into subpackage

---
 src/pip/_internal/commands/install.py         | 22 ++++---
 src/pip/_internal/locations/__init__.py       | 53 +++++++++++++++++
 .../{locations.py => locations/_distutils.py} | 59 ++++---------------
 src/pip/_internal/locations/base.py           | 51 ++++++++++++++++
 src/pip/_internal/locations/sysconfig.py      |  0
 src/pip/_internal/req/req_uninstall.py        | 10 ++--
 6 files changed, 136 insertions(+), 59 deletions(-)
 create mode 100644 src/pip/_internal/locations/__init__.py
 rename src/pip/_internal/{locations.py => locations/_distutils.py} (77%)
 create mode 100644 src/pip/_internal/locations/base.py
 create mode 100644 src/pip/_internal/locations/sysconfig.py

diff --git a/src/pip/_internal/commands/install.py b/src/pip/_internal/commands/install.py
index 78cd0b5cf68..dc743ee0b50 100644
--- a/src/pip/_internal/commands/install.py
+++ b/src/pip/_internal/commands/install.py
@@ -15,7 +15,7 @@
 from pip._internal.cli.req_command import RequirementCommand, with_cleanup
 from pip._internal.cli.status_codes import ERROR, SUCCESS
 from pip._internal.exceptions import CommandError, InstallationError
-from pip._internal.locations import distutils_scheme
+from pip._internal.locations import get_scheme
 from pip._internal.metadata import get_environment
 from pip._internal.models.format_control import FormatControl
 from pip._internal.operations.check import ConflictDetails, check_install_conflicts
@@ -455,10 +455,10 @@ def _handle_target_dir(self, target_dir, target_temp_dir, upgrade):
 
         # Checking both purelib and platlib directories for installed
         # packages to be moved to target directory
-        scheme = distutils_scheme('', home=target_temp_dir.path)
-        purelib_dir = scheme['purelib']
-        platlib_dir = scheme['platlib']
-        data_dir = scheme['data']
+        scheme = get_scheme('', home=target_temp_dir.path)
+        purelib_dir = scheme.purelib
+        platlib_dir = scheme.platlib
+        data_dir = scheme.data
 
         if os.path.exists(purelib_dir):
             lib_dir_list.append(purelib_dir)
@@ -574,9 +574,15 @@ def get_lib_location_guesses(
         prefix=None  # type: Optional[str]
 ):
     # type:(...) -> List[str]
-    scheme = distutils_scheme('', user=user, home=home, root=root,
-                              isolated=isolated, prefix=prefix)
-    return [scheme['purelib'], scheme['platlib']]
+    scheme = get_scheme(
+        '',
+        user=user,
+        home=home,
+        root=root,
+        isolated=isolated,
+        prefix=prefix,
+    )
+    return [scheme.purelib, scheme.platlib]
 
 
 def site_packages_writable(root, isolated):
diff --git a/src/pip/_internal/locations/__init__.py b/src/pip/_internal/locations/__init__.py
new file mode 100644
index 00000000000..9285165d7ee
--- /dev/null
+++ b/src/pip/_internal/locations/__init__.py
@@ -0,0 +1,53 @@
+from typing import Optional
+
+from pip._internal.models.scheme import Scheme
+
+from . import distutils as _distutils
+from .base import (
+    USER_CACHE_DIR,
+    get_major_minor_version,
+    get_src_prefix,
+    site_packages,
+    user_site,
+)
+
+__all__ = [
+    "USER_CACHE_DIR",
+    "get_bin_prefix",
+    "get_bin_user",
+    "get_major_minor_version",
+    "get_scheme",
+    "get_src_prefix",
+    "init_backend",
+    "site_packages",
+    "user_site",
+]
+
+
+def get_scheme(
+    dist_name,  # type: str
+    user=False,  # type: bool
+    home=None,  # type: Optional[str]
+    root=None,  # type: Optional[str]
+    isolated=False,  # type: bool
+    prefix=None,  # type: Optional[str]
+):
+    # type: (...) -> Scheme
+    return _distutils.get_scheme(
+        dist_name,
+        user=user,
+        home=home,
+        root=root,
+        isolated=isolated,
+        prefix=prefix,
+    )
+
+
+def get_bin_prefix():
+    # type: () -> str
+    return _distutils.get_bin_prefix()
+
+
+def get_bin_user():
+    # type: () -> str
+    return _distutils.get_bin_user()
diff --git a/src/pip/_internal/locations.py b/src/pip/_internal/locations/_distutils.py
similarity index 77%
rename from src/pip/_internal/locations.py
rename to src/pip/_internal/locations/_distutils.py
index 19c039eabf8..efc12e8da29 100644
--- a/src/pip/_internal/locations.py
+++ b/src/pip/_internal/locations/_distutils.py
@@ -5,62 +5,19 @@
 
 import os
 import os.path
-import site
 import sys
-import sysconfig
 from distutils.cmd import Command as DistutilsCommand
 from distutils.command.install import SCHEME_KEYS
 from distutils.command.install import install as distutils_install_command
 from typing import Dict, List, Optional, Union, cast
 
 from pip._internal.models.scheme import Scheme
-from pip._internal.utils import appdirs
 from pip._internal.utils.compat import WINDOWS
 from pip._internal.utils.virtualenv import running_under_virtualenv
 
-# Application Directories
-USER_CACHE_DIR = appdirs.user_cache_dir("pip")
+from .base import get_major_minor_version, user_site
 
 
-def get_major_minor_version():
-    # type: () -> str
-    """
-    Return the major-minor version of the current Python as a string, e.g.
-    "3.7" or "3.10".
-    """
-    return '{}.{}'.format(*sys.version_info)
-
-
-def get_src_prefix():
-    # type: () -> str
-    if running_under_virtualenv():
-        src_prefix = os.path.join(sys.prefix, 'src')
-    else:
-        # FIXME: keep src in cwd for now (it is not a temporary folder)
-        try:
-            src_prefix = os.path.join(os.getcwd(), 'src')
-        except OSError:
-            # In case the current working directory has been renamed or deleted
-            sys.exit(
-                "The folder you are executing pip from can no longer be found."
-            )
-
-    # under macOS + virtualenv sys.prefix is not properly resolved
-    # it is something like /path/to/python/bin/..
-    return os.path.abspath(src_prefix)
-
-
-# FIXME doesn't account for venv linked to global site-packages
-
-site_packages = sysconfig.get_path("purelib")  # type: Optional[str]
-
-try:
-    # Use getusersitepackages if this is present, as it ensures that the
-    # value is initialised properly.
-    user_site = site.getusersitepackages()
-except AttributeError:
-    user_site = site.USER_SITE
-
 if WINDOWS:
     bin_py = os.path.join(sys.prefix, 'Scripts')
     bin_user = os.path.join(user_site, 'Scripts')
@@ -78,7 +35,7 @@ def get_src_prefix():
         bin_py = '/usr/local/bin'
 
 
-def distutils_scheme(
+def _distutils_scheme(
     dist_name, user=False, home=None, root=None, isolated=False, prefix=None
 ):
     # type:(str, bool, str, str, bool, str) -> Dict[str, str]
@@ -168,7 +125,7 @@ def get_scheme(
     :param prefix: indicates to use the "prefix" scheme and provides the
         base directory for the same
     """
-    scheme = distutils_scheme(
+    scheme = _distutils_scheme(
         dist_name, user, home, root, isolated, prefix
     )
     return Scheme(
@@ -178,3 +135,13 @@ def get_scheme(
         scripts=scheme["scripts"],
         data=scheme["data"],
     )
+
+
+def get_bin_prefix():
+    # type: () -> str
+    return bin_py
+
+
+def get_bin_user():
+    # type: () -> str
+    return bin_user
diff --git a/src/pip/_internal/locations/base.py b/src/pip/_internal/locations/base.py
new file mode 100644
index 00000000000..8c9f7751179
--- /dev/null
+++ b/src/pip/_internal/locations/base.py
@@ -0,0 +1,51 @@
+import os
+import site
+import sys
+import sysconfig
+import typing
+
+from pip._internal.utils import appdirs
+from pip._internal.utils.virtualenv import running_under_virtualenv
+
+
+# Application Directories
+USER_CACHE_DIR = appdirs.user_cache_dir("pip")
+
+# FIXME doesn't account for venv linked to global site-packages
+site_packages = sysconfig.get_path("purelib")  # type: typing.Optional[str]
+
+
+def get_major_minor_version():
+    # type: () -> str
+    """
+    Return the major-minor version of the current Python as a string, e.g.
+    "3.7" or "3.10".
+    """
+    return '{}.{}'.format(*sys.version_info)
+
+
+def get_src_prefix():
+    # type: () -> str
+    if running_under_virtualenv():
+        src_prefix = os.path.join(sys.prefix, 'src')
+    else:
+        # FIXME: keep src in cwd for now (it is not a temporary folder)
+        try:
+            src_prefix = os.path.join(os.getcwd(), 'src')
+        except OSError:
+            # In case the current working directory has been renamed or deleted
+            sys.exit(
+                "The folder you are executing pip from can no longer be found."
+            )
+
+    # under macOS + virtualenv sys.prefix is not properly resolved
+    # it is something like /path/to/python/bin/..
+    return os.path.abspath(src_prefix)
+
+
+try:
+    # Use getusersitepackages if this is present, as it ensures that the
+    # value is initialised properly.
+    user_site = site.getusersitepackages()
+except AttributeError:
+    user_site = site.USER_SITE
diff --git a/src/pip/_internal/locations/sysconfig.py b/src/pip/_internal/locations/sysconfig.py
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/src/pip/_internal/req/req_uninstall.py b/src/pip/_internal/req/req_uninstall.py
index 519b79166a4..776652de1cc 100644
--- a/src/pip/_internal/req/req_uninstall.py
+++ b/src/pip/_internal/req/req_uninstall.py
@@ -11,7 +11,7 @@
 from pip._vendor.pkg_resources import Distribution
 
 from pip._internal.exceptions import UninstallationError
-from pip._internal.locations import bin_py, bin_user
+from pip._internal.locations import get_bin_prefix, get_bin_user
 from pip._internal.utils.compat import WINDOWS
 from pip._internal.utils.logging import indent_log
 from pip._internal.utils.misc import (
@@ -36,9 +36,9 @@ def _script_names(dist, script_name, is_gui):
     Returns the list of file names
     """
     if dist_in_usersite(dist):
-        bin_dir = bin_user
+        bin_dir = get_bin_user()
     else:
-        bin_dir = bin_py
+        bin_dir = get_bin_prefix()
     exe_name = os.path.join(bin_dir, script_name)
     paths_to_remove = [exe_name]
     if WINDOWS:
@@ -551,9 +551,9 @@ def from_dist(cls, dist):
         if dist.has_metadata('scripts') and dist.metadata_isdir('scripts'):
             for script in dist.metadata_listdir('scripts'):
                 if dist_in_usersite(dist):
-                    bin_dir = bin_user
+                    bin_dir = get_bin_user()
                 else:
-                    bin_dir = bin_py
+                    bin_dir = get_bin_prefix()
                 paths_to_remove.add(os.path.join(bin_dir, script))
                 if WINDOWS:
                     paths_to_remove.add(os.path.join(bin_dir, script) + '.bat')

From 7662c5961e5d2fe8a9cbceb48206b3f7daaec169 Mon Sep 17 00:00:00 2001
From: Tzu-ping Chung <uranusjr@gmail.com>
Date: Thu, 18 Feb 2021 20:56:45 +0800
Subject: [PATCH 02/17] Implement sysconfig locations and warn on mismatch

---
 src/pip/_internal/exceptions.py           |  15 +++
 src/pip/_internal/locations/__init__.py   |  61 +++++++++-
 src/pip/_internal/locations/_sysconfig.py | 137 ++++++++++++++++++++++
 src/pip/_internal/locations/sysconfig.py  |   0
 tests/unit/test_locations.py              |  19 +--
 5 files changed, 220 insertions(+), 12 deletions(-)
 create mode 100644 src/pip/_internal/locations/_sysconfig.py
 delete mode 100644 src/pip/_internal/locations/sysconfig.py

diff --git a/src/pip/_internal/exceptions.py b/src/pip/_internal/exceptions.py
index 01ee4b76984..8aacf812014 100644
--- a/src/pip/_internal/exceptions.py
+++ b/src/pip/_internal/exceptions.py
@@ -59,6 +59,21 @@ def __str__(self):
         )
 
 
+class UserInstallationInvalid(InstallationError):
+    """A --user install is requested on an environment without user site."""
+
+    def __str__(self):
+        # type: () -> str
+        return "User base directory is not specified"
+
+
+class InvalidSchemeCombination(InstallationError):
+    def __str__(self):
+        # type: () -> str
+        before = ", ".join(str(a) for a in self.args[:-1])
+        return f"Cannot set {before} and {self.args[-1]} together"
+
+
 class DistributionNotFound(InstallationError):
     """Raised when a distribution cannot be found to satisfy a requirement"""
 
diff --git a/src/pip/_internal/locations/__init__.py b/src/pip/_internal/locations/__init__.py
index 9285165d7ee..6fc4d53e15d 100644
--- a/src/pip/_internal/locations/__init__.py
+++ b/src/pip/_internal/locations/__init__.py
@@ -1,8 +1,11 @@
+import logging
+import pathlib
+import sysconfig
 from typing import Optional
 
-from pip._internal.models.scheme import Scheme
+from pip._internal.models.scheme import SCHEME_KEYS, Scheme
 
-from . import distutils as _distutils
+from . import _distutils, _sysconfig
 from .base import (
     USER_CACHE_DIR,
     get_major_minor_version,
@@ -24,6 +27,31 @@
 ]
 
 
+logger = logging.getLogger(__name__)
+
+
+def _default_base(*, user):
+    # type: (bool) -> str
+    if user:
+        base = sysconfig.get_config_var("userbase")
+    else:
+        base = sysconfig.get_config_var("base")
+    assert base is not None
+    return base
+
+
+def _warn_if_mismatch(old, new, *, key):
+    # type: (pathlib.Path, pathlib.Path, str) -> None
+    if old == new:
+        return
+    message = (
+        "Value for %s does not match. Please report this: <URL HERE>"
+        "\ndistutils: %s"
+        "\nsysconfig: %s"
+    )
+    logger.warning(message, key, old, new)
+
+
 def get_scheme(
     dist_name,  # type: str
     user=False,  # type: bool
@@ -33,7 +61,15 @@ def get_scheme(
     prefix=None,  # type: Optional[str]
 ):
     # type: (...) -> Scheme
-    return _distutils.get_scheme(
+    old = _distutils.get_scheme(
+        dist_name,
+        user=user,
+        home=home,
+        root=root,
+        isolated=isolated,
+        prefix=prefix,
+    )
+    new = _sysconfig.get_scheme(
         dist_name,
         user=user,
         home=home,
@@ -42,12 +78,27 @@ def get_scheme(
         prefix=prefix,
     )
 
+    base = prefix or home or _default_base(user=user)
+    for k in SCHEME_KEYS:
+        # Extra join because distutils can return relative paths.
+        old_v = pathlib.Path(base, getattr(old, k))
+        new_v = pathlib.Path(getattr(new, k))
+        _warn_if_mismatch(old_v, new_v, key=f"scheme.{k}")
+
+    return old
+
 
 def get_bin_prefix():
     # type: () -> str
-    return _distutils.get_bin_prefix()
+    old = _distutils.get_bin_prefix()
+    new = _sysconfig.get_bin_prefix()
+    _warn_if_mismatch(pathlib.Path(old), pathlib.Path(new), key="bin_prefix")
+    return old
 
 
 def get_bin_user():
     # type: () -> str
-    return _distutils.get_bin_user()
+    old = _distutils.get_bin_user()
+    new = _sysconfig.get_bin_user()
+    _warn_if_mismatch(pathlib.Path(old), pathlib.Path(new), key="bin_user")
+    return old
diff --git a/src/pip/_internal/locations/_sysconfig.py b/src/pip/_internal/locations/_sysconfig.py
new file mode 100644
index 00000000000..b9687870466
--- /dev/null
+++ b/src/pip/_internal/locations/_sysconfig.py
@@ -0,0 +1,137 @@
+import distutils.util  # FIXME: For change_root.
+import logging
+import os
+import sys
+import sysconfig
+import typing
+
+from pip._internal.exceptions import (
+    InvalidSchemeCombination,
+    UserInstallationInvalid,
+)
+from pip._internal.models.scheme import SCHEME_KEYS, Scheme
+from pip._internal.utils.virtualenv import running_under_virtualenv
+
+from .base import get_major_minor_version
+
+
+logger = logging.getLogger(__name__)
+
+
+_AVAILABLE_SCHEMES = set(sysconfig.get_scheme_names())
+
+
+def _infer_scheme(variant):
+    # (typing.Literal["home", "prefix", "user"]) -> str
+    """Try to find a scheme for the current platform.
+
+    Unfortunately ``_get_default_scheme()`` is private, so there's no way to
+    ask things like "what is the '_home' scheme on this platform". This tries
+    to answer that with some heuristics while accounting for ad-hoc platforms
+    not covered by CPython's default sysconfig implementation.
+    """
+    # Most schemes are named like this e.g. "posix_home", "nt_user".
+    suffixed = f"{os.name}_{variant}"
+    if suffixed in _AVAILABLE_SCHEMES:
+        return suffixed
+
+    # The user scheme is not available.
+    if variant == "user" and sysconfig.get_config_var("userbase") is None:
+        raise UserInstallationInvalid()
+
+    # On Windows, prefx and home schemes are the same and just called "nt".
+    if os.name in _AVAILABLE_SCHEMES:
+        return os.name
+
+    # Not sure what's happening, some obscure platform that does not fully
+    # implement sysconfig? Just use the POSIX scheme.
+    logger.warning("No %r scheme for %r; fallback to POSIX.", variant, os.name)
+    return f"posix_{variant}"
+
+
+# Update these keys if the user sets a custom home.
+_HOME_KEYS = (
+    "installed_base",
+    "base",
+    "installed_platbase",
+    "platbase",
+    "prefix",
+    "exec_prefix",
+)
+if sysconfig.get_config_var("userbase") is not None:
+    _HOME_KEYS += ("userbase",)
+
+
+def get_scheme(
+    dist_name,  # type: str
+    user=False,  # type: bool
+    home=None,  # type: typing.Optional[str]
+    root=None,  # type: typing.Optional[str]
+    isolated=False,  # type: bool
+    prefix=None,  # type: typing.Optional[str]
+):
+    # type: (...) -> Scheme
+    """
+    Get the "scheme" corresponding to the input parameters.
+
+    :param dist_name: the name of the package to retrieve the scheme for, used
+        in the headers scheme path
+    :param user: indicates to use the "user" scheme
+    :param home: indicates to use the "home" scheme
+    :param root: root under which other directories are re-based
+    :param isolated: ignored, but kept for distutils compatibility (where
+        this controls whether the user-site pydistutils.cfg is honored)
+    :param prefix: indicates to use the "prefix" scheme and provides the
+        base directory for the same
+    """
+    if user and prefix:
+        raise InvalidSchemeCombination("--user", "--prefix")
+    if home and prefix:
+        raise InvalidSchemeCombination("--home", "--prefix")
+
+    if home is not None:
+        scheme = _infer_scheme("home")
+    elif user:
+        scheme = _infer_scheme("user")
+    else:
+        scheme = _infer_scheme("prefix")
+
+    if home is not None:
+        variables = {k: home for k in _HOME_KEYS}
+    elif prefix is not None:
+        variables = {k: prefix for k in _HOME_KEYS}
+    else:
+        variables = {}
+
+    paths = sysconfig.get_paths(scheme=scheme, vars=variables)
+
+    # Special header path for compatibility to distutils.
+    if running_under_virtualenv():
+        base = variables.get("base", sys.prefix)
+        python_xy = f"python{get_major_minor_version()}"
+        paths["include"] = os.path.join(base, "include", "site", python_xy)
+
+    scheme = Scheme(
+        platlib=paths["platlib"],
+        purelib=paths["purelib"],
+        headers=os.path.join(paths["include"], dist_name),
+        scripts=paths["scripts"],
+        data=paths["data"],
+    )
+    if root is not None:
+        for key in SCHEME_KEYS:
+            value = distutils.util.change_root(root, getattr(scheme, key))
+            setattr(scheme, key, value)
+    return scheme
+
+
+def get_bin_prefix():
+    # type: () -> str
+    # Forcing to use /usr/local/bin for standard macOS framework installs.
+    if sys.platform[:6] == "darwin" and sys.prefix[:16] == "/System/Library/":
+        return "/usr/local/bin"
+    return sysconfig.get_path("scripts", scheme=_infer_scheme("prefix"))
+
+
+def get_bin_user():
+    return sysconfig.get_path("scripts", scheme=_infer_scheme("user"))
diff --git a/src/pip/_internal/locations/sysconfig.py b/src/pip/_internal/locations/sysconfig.py
deleted file mode 100644
index e69de29bb2d..00000000000
diff --git a/tests/unit/test_locations.py b/tests/unit/test_locations.py
index 3d4ec946245..067f4e84486 100644
--- a/tests/unit/test_locations.py
+++ b/tests/unit/test_locations.py
@@ -11,7 +11,7 @@
 
 import pytest
 
-from pip._internal.locations import distutils_scheme
+from pip._internal.locations import SCHEME_KEYS, get_scheme
 
 if sys.platform == 'win32':
     pwd = Mock()
@@ -19,6 +19,11 @@
     import pwd
 
 
+def _get_scheme_dict(*args, **kwargs):
+    scheme = get_scheme(*args, **kwargs)
+    return {k: getattr(scheme, k) for k in SCHEME_KEYS}
+
+
 class TestLocations:
     def setup(self):
         self.tempdir = tempfile.mkdtemp()
@@ -83,8 +88,8 @@ def test_root_modifies_appropriately(self, monkeypatch):
         # root is c:\somewhere\else or /somewhere/else
         root = os.path.normcase(os.path.abspath(
             os.path.join(os.path.sep, 'somewhere', 'else')))
-        norm_scheme = distutils_scheme("example")
-        root_scheme = distutils_scheme("example", root=root)
+        norm_scheme = _get_scheme_dict("example")
+        root_scheme = _get_scheme_dict("example", root=root)
 
         for key, value in norm_scheme.items():
             drive, path = os.path.splitdrive(os.path.abspath(value))
@@ -107,7 +112,7 @@ def test_distutils_config_file_read(self, tmpdir, monkeypatch):
             'find_config_files',
             lambda self: [f],
         )
-        scheme = distutils_scheme('example')
+        scheme = _get_scheme_dict('example')
         assert scheme['scripts'] == install_scripts
 
     @pytest.mark.incompatible_with_venv
@@ -129,15 +134,15 @@ def test_install_lib_takes_precedence(self, tmpdir, monkeypatch):
             'find_config_files',
             lambda self: [f],
         )
-        scheme = distutils_scheme('example')
+        scheme = _get_scheme_dict('example')
         assert scheme['platlib'] == install_lib + os.path.sep
         assert scheme['purelib'] == install_lib + os.path.sep
 
     def test_prefix_modifies_appropriately(self):
         prefix = os.path.abspath(os.path.join('somewhere', 'else'))
 
-        normal_scheme = distutils_scheme("example")
-        prefix_scheme = distutils_scheme("example", prefix=prefix)
+        normal_scheme = _get_scheme_dict("example")
+        prefix_scheme = _get_scheme_dict("example", prefix=prefix)
 
         def _calculate_expected(value):
             path = os.path.join(prefix, os.path.relpath(value, sys.prefix))

From edbda257c6b19cbabd5a3f3328427a68f10fc780 Mon Sep 17 00:00:00 2001
From: Tzu-ping Chung <uranusjr@gmail.com>
Date: Fri, 19 Feb 2021 17:14:11 +0800
Subject: [PATCH 03/17] Abstract out get_python_lib() usages

---
 src/pip/_internal/build_env.py            | 16 ++---------
 src/pip/_internal/locations/__init__.py   | 35 ++++++++++++++++++++++-
 src/pip/_internal/locations/_distutils.py | 21 +++++++++++++-
 src/pip/_internal/locations/_sysconfig.py | 16 +++++++++++
 4 files changed, 73 insertions(+), 15 deletions(-)

diff --git a/src/pip/_internal/build_env.py b/src/pip/_internal/build_env.py
index b1c877cfd9b..f4a1c93353a 100644
--- a/src/pip/_internal/build_env.py
+++ b/src/pip/_internal/build_env.py
@@ -6,7 +6,6 @@
 import sys
 import textwrap
 from collections import OrderedDict
-from distutils.sysconfig import get_python_lib
 from sysconfig import get_paths
 from types import TracebackType
 from typing import TYPE_CHECKING, Iterable, List, Optional, Set, Tuple, Type
@@ -15,6 +14,7 @@
 
 from pip import __file__ as pip_location
 from pip._internal.cli.spinners import open_spinner
+from pip._internal.locations import get_platlib, get_prefixed_libs, get_purelib
 from pip._internal.utils.subprocess import call_subprocess
 from pip._internal.utils.temp_dir import TempDirectory, tempdir_kinds
 
@@ -34,14 +34,7 @@ def __init__(self, path):
             'nt' if os.name == 'nt' else 'posix_prefix',
             vars={'base': path, 'platbase': path}
         )['scripts']
-        # Note: prefer distutils' sysconfig to get the
-        # library paths so PyPy is correctly supported.
-        purelib = get_python_lib(plat_specific=False, prefix=path)
-        platlib = get_python_lib(plat_specific=True, prefix=path)
-        if purelib == platlib:
-            self.lib_dirs = [purelib]
-        else:
-            self.lib_dirs = [purelib, platlib]
+        self.lib_dirs = get_prefixed_libs(path)
 
 
 class BuildEnvironment:
@@ -69,10 +62,7 @@ def __init__(self):
         # - ensure .pth files are honored
         # - prevent access to system site packages
         system_sites = {
-            os.path.normcase(site) for site in (
-                get_python_lib(plat_specific=False),
-                get_python_lib(plat_specific=True),
-            )
+            os.path.normcase(site) for site in (get_purelib(), get_platlib())
         }
         self._site_dir = os.path.join(temp_dir.path, 'site')
         if not os.path.exists(self._site_dir):
diff --git a/src/pip/_internal/locations/__init__.py b/src/pip/_internal/locations/__init__.py
index 6fc4d53e15d..f57bb9a8b07 100644
--- a/src/pip/_internal/locations/__init__.py
+++ b/src/pip/_internal/locations/__init__.py
@@ -1,7 +1,7 @@
 import logging
 import pathlib
 import sysconfig
-from typing import Optional
+from typing import List, Optional
 
 from pip._internal.models.scheme import SCHEME_KEYS, Scheme
 
@@ -19,6 +19,9 @@
     "get_bin_prefix",
     "get_bin_user",
     "get_major_minor_version",
+    "get_platlib",
+    "get_prefixed_libs",
+    "get_purelib",
     "get_scheme",
     "get_src_prefix",
     "init_backend",
@@ -102,3 +105,33 @@ def get_bin_user():
     new = _sysconfig.get_bin_user()
     _warn_if_mismatch(pathlib.Path(old), pathlib.Path(new), key="bin_user")
     return old
+
+
+def get_purelib():
+    # type: () -> str
+    """Return the default pure-Python lib location."""
+    old = _distutils.get_purelib()
+    new = _sysconfig.get_purelib()
+    _warn_if_mismatch(pathlib.Path(old), pathlib.Path(new), key="purelib")
+    return old
+
+
+def get_platlib():
+    # type: () -> str
+    """Return the default platform-shared lib location."""
+    old = _distutils.get_platlib()
+    new = _sysconfig.get_platlib()
+    _warn_if_mismatch(pathlib.Path(old), pathlib.Path(new), key="platlib")
+    return old
+
+
+def get_prefixed_libs(prefix):
+    # type: (str) -> List[str]
+    """Return the lib locations under ``prefix``."""
+    old_pure, old_plat = _distutils.get_prefixed_libs(prefix)
+    new_pure, new_plat = _sysconfig.get_prefixed_libs(prefix)
+    _warn_if_mismatch(old_pure, new_pure, key="prefixed-purelib")
+    _warn_if_mismatch(old_plat, new_plat, key="prefixed-platlib")
+    if old_pure == old_plat:
+        return [old_pure]
+    return [old_pure, old_plat]
diff --git a/src/pip/_internal/locations/_distutils.py b/src/pip/_internal/locations/_distutils.py
index efc12e8da29..bf3ff728f6d 100644
--- a/src/pip/_internal/locations/_distutils.py
+++ b/src/pip/_internal/locations/_distutils.py
@@ -9,7 +9,8 @@
 from distutils.cmd import Command as DistutilsCommand
 from distutils.command.install import SCHEME_KEYS
 from distutils.command.install import install as distutils_install_command
-from typing import Dict, List, Optional, Union, cast
+from distutils.sysconfig import get_python_lib
+from typing import Dict, List, Optional, Tuple, Union, cast
 
 from pip._internal.models.scheme import Scheme
 from pip._internal.utils.compat import WINDOWS
@@ -145,3 +146,21 @@ def get_bin_prefix():
 def get_bin_user():
     # type: () -> str
     return bin_user
+
+
+def get_purelib():
+    # type: () -> str
+    return get_python_lib(plat_specific=False)
+
+
+def get_platlib():
+    # type: () -> str
+    return get_python_lib(plat_specific=True)
+
+
+def get_prefixed_libs(prefix):
+    # type: (str) -> Tuple[str, str]
+    return (
+        get_python_lib(plat_specific=False, prefix=prefix),
+        get_python_lib(plat_specific=True, prefix=prefix),
+    )
diff --git a/src/pip/_internal/locations/_sysconfig.py b/src/pip/_internal/locations/_sysconfig.py
index b9687870466..bc49cb75f77 100644
--- a/src/pip/_internal/locations/_sysconfig.py
+++ b/src/pip/_internal/locations/_sysconfig.py
@@ -135,3 +135,19 @@ def get_bin_prefix():
 
 def get_bin_user():
     return sysconfig.get_path("scripts", scheme=_infer_scheme("user"))
+
+
+def get_purelib():
+    # type: () -> str
+    return sysconfig.get_path("purelib")
+
+
+def get_platlib():
+    # type: () -> str
+    return sysconfig.get_path("platlib")
+
+
+def get_prefixed_libs(prefix):
+    # type: (str) -> typing.Tuple[str, str]
+    paths = sysconfig.get_paths(vars={"base": prefix, "platbase": prefix})
+    return (paths["purelib"], paths["platlib"])

From 686734fdd1e3c6e280b84157e9abc3da41357bfb Mon Sep 17 00:00:00 2001
From: Tzu-ping Chung <uranusjr@gmail.com>
Date: Fri, 19 Feb 2021 17:53:27 +0800
Subject: [PATCH 04/17] Provide issue URL to report

---
 src/pip/_internal/locations/__init__.py | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/pip/_internal/locations/__init__.py b/src/pip/_internal/locations/__init__.py
index f57bb9a8b07..db1c1309ee4 100644
--- a/src/pip/_internal/locations/__init__.py
+++ b/src/pip/_internal/locations/__init__.py
@@ -47,12 +47,13 @@ def _warn_if_mismatch(old, new, *, key):
     # type: (pathlib.Path, pathlib.Path, str) -> None
     if old == new:
         return
+    issue_url = "https://github.com/pypa/pip/issues/9617"
     message = (
-        "Value for %s does not match. Please report this: <URL HERE>"
+        "Value for %s does not match. Please report this to %s"
         "\ndistutils: %s"
         "\nsysconfig: %s"
     )
-    logger.warning(message, key, old, new)
+    logger.warning(message, key, issue_url, old, new)
 
 
 def get_scheme(

From b4545a01441ec3df4130664cbe2c15f3dfd783cc Mon Sep 17 00:00:00 2001
From: Tzu-ping Chung <uranusjr@gmail.com>
Date: Fri, 19 Feb 2021 17:59:18 +0800
Subject: [PATCH 05/17] News

---
 news/9617.process.rst | 3 +++
 1 file changed, 3 insertions(+)
 create mode 100644 news/9617.process.rst

diff --git a/news/9617.process.rst b/news/9617.process.rst
new file mode 100644
index 00000000000..f505c460541
--- /dev/null
+++ b/news/9617.process.rst
@@ -0,0 +1,3 @@
+Start installation scheme migration from ``distutils`` to ``sysconfig``. A
+warning is implemented to detect differences between the two implementations to
+encourage user reports, so we can avoid breakages before they happen.

From 08c282919f28c324de48118828e670f8fac88fd3 Mon Sep 17 00:00:00 2001
From: Tzu-ping Chung <uranusjr@gmail.com>
Date: Fri, 19 Feb 2021 18:11:20 +0800
Subject: [PATCH 06/17] Blackify

---
 src/pip/_internal/locations/_distutils.py | 39 +++++++++++------------
 src/pip/_internal/locations/_sysconfig.py |  6 +---
 src/pip/_internal/locations/base.py       | 11 +++----
 3 files changed, 23 insertions(+), 33 deletions(-)

diff --git a/src/pip/_internal/locations/_distutils.py b/src/pip/_internal/locations/_distutils.py
index bf3ff728f6d..70b7e8d6dd3 100644
--- a/src/pip/_internal/locations/_distutils.py
+++ b/src/pip/_internal/locations/_distutils.py
@@ -20,20 +20,20 @@
 
 
 if WINDOWS:
-    bin_py = os.path.join(sys.prefix, 'Scripts')
-    bin_user = os.path.join(user_site, 'Scripts')
+    bin_py = os.path.join(sys.prefix, "Scripts")
+    bin_user = os.path.join(user_site, "Scripts")
     # buildout uses 'bin' on Windows too?
     if not os.path.exists(bin_py):
-        bin_py = os.path.join(sys.prefix, 'bin')
-        bin_user = os.path.join(user_site, 'bin')
+        bin_py = os.path.join(sys.prefix, "bin")
+        bin_user = os.path.join(user_site, "bin")
 else:
-    bin_py = os.path.join(sys.prefix, 'bin')
-    bin_user = os.path.join(user_site, 'bin')
+    bin_py = os.path.join(sys.prefix, "bin")
+    bin_user = os.path.join(user_site, "bin")
 
     # Forcing to use /usr/local/bin for standard macOS framework installs
     # Also log to ~/Library/Logs/ for use with the Console.app log viewer
-    if sys.platform[:6] == 'darwin' and sys.prefix[:16] == '/System/Library/':
-        bin_py = '/usr/local/bin'
+    if sys.platform[:6] == "darwin" and sys.prefix[:16] == "/System/Library/":
+        bin_py = "/usr/local/bin"
 
 
 def _distutils_scheme(
@@ -45,14 +45,14 @@ def _distutils_scheme(
     """
     from distutils.dist import Distribution
 
-    dist_args = {'name': dist_name}  # type: Dict[str, Union[str, List[str]]]
+    dist_args = {"name": dist_name}  # type: Dict[str, Union[str, List[str]]]
     if isolated:
         dist_args["script_args"] = ["--no-user-cfg"]
 
     d = Distribution(dist_args)
     d.parse_config_files()
     obj = None  # type: Optional[DistutilsCommand]
-    obj = d.get_command_obj('install', create=True)
+    obj = d.get_command_obj("install", create=True)
     assert obj is not None
     i = cast(distutils_install_command, obj)
     # NOTE: setting user or home has the side-effect of creating the home dir
@@ -70,28 +70,27 @@ def _distutils_scheme(
 
     scheme = {}
     for key in SCHEME_KEYS:
-        scheme[key] = getattr(i, 'install_' + key)
+        scheme[key] = getattr(i, "install_" + key)
 
     # install_lib specified in setup.cfg should install *everything*
     # into there (i.e. it takes precedence over both purelib and
     # platlib).  Note, i.install_lib is *always* set after
     # finalize_options(); we only want to override here if the user
     # has explicitly requested it hence going back to the config
-    if 'install_lib' in d.get_option_dict('install'):
+    if "install_lib" in d.get_option_dict("install"):
         scheme.update(dict(purelib=i.install_lib, platlib=i.install_lib))
 
     if running_under_virtualenv():
-        scheme['headers'] = os.path.join(
+        scheme["headers"] = os.path.join(
             i.prefix,
-            'include',
-            'site',
-            f'python{get_major_minor_version()}',
+            "include",
+            "site",
+            f"python{get_major_minor_version()}",
             dist_name,
         )
 
         if root is not None:
-            path_no_drive = os.path.splitdrive(
-                os.path.abspath(scheme["headers"]))[1]
+            path_no_drive = os.path.splitdrive(os.path.abspath(scheme["headers"]))[1]
             scheme["headers"] = os.path.join(
                 root,
                 path_no_drive[1:],
@@ -126,9 +125,7 @@ def get_scheme(
     :param prefix: indicates to use the "prefix" scheme and provides the
         base directory for the same
     """
-    scheme = _distutils_scheme(
-        dist_name, user, home, root, isolated, prefix
-    )
+    scheme = _distutils_scheme(dist_name, user, home, root, isolated, prefix)
     return Scheme(
         platlib=scheme["platlib"],
         purelib=scheme["purelib"],
diff --git a/src/pip/_internal/locations/_sysconfig.py b/src/pip/_internal/locations/_sysconfig.py
index bc49cb75f77..d29580dcc9f 100644
--- a/src/pip/_internal/locations/_sysconfig.py
+++ b/src/pip/_internal/locations/_sysconfig.py
@@ -5,16 +5,12 @@
 import sysconfig
 import typing
 
-from pip._internal.exceptions import (
-    InvalidSchemeCombination,
-    UserInstallationInvalid,
-)
+from pip._internal.exceptions import InvalidSchemeCombination, UserInstallationInvalid
 from pip._internal.models.scheme import SCHEME_KEYS, Scheme
 from pip._internal.utils.virtualenv import running_under_virtualenv
 
 from .base import get_major_minor_version
 
-
 logger = logging.getLogger(__name__)
 
 
diff --git a/src/pip/_internal/locations/base.py b/src/pip/_internal/locations/base.py
index 8c9f7751179..7f8ef670aac 100644
--- a/src/pip/_internal/locations/base.py
+++ b/src/pip/_internal/locations/base.py
@@ -7,7 +7,6 @@
 from pip._internal.utils import appdirs
 from pip._internal.utils.virtualenv import running_under_virtualenv
 
-
 # Application Directories
 USER_CACHE_DIR = appdirs.user_cache_dir("pip")
 
@@ -21,22 +20,20 @@ def get_major_minor_version():
     Return the major-minor version of the current Python as a string, e.g.
     "3.7" or "3.10".
     """
-    return '{}.{}'.format(*sys.version_info)
+    return "{}.{}".format(*sys.version_info)
 
 
 def get_src_prefix():
     # type: () -> str
     if running_under_virtualenv():
-        src_prefix = os.path.join(sys.prefix, 'src')
+        src_prefix = os.path.join(sys.prefix, "src")
     else:
         # FIXME: keep src in cwd for now (it is not a temporary folder)
         try:
-            src_prefix = os.path.join(os.getcwd(), 'src')
+            src_prefix = os.path.join(os.getcwd(), "src")
         except OSError:
             # In case the current working directory has been renamed or deleted
-            sys.exit(
-                "The folder you are executing pip from can no longer be found."
-            )
+            sys.exit("The folder you are executing pip from can no longer be found.")
 
     # under macOS + virtualenv sys.prefix is not properly resolved
     # it is something like /path/to/python/bin/..

From 790bd02ede18995195ed08fc0ac2aa142c56537c Mon Sep 17 00:00:00 2001
From: Tzu-ping Chung <uranusjr@gmail.com>
Date: Fri, 19 Feb 2021 18:15:45 +0800
Subject: [PATCH 07/17] Type fixes

---
 src/pip/_internal/locations/__init__.py   | 12 +++++++++--
 src/pip/_internal/locations/_sysconfig.py | 25 ++++++++++++-----------
 src/pip/_internal/locations/base.py       |  2 +-
 3 files changed, 24 insertions(+), 15 deletions(-)

diff --git a/src/pip/_internal/locations/__init__.py b/src/pip/_internal/locations/__init__.py
index db1c1309ee4..327d75a957b 100644
--- a/src/pip/_internal/locations/__init__.py
+++ b/src/pip/_internal/locations/__init__.py
@@ -131,8 +131,16 @@ def get_prefixed_libs(prefix):
     """Return the lib locations under ``prefix``."""
     old_pure, old_plat = _distutils.get_prefixed_libs(prefix)
     new_pure, new_plat = _sysconfig.get_prefixed_libs(prefix)
-    _warn_if_mismatch(old_pure, new_pure, key="prefixed-purelib")
-    _warn_if_mismatch(old_plat, new_plat, key="prefixed-platlib")
+    _warn_if_mismatch(
+        pathlib.Path(old_pure),
+        pathlib.Path(new_pure),
+        key="prefixed-purelib",
+    )
+    _warn_if_mismatch(
+        pathlib.Path(old_plat),
+        pathlib.Path(new_plat),
+        key="prefixed-platlib",
+    )
     if old_pure == old_plat:
         return [old_pure]
     return [old_pure, old_plat]
diff --git a/src/pip/_internal/locations/_sysconfig.py b/src/pip/_internal/locations/_sysconfig.py
index d29580dcc9f..64aa487dc68 100644
--- a/src/pip/_internal/locations/_sysconfig.py
+++ b/src/pip/_internal/locations/_sysconfig.py
@@ -18,7 +18,7 @@
 
 
 def _infer_scheme(variant):
-    # (typing.Literal["home", "prefix", "user"]) -> str
+    # type: (typing.Literal["home", "prefix", "user"]) -> str
     """Try to find a scheme for the current platform.
 
     Unfortunately ``_get_default_scheme()`` is private, so there's no way to
@@ -46,16 +46,16 @@ def _infer_scheme(variant):
 
 
 # Update these keys if the user sets a custom home.
-_HOME_KEYS = (
+_HOME_KEYS = [
     "installed_base",
     "base",
     "installed_platbase",
     "platbase",
     "prefix",
     "exec_prefix",
-)
+]
 if sysconfig.get_config_var("userbase") is not None:
-    _HOME_KEYS += ("userbase",)
+    _HOME_KEYS.append("userbase")
 
 
 def get_scheme(
@@ -86,11 +86,11 @@ def get_scheme(
         raise InvalidSchemeCombination("--home", "--prefix")
 
     if home is not None:
-        scheme = _infer_scheme("home")
+        scheme_name = _infer_scheme("home")
     elif user:
-        scheme = _infer_scheme("user")
+        scheme_name = _infer_scheme("user")
     else:
-        scheme = _infer_scheme("prefix")
+        scheme_name = _infer_scheme("prefix")
 
     if home is not None:
         variables = {k: home for k in _HOME_KEYS}
@@ -99,7 +99,7 @@ def get_scheme(
     else:
         variables = {}
 
-    paths = sysconfig.get_paths(scheme=scheme, vars=variables)
+    paths = sysconfig.get_paths(scheme=scheme_name, vars=variables)
 
     # Special header path for compatibility to distutils.
     if running_under_virtualenv():
@@ -126,21 +126,22 @@ def get_bin_prefix():
     # Forcing to use /usr/local/bin for standard macOS framework installs.
     if sys.platform[:6] == "darwin" and sys.prefix[:16] == "/System/Library/":
         return "/usr/local/bin"
-    return sysconfig.get_path("scripts", scheme=_infer_scheme("prefix"))
+    return sysconfig.get_paths(scheme=_infer_scheme("prefix"))["scripts"]
 
 
 def get_bin_user():
-    return sysconfig.get_path("scripts", scheme=_infer_scheme("user"))
+    # type: () -> str
+    return sysconfig.get_paths(scheme=_infer_scheme("user"))["scripts"]
 
 
 def get_purelib():
     # type: () -> str
-    return sysconfig.get_path("purelib")
+    return sysconfig.get_paths()["purelib"]
 
 
 def get_platlib():
     # type: () -> str
-    return sysconfig.get_path("platlib")
+    return sysconfig.get_paths()["platlib"]
 
 
 def get_prefixed_libs(prefix):
diff --git a/src/pip/_internal/locations/base.py b/src/pip/_internal/locations/base.py
index 7f8ef670aac..98557abbe63 100644
--- a/src/pip/_internal/locations/base.py
+++ b/src/pip/_internal/locations/base.py
@@ -43,6 +43,6 @@ def get_src_prefix():
 try:
     # Use getusersitepackages if this is present, as it ensures that the
     # value is initialised properly.
-    user_site = site.getusersitepackages()
+    user_site = site.getusersitepackages()  # type: typing.Optional[str]
 except AttributeError:
     user_site = site.USER_SITE

From 7563a9c6cc1c576020d3e92ca0ddcae7e19e228d Mon Sep 17 00:00:00 2001
From: Tzu-ping Chung <uranusjr@gmail.com>
Date: Tue, 23 Feb 2021 02:04:15 +0800
Subject: [PATCH 08/17] Goodness

---
 src/pip/_internal/locations/_distutils.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/src/pip/_internal/locations/_distutils.py b/src/pip/_internal/locations/_distutils.py
index 70b7e8d6dd3..d4cb6b17948 100644
--- a/src/pip/_internal/locations/_distutils.py
+++ b/src/pip/_internal/locations/_distutils.py
@@ -18,7 +18,6 @@
 
 from .base import get_major_minor_version, user_site
 
-
 if WINDOWS:
     bin_py = os.path.join(sys.prefix, "Scripts")
     bin_user = os.path.join(user_site, "Scripts")

From 1e1289e550e9aa329b1e3aa3f49edb6022dde525 Mon Sep 17 00:00:00 2001
From: Tzu-ping Chung <uranusjr@gmail.com>
Date: Tue, 23 Feb 2021 03:51:12 +0800
Subject: [PATCH 09/17] User-site special case fixes

---
 src/pip/_internal/locations/_distutils.py | 20 +-------------------
 src/pip/_internal/locations/_sysconfig.py | 21 +++++++++++++++++----
 src/pip/_internal/locations/base.py       | 18 ++++++++++++++++++
 3 files changed, 36 insertions(+), 23 deletions(-)

diff --git a/src/pip/_internal/locations/_distutils.py b/src/pip/_internal/locations/_distutils.py
index d4cb6b17948..7eecf8d8fa4 100644
--- a/src/pip/_internal/locations/_distutils.py
+++ b/src/pip/_internal/locations/_distutils.py
@@ -5,7 +5,6 @@
 
 import os
 import os.path
-import sys
 from distutils.cmd import Command as DistutilsCommand
 from distutils.command.install import SCHEME_KEYS
 from distutils.command.install import install as distutils_install_command
@@ -13,26 +12,9 @@
 from typing import Dict, List, Optional, Tuple, Union, cast
 
 from pip._internal.models.scheme import Scheme
-from pip._internal.utils.compat import WINDOWS
 from pip._internal.utils.virtualenv import running_under_virtualenv
 
-from .base import get_major_minor_version, user_site
-
-if WINDOWS:
-    bin_py = os.path.join(sys.prefix, "Scripts")
-    bin_user = os.path.join(user_site, "Scripts")
-    # buildout uses 'bin' on Windows too?
-    if not os.path.exists(bin_py):
-        bin_py = os.path.join(sys.prefix, "bin")
-        bin_user = os.path.join(user_site, "bin")
-else:
-    bin_py = os.path.join(sys.prefix, "bin")
-    bin_user = os.path.join(user_site, "bin")
-
-    # Forcing to use /usr/local/bin for standard macOS framework installs
-    # Also log to ~/Library/Logs/ for use with the Console.app log viewer
-    if sys.platform[:6] == "darwin" and sys.prefix[:16] == "/System/Library/":
-        bin_py = "/usr/local/bin"
+from .base import bin_py, bin_user, get_major_minor_version
 
 
 def _distutils_scheme(
diff --git a/src/pip/_internal/locations/_sysconfig.py b/src/pip/_internal/locations/_sysconfig.py
index 64aa487dc68..a71cd8adc43 100644
--- a/src/pip/_internal/locations/_sysconfig.py
+++ b/src/pip/_internal/locations/_sysconfig.py
@@ -9,7 +9,7 @@
 from pip._internal.models.scheme import SCHEME_KEYS, Scheme
 from pip._internal.utils.virtualenv import running_under_virtualenv
 
-from .base import get_major_minor_version
+from .base import bin_user, get_major_minor_version
 
 logger = logging.getLogger(__name__)
 
@@ -101,12 +101,22 @@ def get_scheme(
 
     paths = sysconfig.get_paths(scheme=scheme_name, vars=variables)
 
-    # Special header path for compatibility to distutils.
+    # Pip historically uses a special header path in virtual environments.
     if running_under_virtualenv():
-        base = variables.get("base", sys.prefix)
+        if user:
+            base = variables.get("userbase", sys.prefix)
+        else:
+            base = variables.get("base", sys.prefix)
         python_xy = f"python{get_major_minor_version()}"
         paths["include"] = os.path.join(base, "include", "site", python_xy)
 
+    # Special user scripts path on Windows for compatibility to distutils.
+    # See ``distutils.commands.install.INSTALL_SCHEMES["nt_user"]["scritps"]``.
+    if scheme_name == "nt_user":
+        base = variables.get("userbase", sys.prefix)
+        python_xy = f"Python{sys.version_info.major}{sys.version_info.minor}"
+        paths["scripts"] = os.path.join(base, python_xy, "Scripts")
+
     scheme = Scheme(
         platlib=paths["platlib"],
         purelib=paths["purelib"],
@@ -131,7 +141,10 @@ def get_bin_prefix():
 
 def get_bin_user():
     # type: () -> str
-    return sysconfig.get_paths(scheme=_infer_scheme("user"))["scripts"]
+    # pip puts the scripts directory in site-packages, not under userbase.
+    # I'm honestly not sure if this is a bug (because ``get_scheme()`` puts it
+    # correctly under userbase), but we need to be compatible.
+    return bin_user
 
 
 def get_purelib():
diff --git a/src/pip/_internal/locations/base.py b/src/pip/_internal/locations/base.py
index 98557abbe63..5035662e3a4 100644
--- a/src/pip/_internal/locations/base.py
+++ b/src/pip/_internal/locations/base.py
@@ -5,6 +5,7 @@
 import typing
 
 from pip._internal.utils import appdirs
+from pip._internal.utils.compat import WINDOWS
 from pip._internal.utils.virtualenv import running_under_virtualenv
 
 # Application Directories
@@ -46,3 +47,20 @@ def get_src_prefix():
     user_site = site.getusersitepackages()  # type: typing.Optional[str]
 except AttributeError:
     user_site = site.USER_SITE
+
+
+if WINDOWS:
+    bin_py = os.path.join(sys.prefix, "Scripts")
+    bin_user = os.path.join(user_site, "Scripts")
+    # buildout uses 'bin' on Windows too?
+    if not os.path.exists(bin_py):
+        bin_py = os.path.join(sys.prefix, "bin")
+        bin_user = os.path.join(user_site, "bin")
+else:
+    bin_py = os.path.join(sys.prefix, "bin")
+    bin_user = os.path.join(user_site, "bin")
+
+    # Forcing to use /usr/local/bin for standard macOS framework installs
+    # Also log to ~/Library/Logs/ for use with the Console.app log viewer
+    if sys.platform[:6] == "darwin" and sys.prefix[:16] == "/System/Library/":
+        bin_py = "/usr/local/bin"

From b7068f643e1b0b4fddab040878319ae8276efd0c Mon Sep 17 00:00:00 2001
From: Tzu-ping Chung <uranusjr@gmail.com>
Date: Tue, 23 Feb 2021 04:24:17 +0800
Subject: [PATCH 10/17] Split bin_user and bin_prefix implementations

Module-level logic is bad.
---
 src/pip/_internal/locations/__init__.py   |  9 +-------
 src/pip/_internal/locations/_distutils.py | 22 +++++++++++-------
 src/pip/_internal/locations/_sysconfig.py | 10 +-------
 src/pip/_internal/locations/base.py       | 28 +++++++++++------------
 4 files changed, 30 insertions(+), 39 deletions(-)

diff --git a/src/pip/_internal/locations/__init__.py b/src/pip/_internal/locations/__init__.py
index 327d75a957b..78969546345 100644
--- a/src/pip/_internal/locations/__init__.py
+++ b/src/pip/_internal/locations/__init__.py
@@ -8,6 +8,7 @@
 from . import _distutils, _sysconfig
 from .base import (
     USER_CACHE_DIR,
+    get_bin_user,
     get_major_minor_version,
     get_src_prefix,
     site_packages,
@@ -100,14 +101,6 @@ def get_bin_prefix():
     return old
 
 
-def get_bin_user():
-    # type: () -> str
-    old = _distutils.get_bin_user()
-    new = _sysconfig.get_bin_user()
-    _warn_if_mismatch(pathlib.Path(old), pathlib.Path(new), key="bin_user")
-    return old
-
-
 def get_purelib():
     # type: () -> str
     """Return the default pure-Python lib location."""
diff --git a/src/pip/_internal/locations/_distutils.py b/src/pip/_internal/locations/_distutils.py
index 7eecf8d8fa4..2d7ab73213c 100644
--- a/src/pip/_internal/locations/_distutils.py
+++ b/src/pip/_internal/locations/_distutils.py
@@ -4,7 +4,7 @@
 # mypy: strict-optional=False
 
 import os
-import os.path
+import sys
 from distutils.cmd import Command as DistutilsCommand
 from distutils.command.install import SCHEME_KEYS
 from distutils.command.install import install as distutils_install_command
@@ -12,9 +12,10 @@
 from typing import Dict, List, Optional, Tuple, Union, cast
 
 from pip._internal.models.scheme import Scheme
+from pip._internal.utils.compat import WINDOWS
 from pip._internal.utils.virtualenv import running_under_virtualenv
 
-from .base import bin_py, bin_user, get_major_minor_version
+from .base import get_major_minor_version
 
 
 def _distutils_scheme(
@@ -118,12 +119,17 @@ def get_scheme(
 
 def get_bin_prefix():
     # type: () -> str
-    return bin_py
-
-
-def get_bin_user():
-    # type: () -> str
-    return bin_user
+    if WINDOWS:
+        bin_py = os.path.join(sys.prefix, "Scripts")
+        # buildout uses 'bin' on Windows too?
+        if not os.path.exists(bin_py):
+            bin_py = os.path.join(sys.prefix, "bin")
+        return bin_py
+    # Forcing to use /usr/local/bin for standard macOS framework installs
+    # Also log to ~/Library/Logs/ for use with the Console.app log viewer
+    if sys.platform[:6] == "darwin" and sys.prefix[:16] == "/System/Library/":
+        return "/usr/local/bin"
+    return os.path.join(sys.prefix, "bin")
 
 
 def get_purelib():
diff --git a/src/pip/_internal/locations/_sysconfig.py b/src/pip/_internal/locations/_sysconfig.py
index a71cd8adc43..8ef72813b3b 100644
--- a/src/pip/_internal/locations/_sysconfig.py
+++ b/src/pip/_internal/locations/_sysconfig.py
@@ -9,7 +9,7 @@
 from pip._internal.models.scheme import SCHEME_KEYS, Scheme
 from pip._internal.utils.virtualenv import running_under_virtualenv
 
-from .base import bin_user, get_major_minor_version
+from .base import get_major_minor_version
 
 logger = logging.getLogger(__name__)
 
@@ -139,14 +139,6 @@ def get_bin_prefix():
     return sysconfig.get_paths(scheme=_infer_scheme("prefix"))["scripts"]
 
 
-def get_bin_user():
-    # type: () -> str
-    # pip puts the scripts directory in site-packages, not under userbase.
-    # I'm honestly not sure if this is a bug (because ``get_scheme()`` puts it
-    # correctly under userbase), but we need to be compatible.
-    return bin_user
-
-
 def get_purelib():
     # type: () -> str
     return sysconfig.get_paths()["purelib"]
diff --git a/src/pip/_internal/locations/base.py b/src/pip/_internal/locations/base.py
index 5035662e3a4..3a03a79565c 100644
--- a/src/pip/_internal/locations/base.py
+++ b/src/pip/_internal/locations/base.py
@@ -49,18 +49,18 @@ def get_src_prefix():
     user_site = site.USER_SITE
 
 
-if WINDOWS:
-    bin_py = os.path.join(sys.prefix, "Scripts")
-    bin_user = os.path.join(user_site, "Scripts")
-    # buildout uses 'bin' on Windows too?
-    if not os.path.exists(bin_py):
-        bin_py = os.path.join(sys.prefix, "bin")
-        bin_user = os.path.join(user_site, "bin")
-else:
-    bin_py = os.path.join(sys.prefix, "bin")
-    bin_user = os.path.join(user_site, "bin")
+def get_bin_user():
+    # type: () -> str
+    """Get the user-site scripts directory.
 
-    # Forcing to use /usr/local/bin for standard macOS framework installs
-    # Also log to ~/Library/Logs/ for use with the Console.app log viewer
-    if sys.platform[:6] == "darwin" and sys.prefix[:16] == "/System/Library/":
-        bin_py = "/usr/local/bin"
+    Pip puts the scripts directory in site-packages, not under userbase.
+    I'm honestly not sure if this is a bug (because ``get_scheme()`` puts it
+    correctly under userbase), but we need to keep backwards compatibility.
+    """
+    assert user_site is not None, "user site unavailable"
+    if not WINDOWS:
+        return os.path.join(user_site, "bin")
+    # Special case for buildout, which uses 'bin' on Windows too?
+    if not os.path.exists(os.path.join(sys.prefix, "Scripts")):
+        os.path.join(user_site, "bin")
+    return os.path.join(user_site, "Scripts")

From 60a82e7a0eb47191ad92f045c1522dfce6faaeb7 Mon Sep 17 00:00:00 2001
From: Tzu-ping Chung <uranusjr@gmail.com>
Date: Tue, 23 Feb 2021 04:38:32 +0800
Subject: [PATCH 11/17] Better erroring

---
 src/pip/_internal/locations/base.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/pip/_internal/locations/base.py b/src/pip/_internal/locations/base.py
index 3a03a79565c..e373ab30c75 100644
--- a/src/pip/_internal/locations/base.py
+++ b/src/pip/_internal/locations/base.py
@@ -4,6 +4,7 @@
 import sysconfig
 import typing
 
+from pip._internal.exceptions import UserInstallationInvalid
 from pip._internal.utils import appdirs
 from pip._internal.utils.compat import WINDOWS
 from pip._internal.utils.virtualenv import running_under_virtualenv
@@ -57,7 +58,8 @@ def get_bin_user():
     I'm honestly not sure if this is a bug (because ``get_scheme()`` puts it
     correctly under userbase), but we need to keep backwards compatibility.
     """
-    assert user_site is not None, "user site unavailable"
+    if user_site is None:
+        raise UserInstallationInvalid()
     if not WINDOWS:
         return os.path.join(user_site, "bin")
     # Special case for buildout, which uses 'bin' on Windows too?

From 0c4bd55706cb85166c6b9634284c5b03c39c0103 Mon Sep 17 00:00:00 2001
From: Tzu-ping Chung <uranusjr@gmail.com>
Date: Tue, 23 Feb 2021 05:04:21 +0800
Subject: [PATCH 12/17] Stray name

---
 src/pip/_internal/locations/__init__.py | 1 -
 1 file changed, 1 deletion(-)

diff --git a/src/pip/_internal/locations/__init__.py b/src/pip/_internal/locations/__init__.py
index 78969546345..57fbbfd250c 100644
--- a/src/pip/_internal/locations/__init__.py
+++ b/src/pip/_internal/locations/__init__.py
@@ -25,7 +25,6 @@
     "get_purelib",
     "get_scheme",
     "get_src_prefix",
-    "init_backend",
     "site_packages",
     "user_site",
 ]

From a0a3bde15275d0d0b976c9080f210395a33f6d81 Mon Sep 17 00:00:00 2001
From: Tzu-ping Chung <uranusjr@gmail.com>
Date: Tue, 23 Feb 2021 19:27:27 +0800
Subject: [PATCH 13/17] Split scheme inference functions for clarity

This also helps catch bugs in the logic, which are also fixed in this
commit.
---
 src/pip/_internal/locations/_sysconfig.py | 59 ++++++++++++++---------
 1 file changed, 35 insertions(+), 24 deletions(-)

diff --git a/src/pip/_internal/locations/_sysconfig.py b/src/pip/_internal/locations/_sysconfig.py
index 8ef72813b3b..b78366a9db9 100644
--- a/src/pip/_internal/locations/_sysconfig.py
+++ b/src/pip/_internal/locations/_sysconfig.py
@@ -14,35 +14,46 @@
 logger = logging.getLogger(__name__)
 
 
-_AVAILABLE_SCHEMES = set(sysconfig.get_scheme_names())
+# Notes on _infer_* functions.
+# Unfortunately ``_get_default_scheme()`` is private, so there's no way to
+# ask things like "what is the '_prefix' scheme on this platform". These
+# functions try to answer that with some heuristics while accounting for ad-hoc
+# platforms not covered by CPython's default sysconfig implementation. If the
+# ad-hoc implementation does not fully implement sysconfig, we'll fall back to
+# a POSIX scheme.
 
+_AVAILABLE_SCHEMES = set(sysconfig.get_scheme_names())
 
-def _infer_scheme(variant):
-    # type: (typing.Literal["home", "prefix", "user"]) -> str
-    """Try to find a scheme for the current platform.
 
-    Unfortunately ``_get_default_scheme()`` is private, so there's no way to
-    ask things like "what is the '_home' scheme on this platform". This tries
-    to answer that with some heuristics while accounting for ad-hoc platforms
-    not covered by CPython's default sysconfig implementation.
-    """
-    # Most schemes are named like this e.g. "posix_home", "nt_user".
-    suffixed = f"{os.name}_{variant}"
+def _infer_prefix():
+    # type: () -> str
+    """Try to find a prefix scheme for the current platform."""
+    suffixed = f"{os.name}_prefix"
     if suffixed in _AVAILABLE_SCHEMES:
         return suffixed
+    if os.name in _AVAILABLE_SCHEMES:  # On Windows, prefx is just called "nt".
+        return os.name
+    return "posix_prefix"
+
 
-    # The user scheme is not available.
-    if variant == "user" and sysconfig.get_config_var("userbase") is None:
+def _infer_user():
+    # type: () -> str
+    """Try to find a user scheme for the current platform."""
+    suffixed = f"{os.name}_user"
+    if suffixed in _AVAILABLE_SCHEMES:
+        return suffixed
+    if "posix_user" not in _AVAILABLE_SCHEMES:  # User scheme unavailable.
         raise UserInstallationInvalid()
+    return "posix_user"
 
-    # On Windows, prefx and home schemes are the same and just called "nt".
-    if os.name in _AVAILABLE_SCHEMES:
-        return os.name
 
-    # Not sure what's happening, some obscure platform that does not fully
-    # implement sysconfig? Just use the POSIX scheme.
-    logger.warning("No %r scheme for %r; fallback to POSIX.", variant, os.name)
-    return f"posix_{variant}"
+def _infer_home():
+    # type: () -> str
+    """Try to find a home for the current platform."""
+    suffixed = f"{os.name}_home"
+    if suffixed in _AVAILABLE_SCHEMES:
+        return suffixed
+    return "posix_home"
 
 
 # Update these keys if the user sets a custom home.
@@ -86,11 +97,11 @@ def get_scheme(
         raise InvalidSchemeCombination("--home", "--prefix")
 
     if home is not None:
-        scheme_name = _infer_scheme("home")
+        scheme_name = _infer_home()
     elif user:
-        scheme_name = _infer_scheme("user")
+        scheme_name = _infer_user()
     else:
-        scheme_name = _infer_scheme("prefix")
+        scheme_name = _infer_prefix()
 
     if home is not None:
         variables = {k: home for k in _HOME_KEYS}
@@ -136,7 +147,7 @@ def get_bin_prefix():
     # Forcing to use /usr/local/bin for standard macOS framework installs.
     if sys.platform[:6] == "darwin" and sys.prefix[:16] == "/System/Library/":
         return "/usr/local/bin"
-    return sysconfig.get_paths(scheme=_infer_scheme("prefix"))["scripts"]
+    return sysconfig.get_paths()["scripts"]
 
 
 def get_purelib():

From e0d6028ebf5f136e16223714c4fb87decea3f456 Mon Sep 17 00:00:00 2001
From: Tzu-ping Chung <uranusjr@gmail.com>
Date: Wed, 24 Feb 2021 17:46:05 +0800
Subject: [PATCH 14/17] Additional logging to get context

---
 src/pip/_internal/locations/__init__.py | 61 ++++++++++++++++++-------
 1 file changed, 44 insertions(+), 17 deletions(-)

diff --git a/src/pip/_internal/locations/__init__.py b/src/pip/_internal/locations/__init__.py
index 57fbbfd250c..9513d8b0fea 100644
--- a/src/pip/_internal/locations/__init__.py
+++ b/src/pip/_internal/locations/__init__.py
@@ -44,16 +44,30 @@ def _default_base(*, user):
 
 
 def _warn_if_mismatch(old, new, *, key):
-    # type: (pathlib.Path, pathlib.Path, str) -> None
+    # type: (pathlib.Path, pathlib.Path, str) -> bool
     if old == new:
-        return
+        return False
     issue_url = "https://github.com/pypa/pip/issues/9617"
     message = (
-        "Value for %s does not match. Please report this to %s"
+        "Value for %s does not match. Please report this to <%s>"
         "\ndistutils: %s"
         "\nsysconfig: %s"
     )
     logger.warning(message, key, issue_url, old, new)
+    return True
+
+
+def _log_context(
+    *,
+    user: bool = False,
+    home: Optional[str] = None,
+    root: Optional[str] = None,
+    prefix: Optional[str] = None,
+) -> None:
+    message = (
+        "Additional context:" "\nuser = %r" "\nhome = %r" "\nroot = %r" "\nprefix = %r"
+    )
+    logger.warning(message, user, home, root, prefix)
 
 
 def get_scheme(
@@ -83,11 +97,15 @@ def get_scheme(
     )
 
     base = prefix or home or _default_base(user=user)
+    warned = []
     for k in SCHEME_KEYS:
         # Extra join because distutils can return relative paths.
         old_v = pathlib.Path(base, getattr(old, k))
         new_v = pathlib.Path(getattr(new, k))
-        _warn_if_mismatch(old_v, new_v, key=f"scheme.{k}")
+        warned.append(_warn_if_mismatch(old_v, new_v, key=f"scheme.{k}"))
+
+    if any(warned):
+        _log_context(user=user, home=home, root=root, prefix=prefix)
 
     return old
 
@@ -96,7 +114,8 @@ def get_bin_prefix():
     # type: () -> str
     old = _distutils.get_bin_prefix()
     new = _sysconfig.get_bin_prefix()
-    _warn_if_mismatch(pathlib.Path(old), pathlib.Path(new), key="bin_prefix")
+    if _warn_if_mismatch(pathlib.Path(old), pathlib.Path(new), key="bin_prefix"):
+        _log_context()
     return old
 
 
@@ -105,7 +124,8 @@ def get_purelib():
     """Return the default pure-Python lib location."""
     old = _distutils.get_purelib()
     new = _sysconfig.get_purelib()
-    _warn_if_mismatch(pathlib.Path(old), pathlib.Path(new), key="purelib")
+    if _warn_if_mismatch(pathlib.Path(old), pathlib.Path(new), key="purelib"):
+        _log_context()
     return old
 
 
@@ -114,7 +134,8 @@ def get_platlib():
     """Return the default platform-shared lib location."""
     old = _distutils.get_platlib()
     new = _sysconfig.get_platlib()
-    _warn_if_mismatch(pathlib.Path(old), pathlib.Path(new), key="platlib")
+    if _warn_if_mismatch(pathlib.Path(old), pathlib.Path(new), key="platlib"):
+        _log_context()
     return old
 
 
@@ -123,16 +144,22 @@ def get_prefixed_libs(prefix):
     """Return the lib locations under ``prefix``."""
     old_pure, old_plat = _distutils.get_prefixed_libs(prefix)
     new_pure, new_plat = _sysconfig.get_prefixed_libs(prefix)
-    _warn_if_mismatch(
-        pathlib.Path(old_pure),
-        pathlib.Path(new_pure),
-        key="prefixed-purelib",
-    )
-    _warn_if_mismatch(
-        pathlib.Path(old_plat),
-        pathlib.Path(new_plat),
-        key="prefixed-platlib",
-    )
+
+    warned = [
+        _warn_if_mismatch(
+            pathlib.Path(old_pure),
+            pathlib.Path(new_pure),
+            key="prefixed-purelib",
+        ),
+        _warn_if_mismatch(
+            pathlib.Path(old_plat),
+            pathlib.Path(new_plat),
+            key="prefixed-platlib",
+        ),
+    ]
+    if any(warned):
+        _log_context(prefix=prefix)
+
     if old_pure == old_plat:
         return [old_pure]
     return [old_pure, old_plat]

From b4ce28923710dd17f2b60d849fa97036dfd2020c Mon Sep 17 00:00:00 2001
From: Tzu-ping Chung <uranusjr@gmail.com>
Date: Sun, 28 Feb 2021 06:42:39 +0800
Subject: [PATCH 15/17] Add a couple of special cases for PyPy

---
 src/pip/_internal/locations/_sysconfig.py | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/src/pip/_internal/locations/_sysconfig.py b/src/pip/_internal/locations/_sysconfig.py
index b78366a9db9..ee75a6fb9b6 100644
--- a/src/pip/_internal/locations/_sysconfig.py
+++ b/src/pip/_internal/locations/_sysconfig.py
@@ -27,7 +27,22 @@
 
 def _infer_prefix():
     # type: () -> str
-    """Try to find a prefix scheme for the current platform."""
+    """Try to find a prefix scheme for the current platform.
+
+    This tries:
+
+    * Implementation + OS, used by PyPy on Windows (``pypy_nt``).
+    * Implementation without OS, used by PyPy on POSIX (``pypy``).
+    * OS + "prefix", used by CPython on POSIX (``posix_prefix``).
+    * Just the OS name, used by CPython on Windows (``nt``).
+
+    If none of the above works, fall back to ``posix_prefix``.
+    """
+    implementation_suffixed = f"{sys.implementation.name}_{os.name}"
+    if implementation_suffixed in _AVAILABLE_SCHEMES:
+        return implementation_suffixed
+    if sys.implementation.name in _AVAILABLE_SCHEMES:
+        return sys.implementation.name
     suffixed = f"{os.name}_prefix"
     if suffixed in _AVAILABLE_SCHEMES:
         return suffixed

From 826234e3acf1406e855d69207b15caf17d541f6d Mon Sep 17 00:00:00 2001
From: Tzu-ping Chung <uranusjr@gmail.com>
Date: Sun, 28 Feb 2021 07:37:23 +0800
Subject: [PATCH 16/17] New style type hints

---
 src/pip/_internal/locations/__init__.py | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/src/pip/_internal/locations/__init__.py b/src/pip/_internal/locations/__init__.py
index 688403724bb..580da8eae38 100644
--- a/src/pip/_internal/locations/__init__.py
+++ b/src/pip/_internal/locations/__init__.py
@@ -32,8 +32,7 @@
 logger = logging.getLogger(__name__)
 
 
-def _default_base(*, user):
-    # type: (bool) -> str
+def _default_base(*, user: bool) -> str:
     if user:
         base = sysconfig.get_config_var("userbase")
     else:
@@ -42,8 +41,7 @@ def _default_base(*, user):
     return base
 
 
-def _warn_if_mismatch(old, new, *, key):
-    # type: (pathlib.Path, pathlib.Path, str) -> bool
+def _warn_if_mismatch(old: pathlib.Path, new: pathlib.Path, *, key: str) -> bool:
     if old == new:
         return False
     issue_url = "https://github.com/pypa/pip/issues/9617"

From 6d018bf2209cd4ade569a5333a52487c00be033e Mon Sep 17 00:00:00 2001
From: Tzu-ping Chung <uranusjr@gmail.com>
Date: Sun, 28 Feb 2021 07:47:17 +0800
Subject: [PATCH 17/17] Special-case PyPy's lib location differences

---
 src/pip/_internal/locations/__init__.py | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/src/pip/_internal/locations/__init__.py b/src/pip/_internal/locations/__init__.py
index 580da8eae38..18bf0319f3d 100644
--- a/src/pip/_internal/locations/__init__.py
+++ b/src/pip/_internal/locations/__init__.py
@@ -1,5 +1,6 @@
 import logging
 import pathlib
+import sys
 import sysconfig
 from typing import List, Optional
 
@@ -99,6 +100,22 @@ def get_scheme(
         # Extra join because distutils can return relative paths.
         old_v = pathlib.Path(base, getattr(old, k))
         new_v = pathlib.Path(getattr(new, k))
+
+        # distutils incorrectly put PyPy packages under ``site-packages/python``
+        # in the ``posix_home`` scheme, but PyPy devs said they expect the
+        # directory name to be ``pypy`` instead. So we treat this as a bug fix
+        # and not warn about it. See bpo-43307 and python/cpython#24628.
+        skip_pypy_special_case = (
+            sys.implementation.name == "pypy"
+            and home is not None
+            and k in ("platlib", "purelib")
+            and old_v.parent == new_v.parent
+            and old_v.name == "python"
+            and new_v.name == "pypy"
+        )
+        if skip_pypy_special_case:
+            continue
+
         warned.append(_warn_if_mismatch(old_v, new_v, key=f"scheme.{k}"))
 
     if any(warned):