bpo-45413: Define "posix_venv", "nt_venv" and "venv" sysconfig installation schemes

Lib/sysconfig.py

@@ -58,6 +58,28 @@
         },
     }
 
+# Downstream distributors can overwrite the default install scheme.
+# This is done to support downstream modifications where distributors change
+# the installation layout (eg. different site-packages directory).
+# So, distributors will change the default scheme to one that correctly
+# represents their layout.
+# This presents an issue for projects/people that need to bootstrap virtual
+# environments, like virtualenv. As distributors might now be customizing
+# the default install scheme, there is no guarantee that the information
+# returned by sysconfig.get_default_scheme/get_paths is correct for
+# a virtual environment, the only guarantee we have is that it correct
+# for the *current* environment. When bootstrapping a virtual environment,
+# we need to know its layout, so that we can place the files in the
+# correct locations.
+# The "venv" install scheme is a scheme to bootstrap virtual environments,
+# essentially identical to the default posix_prefix/nt schemes.
+# Downstream distributors will need to change this,
+# if their posix_prefix/nt scheme is not venv-compatible.
+if sys.platform == 'win32':
+    _INSTALL_SCHEMES['venv'] = dict(_INSTALL_SCHEMES['nt'])
+else:
+    _INSTALL_SCHEMES['venv'] = dict(_INSTALL_SCHEMES['posix_prefix'])
+
 
 # NOTE: site.py has copy of this function.
 # Sync it when modify this function.

Lib/test/test_sysconfig.py

@@ -267,7 +267,7 @@ def test_get_config_h_filename(self):
         self.assertTrue(os.path.isfile(config_h), config_h)
 
     def test_get_scheme_names(self):
-        wanted = ['nt', 'posix_home', 'posix_prefix']
+        wanted = ['nt', 'posix_home', 'posix_prefix', 'venv']
         if HAS_USER_BASE:
             wanted.extend(['nt_user', 'osx_framework_user', 'posix_user'])
         self.assertEqual(get_scheme_names(), tuple(sorted(wanted)))

Lib/venv/__init__.py

@@ -16,6 +16,9 @@
 CORE_VENV_DEPS = ('pip', 'setuptools')
 logger = logging.getLogger(__name__)
 
+def _venv_path(name):
+    return sysconfig.get_path(name, scheme='venv').removeprefix(sys.prefix + os.sep)
+
 
 class EnvBuilder:
     """
@@ -120,16 +123,10 @@ def create_if_needed(d):
         context.executable = executable
         context.python_dir = dirname
         context.python_exe = exename
-        if sys.platform == 'win32':
-            binname = 'Scripts'
-            incpath = 'Include'
-            libpath = os.path.join(env_dir, 'Lib', 'site-packages')
-        else:
-            binname = 'bin'
-            incpath = 'include'
-            libpath = os.path.join(env_dir, 'lib',
-                                   'python%d.%d' % sys.version_info[:2],
-                                   'site-packages')
+        binname = _venv_path('scripts')
+        incpath = _venv_path('include')
+        libpath = os.path.join(env_dir, _venv_path('purelib'))
+
         context.inc_path = path = os.path.join(env_dir, incpath)
         create_if_needed(path)
         create_if_needed(libpath)

Misc/NEWS.d/next/Library/2022-01-31-15-19-38.bpo-45413.1vaS0V.rst

@@ -0,0 +1,9 @@
+Define a ``"venv"`` :mod:`sysconfig` installation scheme to be used for
+bootstrapping new virtual environments. It is a copy of the
+``"posix_prefix"`` or ``"nt"`` install scheme depending on the platform. The
+:mod:`venv` module now uses that scheme to create new virtual environments
+instead of hardcoding the paths depending only on the platform. Downstream
+Python distributors customizing the ``"posix_prefix"`` or ``"nt"`` install
+scheme in a way that is not compatible with the install scheme used in
+virtual environments are encouraged to supply a modified working ``"venv"``
+scheme.