Make default scheme function public

Doc/library/sysconfig.rst

@@ -107,6 +107,26 @@ identifier.  Python currently uses eight paths:
    :mod:`sysconfig`.
 
 
+.. function:: get_default_scheme()
+
+   Return the default scheme name for the current platform.
+
+   .. versionadded:: 3.10
+
+
+.. function:: get_preferred_scheme(key)
+
+   Return a preferred scheme name for an installation layout specified by *key*.
+
+   *key* must be either ``"prefix"``, ``"home"``, or ``"user"``.
+
+   The return value is a scheme name listed in :func:`get_scheme_names`. It
+   can be passed to :mod:`sysconfig` functions that take a *scheme* argument,
+   such as :func:`get_paths`.
+
+   .. versionadded:: 3.10
+
+
 .. function:: get_path_names()
 
    Return a tuple containing all path names currently supported in

Lib/sysconfig.py

@@ -212,13 +212,38 @@ def _expand_vars(scheme, vars):
     return res
 
 
-def _get_default_scheme():
+def get_default_scheme():
     if os.name == 'posix':
         # the default scheme for posix is posix_prefix
         return 'posix_prefix'
     return os.name
 
 
+def _get_preferred_schemes():
+    if os.name == 'nt':
+        return {
+            'prefix': 'nt',
+            'home': 'posix_home',
+            'user': 'nt_user',
+        }
+    if sys.platform == 'darwin' and sys._framework:
+        return {
+            'prefix': 'posix_prefix',
+            'home': 'posix_home',
+            'user': 'osx_framework_user',
+        }
+    return {
+        'prefix': 'posix_prefix',
+        'home': 'posix_home',
+        'user': 'posix_user',
+    }
+
+
+def get_preferred_scheme(key):
+    scheme = _get_preferred_schemes()[key]
+    if scheme not in _INSTALL_SCHEMES:
+        raise KeyError(key)
+    return scheme
 
 
 def _parse_makefile(filename, vars=None):
@@ -517,7 +542,7 @@ def get_path_names():
     return _SCHEME_KEYS
 
 
-def get_paths(scheme=_get_default_scheme(), vars=None, expand=True):
+def get_paths(scheme=get_default_scheme(), vars=None, expand=True):
     """Return a mapping containing an install scheme.
 
     ``scheme`` is the install scheme name. If not provided, it will
@@ -529,7 +554,7 @@ def get_paths(scheme=_get_default_scheme(), vars=None, expand=True):
         return _INSTALL_SCHEMES[scheme]
 
 
-def get_path(name, scheme=_get_default_scheme(), vars=None, expand=True):
+def get_path(name, scheme=get_default_scheme(), vars=None, expand=True):
     """Return a path corresponding to the scheme.
 
     ``scheme`` is the install scheme name.
@@ -731,7 +756,7 @@ def _main():
         return
     print('Platform: "%s"' % get_platform())
     print('Python version: "%s"' % get_python_version())
-    print('Current installation scheme: "%s"' % _get_default_scheme())
+    print('Current installation scheme: "%s"' % get_default_scheme())
     print()
     _print_dict('Paths', get_paths())
     print()

Lib/test/test_sysconfig.py

@@ -14,7 +14,7 @@
 import sysconfig
 from sysconfig import (get_paths, get_platform, get_config_vars,
                        get_path, get_path_names, _INSTALL_SCHEMES,
-                       _get_default_scheme, _expand_vars,
+                       get_default_scheme, _expand_vars,
                        get_scheme_names, get_config_var, _main)
 import _osx_support
 
@@ -94,7 +94,7 @@ def test_get_path_names(self):
 
     def test_get_paths(self):
         scheme = get_paths()
-        default_scheme = _get_default_scheme()
+        default_scheme = get_default_scheme()
         wanted = _expand_vars(default_scheme, None)
         wanted = sorted(wanted.items())
         scheme = sorted(scheme.items())

Misc/NEWS.d/next/Library/2021-02-25-14-43-59.bpo-43312.6dg9_2.rst

@@ -0,0 +1,3 @@
+New functions :func:`sysconfig.get_preferred_scheme` and
+:func:`sysconfig.get_default_scheme` are added to query a platform for its
+preferred "user", "home", and "prefix" (default) scheme names.