bpo-43976: add vendor config

Doc/library/site.rst

@@ -32,14 +32,20 @@ It starts by constructing up to four directories from a head and a tail part.
 For the head part, it uses ``sys.prefix`` and ``sys.exec_prefix``; empty heads
 are skipped.  For the tail part, it uses the empty string and then
 :file:`lib/site-packages` (on Windows) or
-:file:`lib/python{X.Y}/site-packages` (on Unix and macOS).  For each
+:file:`lib/python{X.Y}/site-packages` (on Unix and macOS), and finally
+the ``purelib`` and ``platlib`` paths for each scheme specified in the
+``EXTRA_SITE_INSTALL_SCHEMES`` list variable of the vendor config. For each
 of the distinct head-tail combinations, it sees if it refers to an existing
 directory, and if so, adds it to ``sys.path`` and also inspects the newly
 added path for configuration files.
 
 .. versionchanged:: 3.5
    Support for the "site-python" directory has been removed.
 
+.. versionchanged:: 3.11
+   Extra site install schemes specified in the vendor config
+   (``--with-vendor-config`` configure option) will also be loaded.
+
 If a file named "pyvenv.cfg" exists one directory above sys.executable,
 sys.prefix and sys.exec_prefix are set to that directory and
 it is also checked for site-packages (sys.base_prefix and

Doc/library/sysconfig.rst

@@ -85,6 +85,10 @@ Python currently supports seven schemes:
 - *nt*: scheme for NT platforms like Windows.
 - *nt_user*: scheme for NT platforms, when the *user* option is used.
 
+Additionally to these, Python also supports vendor schemes specified in the
+``EXTRA_INSTALL_SCHEMES`` dictionary variable of the vendor config
+(``--with-vendor-config`` configure option).
+
 Each scheme is itself composed of a series of paths and each path has a unique
 identifier.  Python currently uses eight paths:
 
@@ -129,21 +133,6 @@ identifier.  Python currently uses eight paths:
    .. versionadded:: 3.10
 
 
-.. function:: _get_preferred_schemes()
-
-   Return a dict containing preferred scheme names on the current platform.
-   Python implementers and redistributors may add their preferred schemes to
-   the ``_INSTALL_SCHEMES`` module-level global value, and modify this function
-   to return those scheme names, to e.g. provide different schemes for system
-   and language package managers to use, so packages installed by either do not
-   mix with those by the other.
-
-   End users should not use this function, but :func:`get_default_scheme` and
-   :func:`get_preferred_scheme()` instead.
-
-   .. versionadded:: 3.10
-
-
 .. function:: get_path_names()
 
    Return a tuple containing all path names currently supported in

Doc/using/configure.rst

@@ -116,6 +116,32 @@ General Options
 
    .. versionadded:: 3.10
 
+.. cmdoption:: --with-vendor-config=config.py
+
+   Path to the vendor config (none by default).
+
+   The vendor config is a Python file that allows configuring some aspects of
+   the Python distribution.
+
+   A ``EXTRA_INSTALL_SCHEMES`` dictionary variable can be specified in the
+   config to add extra install schemes. These schemes will be picked up by the
+   :mod:`sysconfig` module.
+
+   A ``EXTRA_SITE_INSTALL_SCHEMES`` list variable can be specified in the config
+   to add extra schemes to the :mod:`site` module initialization. This options
+   allow Python distributors to define custom locations to use for their Python
+   packages.
+
+   A ``get_preferred_schemes()`` function can be specified in the config. This
+   function should return a dict containing preferred scheme names on the
+   current platform. Python implementers and redistributors may add their
+   preferred schemes to the ``EXTRA_INSTALL_SCHEMES`` vendor config variable,
+   and modify this function to return those scheme names, to e.g. provide
+   different schemes for system and language package managers to use, so
+   packages installed by either do not mix with those by the other.
+
+   .. versionadded:: 3.11
+
 
 Install Options
 ---------------