gh-98718: add sys.build_prefix

Signed-off-by: Filipe Laíns <lains@riseup.net>

Author: FFY00

Doc/library/sys.rst

@@ -144,6 +144,15 @@ always available.
    .. versionadded:: 3.3
 
 
+.. data:: build_prefix
+
+   Set during Python startup, before ``site.py`` is run. If running Python from
+   source, the value will be set to the build path, otherwise it will be set to
+   :const:`None`.
+
+   .. versionadded:: 3.12
+
+
 .. data:: byteorder
 
    An indicator of the native byte order.  This will have the value ``'big'`` on

Include/cpython/initconfig.h

@@ -197,6 +197,7 @@ typedef struct PyConfig {
     wchar_t *base_prefix;
     wchar_t *exec_prefix;
     wchar_t *base_exec_prefix;
+    wchar_t *build_prefix;
 
     /* --- Parameter only used by Py_Main() ---------- */
     int skip_source_first_line;

Lib/sysconfig.py

@@ -226,12 +226,9 @@ def is_python_build(check_home=None):
         import warnings
         warnings.warn("check_home argument is deprecated and ignored.",
                       DeprecationWarning, stacklevel=2)
-    for fn in ("Setup", "Setup.local"):
-        if os.path.isfile(os.path.join(_PROJECT_BASE, "Modules", fn)):
-            return True
-    return False
+    return bool(sys.build_prefix)
 
-_PYTHON_BUILD = is_python_build()
+_PYTHON_BUILD = bool(sys.build_prefix)
 
 if _PYTHON_BUILD:
     for scheme in ('posix_prefix', 'posix_home'):

Misc/NEWS.d/next/Library/2022-11-09-03-07-26.gh-issue-98718.rY4eIr.rst

@@ -0,0 +1,2 @@
+:mod:`sys` now exports :attr:`sys.build_prefix`, with the path of the Python
+build directory, if any.

Modules/getpath.py

@@ -509,6 +509,9 @@ def search_up(prefix, *landmarks, test=isfile):
         config['_is_python_build'] = 1
 
 
+config['build_prefix'] = build_prefix
+
+
 # ******************************************************************************
 # CALCULATE prefix AND exec_prefix
 # ******************************************************************************

Python/initconfig.c

@@ -731,6 +731,7 @@ PyConfig_Clear(PyConfig *config)
     CLEAR(config->base_prefix);
     CLEAR(config->exec_prefix);
     CLEAR(config->base_exec_prefix);
+    CLEAR(config->build_prefix);
     CLEAR(config->platlibdir);
 
     CLEAR(config->filesystem_encoding);
@@ -985,6 +986,7 @@ _PyConfig_Copy(PyConfig *config, const PyConfig *config2)
     COPY_WSTR_ATTR(base_prefix);
     COPY_WSTR_ATTR(exec_prefix);
     COPY_WSTR_ATTR(base_exec_prefix);
+    COPY_WSTR_ATTR(build_prefix);
     COPY_WSTR_ATTR(platlibdir);
 
     COPY_ATTR(site_import);
@@ -1094,6 +1096,7 @@ _PyConfig_AsDict(const PyConfig *config)
     SET_ITEM_WSTR(base_prefix);
     SET_ITEM_WSTR(exec_prefix);
     SET_ITEM_WSTR(base_exec_prefix);
+    SET_ITEM_WSTR(build_prefix);
     SET_ITEM_WSTR(platlibdir);
     SET_ITEM_INT(site_import);
     SET_ITEM_INT(bytes_warning);
@@ -1407,6 +1410,7 @@ _PyConfig_FromDict(PyConfig *config, PyObject *dict)
     GET_WSTR_OPT(base_prefix);
     GET_WSTR_OPT(exec_prefix);
     GET_WSTR_OPT(base_exec_prefix);
+    GET_WSTR_OPT(build_prefix);
 
     GET_UINT(skip_source_first_line);
     GET_WSTR_OPT(run_command);
@@ -3172,6 +3176,7 @@ _Py_DumpPathConfig(PyThreadState *tstate)
     DUMP_SYS(_base_executable);
     DUMP_SYS(base_prefix);
     DUMP_SYS(base_exec_prefix);
+    DUMP_SYS(build_prefix);
     DUMP_SYS(platlibdir);
     DUMP_SYS(executable);
     DUMP_SYS(prefix);

Python/sysmodule.c

@@ -3306,6 +3306,9 @@ _PySys_UpdateConfig(PyThreadState *tstate)
         SET_SYS_FROM_WSTR(SYS_ATTR, WSTR); \
     }
 
+#define COPY_BOOL(KEY, VALUE) \
+        SET_SYS(KEY, PyBool_FromLong(VALUE));
+
     if (config->module_search_paths_set) {
         COPY_LIST("path", config->module_search_paths);
     }
@@ -3317,6 +3320,7 @@ _PySys_UpdateConfig(PyThreadState *tstate)
     COPY_WSTR("exec_prefix", config->exec_prefix);
     COPY_WSTR("base_exec_prefix", config->base_exec_prefix);
     COPY_WSTR("platlibdir", config->platlibdir);
+    COPY_WSTR("build_prefix", config->build_prefix);
 
     if (config->pycache_prefix != NULL) {
         SET_SYS_FROM_WSTR("pycache_prefix", config->pycache_prefix);