gh-98718: re-use already calculated is_python_build from getpath

Include/cpython/initconfig.h

@@ -210,6 +210,7 @@ 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/__init__.py

@@ -222,12 +222,9 @@ def _safe_realpath(path):
     _PROJECT_BASE = _safe_realpath(os.environ["_PYTHON_PROJECT_BASE"])
 
 def is_python_build():
-    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'):

Lib/test/test_capi/test_config.py

@@ -37,6 +37,7 @@ def test_config_get(self):
             ("base_executable", str | None, "_base_executable"),
             ("base_prefix", str | None, "base_prefix"),
             ("buffered_stdio", bool, None),
+            ("build_prefix", str | None, "build_prefix"),
             ("bytes_warning", int, None),
             ("check_hash_pycs_mode", str, None),
             ("code_debug_ranges", bool, None),
@@ -254,6 +255,7 @@ def test_config_set_sys_attr(self):
             ('base_exec_prefix', 'base_exec_prefix', str | None),
             ('base_executable', '_base_executable', str | None),
             ('base_prefix', 'base_prefix', str | None),
+            ('build_prefix', 'build_prefix', str | None),
             ('exec_prefix', 'exec_prefix', str | None),
             ('executable', 'executable', str | None),
             ('module_search_paths', 'path', list[str]),

Lib/test/test_embed.py

@@ -799,6 +799,7 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
         'base_prefix': GET_DEFAULT_CONFIG,
         'exec_prefix': GET_DEFAULT_CONFIG,
         'base_exec_prefix': GET_DEFAULT_CONFIG,
+        'build_prefix': IGNORE_CONFIG,
         'module_search_paths': GET_DEFAULT_CONFIG,
         'module_search_paths_set': True,
         'platlibdir': sys.platlibdir,

Modules/getpath.py

@@ -555,6 +555,9 @@ 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

@@ -114,6 +114,7 @@ static const PyConfigSpec PYCONFIG_SPEC[] = {
     SPEC(base_exec_prefix, WSTR_OPT, PUBLIC, SYS_ATTR("base_exec_prefix")),
     SPEC(base_executable, WSTR_OPT, PUBLIC, SYS_ATTR("_base_executable")),
     SPEC(base_prefix, WSTR_OPT, PUBLIC, SYS_ATTR("base_prefix")),
+    SPEC(build_prefix, WSTR_OPT, PUBLIC, SYS_ATTR("build_prefix")),
     SPEC(bytes_warning, UINT, PUBLIC, SYS_FLAG(9)),
     SPEC(cpu_count, INT, PUBLIC, NO_SYS),
     SPEC(lazy_imports, INT, PUBLIC, NO_SYS),
@@ -1088,6 +1089,7 @@ 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->sys_path_0);
 
@@ -3965,6 +3967,7 @@ _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

@@ -4191,6 +4191,9 @@ _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);
     }
@@ -4202,6 +4205,7 @@ _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);