python · FFY00 · Oct 26, 2022 · Nov 9, 2022
diff --git a/Doc/library/sys.rst b/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

@@ -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;

@@ -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'):

diff --git a/Lib/test/test_embed.py b/Lib/test/test_embed.py
@@ -783,6 +783,9 @@ def check_all_configs(self, testname, expected_config=None,
         except json.JSONDecodeError:
             self.fail(f"fail to decode stdout: {out!r}")
 
+        # build_prefix depends on if Python was installed, ignore it here
+        configs['config'].pop('build_prefix', None)
+
         self.check_pre_config(configs, expected_preconfig)
         self.check_config(configs, expected_config)
         self.check_global_config(configs)

diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
@@ -603,6 +603,7 @@ def test_attributes(self):
         self.assertIsInstance(sys.platform, str)
         self.assertIsInstance(sys.prefix, str)
         self.assertIsInstance(sys.base_prefix, str)
+        self.assertIsInstance(sys.build_prefix, (str, None))
         self.assertIsInstance(sys.platlibdir, str)
         self.assertIsInstance(sys.version, str)
         vi = sys.version_info

diff --git a/Misc/NEWS.d/next/Library/2022-11-09-03-07-26.gh-issue-98718.rY4eIr.rst b/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.
@@ -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
 # ******************************************************************************

@@ -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);

@@ -3317,6 +3317,7 @@ _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);
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,2 @@
		:mod:`sys` now exports :attr:`sys.build_prefix`, with the path of the Python
		build directory, if any.