@@ -438,10 +438,19 @@ Miscellaneous options
      * Set the :attr:`~sys.flags.dev_mode` attribute of :attr:`sys.flags` to
        ``True``
 
-   * ``-X utf8`` enables UTF-8 mode for operating system interfaces, overriding
+   * ``-X utf8`` enables UTF-8 mode (:pep:`540`) for operating system interfaces, overriding
      the default locale-aware mode. ``-X utf8=0`` explicitly disables UTF-8
      mode (even when it would otherwise activate automatically).
      See :envvar:`PYTHONUTF8` for more details.
+   * ``-X coerce_c_locale`` or ``-X coerce_c_locale=1`` tries to coerce the C
+     locale (:pep:`538`).
+     ``-X coerce_c_locale=0`` skips coercing the legacy ASCII-based C and POSIX
+     locales to a more capable UTF-8 based alternative.
+     ``-X coerce_c_locale=warn`` will cause Python to emit warning messages on
+     ``stderr`` if either the locale coercion activates, or else if a locale
+     that *would* have triggered coercion is still active when the Python
+     runtime is initialized.
+     See :envvar:`PYTHONCOERCECLOCALE` for more details.
 
    It also allows passing arbitrary values and retrieving them through the
    :data:`sys._xoptions` dictionary.
@@ -461,6 +470,9 @@ Miscellaneous options
    .. versionadded:: 3.7
       The ``-X importtime``, ``-X dev`` and ``-X utf8`` options.
 
+   .. versionadded:: 3.7.1
+      The ``-X coerce_c_locale`` option.
+
 
 Options you shouldn't use
 ~~~~~~~~~~~~~~~~~~~~~~~~~
@@ -834,6 +846,8 @@ conflict.
    order to force the interpreter to use ``ASCII`` instead of ``UTF-8`` for
    system interfaces.
 
+   Also available as the :option:`-X` ``coerce_c_locale`` option.
+
    Availability: \*nix
 
    .. versionadded:: 3.7
 
@@ -2494,3 +2494,10 @@ versions, it respected an ill-defined subset of those environment variables,
 while in Python 3.7.0 it didn't read any of them due to :issue:`34247`). If
 this behavior is unwanted, set :c:data:`Py_IgnoreEnvironmentFlag` to 1 before
 calling :c:func:`Py_Initialize`.
+
+:c:func:`Py_Initialize` and :c:func:`Py_Main` cannot enable the C locale
+coercion (:pep:`538`) anymore: it is always disabled. It can now only be
+enabled by the Python program ("python3).
+
+New :option:`-X` ``coerce_c_locale`` command line option to control C locale
+coercion (:pep:`538`).
@@ -119,7 +119,11 @@ PyAPI_FUNC(int) Py_FdIsInteractive(FILE *, const char *);
 /* Bootstrap __main__ (defined in Modules/main.c) */
 PyAPI_FUNC(int) Py_Main(int argc, wchar_t **argv);
 #ifdef Py_BUILD_CORE
+#  ifdef MS_WINDOWS
+PyAPI_FUNC(int) _Py_WindowsMain(int argc, wchar_t **argv);
+#  else
 PyAPI_FUNC(int) _Py_UnixMain(int argc, char **argv);
+#  endif
 #endif
 
 /* In getpath.c */
 
@@ -41,8 +41,6 @@ typedef struct {
     int show_alloc_count;   /* -X showalloccount */
     int dump_refs;          /* PYTHONDUMPREFS */
     int malloc_stats;       /* PYTHONMALLOCSTATS */
-    int coerce_c_locale;    /* PYTHONCOERCECLOCALE, -1 means unknown */
-    int coerce_c_locale_warn; /* PYTHONCOERCECLOCALE=warn */
     int utf8_mode;          /* PYTHONUTF8, -X utf8; -1 means unknown */
 
     wchar_t *program_name;  /* Program name, see also Py_GetProgramName() */
@@ -74,14 +72,17 @@ typedef struct {
 
     /* Private fields */
     int _disable_importlib; /* Needed by freeze_importlib */
+    int _coerce_c_locale;    /* PYTHONCOERCECLOCALE, -1 means unknown */
+    int _coerce_c_locale_warn; /* PYTHONCOERCECLOCALE=warn */
 } _PyCoreConfig;
 
 #define _PyCoreConfig_INIT \
     (_PyCoreConfig){ \
         .install_signal_handlers = -1, \
         .ignore_environment = -1, \
         .use_hash_seed = -1, \
-        .coerce_c_locale = -1, \
+        ._coerce_c_locale = 0, \
+        ._coerce_c_locale_warn = 0, \
         .faulthandler = -1, \
         .tracemalloc = -1, \
         .utf8_mode = -1, \
 
@@ -139,7 +139,7 @@ def _handle_output_variations(data):
         return data
 
     @classmethod
-    def get_child_details(cls, env_vars):
+    def get_child_details(cls, env_vars, xoption=None):
         """Retrieves fsencoding and standard stream details from a child process
 
         Returns (encoding_details, stderr_lines):
@@ -150,10 +150,11 @@ def get_child_details(cls, env_vars):
         The child is run in isolated mode if the current interpreter supports
         that.
         """
-        result, py_cmd = run_python_until_end(
-            "-X", "utf8=0", "-c", cls.CHILD_PROCESS_SCRIPT,
-            **env_vars
-        )
+        args = []
+        if xoption:
+            args.extend(("-X", f"coerce_c_locale={xoption}"))
+        args.extend(("-X", "utf8=0", "-c", cls.CHILD_PROCESS_SCRIPT))
+        result, py_cmd = run_python_until_end(*args, **env_vars)
         if not result.rc == 0:
             result.fail(py_cmd)
         # All subprocess outputs in this test case should be pure ASCII
@@ -212,15 +213,16 @@ def _check_child_encoding_details(self,
                                       expected_fs_encoding,
                                       expected_stream_encoding,
                                       expected_warnings,
-                                      coercion_expected):
+                                      coercion_expected,
+                                      xoption=None):
         """Check the C locale handling for the given process environment
 
         Parameters:
             expected_fs_encoding: expected sys.getfilesystemencoding() result
             expected_stream_encoding: expected encoding for standard streams
             expected_warning: stderr output to expect (if any)
         """
-        result = EncodingDetails.get_child_details(env_vars)
+        result = EncodingDetails.get_child_details(env_vars, xoption)
         encoding_details, stderr_lines = result
         expected_details = EncodingDetails.get_expected_details(
             coercion_expected,
@@ -290,6 +292,7 @@ def _check_c_locale_coercion(self,
                                  coerce_c_locale,
                                  expected_warnings=None,
                                  coercion_expected=True,
+                                 use_xoption=False,
                                  **extra_vars):
         """Check the C locale handling for various configurations
 
@@ -319,8 +322,12 @@ def _check_c_locale_coercion(self,
             "PYTHONCOERCECLOCALE": "",
         }
         base_var_dict.update(extra_vars)
+        xoption = None
         if coerce_c_locale is not None:
-            base_var_dict["PYTHONCOERCECLOCALE"] = coerce_c_locale
+            if use_xoption:
+                xoption = coerce_c_locale
+            else:
+                base_var_dict["PYTHONCOERCECLOCALE"] = coerce_c_locale
 
         # Check behaviour for the default locale
         with self.subTest(default_locale=True,
@@ -342,7 +349,8 @@ def _check_c_locale_coercion(self,
                                                fs_encoding,
                                                stream_encoding,
                                                _expected_warnings,
-                                               _coercion_expected)
+                                               _coercion_expected,
+                                               xoption=xoption)
 
         # Check behaviour for explicitly configured locales
         for locale_to_set in EXPECTED_C_LOCALE_EQUIVALENTS:
@@ -357,7 +365,8 @@ def _check_c_locale_coercion(self,
                                                        fs_encoding,
                                                        stream_encoding,
                                                        expected_warnings,
-                                                       coercion_expected)
+                                                       coercion_expected,
+                                                       xoption=xoption)
 
     def test_PYTHONCOERCECLOCALE_not_set(self):
         # This should coerce to the first available target locale by default
@@ -404,6 +413,32 @@ def test_LC_ALL_set_to_C(self):
                                       expected_warnings=[LEGACY_LOCALE_WARNING],
                                       coercion_expected=False)
 
+    def test_xoption_set_to_1(self):
+        self._check_c_locale_coercion("utf-8", "utf-8", coerce_c_locale="1",
+                                      use_xoption=True)
+
+    def test_xoption_set_to_zero(self):
+        # The setting "0" should result in the locale coercion being disabled
+        self._check_c_locale_coercion(EXPECTED_C_LOCALE_FS_ENCODING,
+                                      EXPECTED_C_LOCALE_STREAM_ENCODING,
+                                      coerce_c_locale="0",
+                                      coercion_expected=False,
+                                      use_xoption=True)
+        # Setting LC_ALL=C shouldn't make any difference to the behaviour
+        self._check_c_locale_coercion(EXPECTED_C_LOCALE_FS_ENCODING,
+                                      EXPECTED_C_LOCALE_STREAM_ENCODING,
+                                      coerce_c_locale="0",
+                                      LC_ALL="C",
+                                      coercion_expected=False,
+                                      use_xoption=True)
+
+    def test_xoption_set_to_warn(self):
+        # -X coerce_c_locale=warn enables runtime warnings for legacy locales
+        self._check_c_locale_coercion("utf-8", "utf-8",
+                                      coerce_c_locale="warn",
+                                      expected_warnings=[CLI_COERCION_WARNING],
+                                      use_xoption=True)
+
 def test_main():
     test.support.run_unittest(
         LocaleConfigurationTests,
 
@@ -159,13 +159,16 @@ def test_undecodable_code(self):
         env = os.environ.copy()
         # Use C locale to get ascii for the locale encoding
         env['LC_ALL'] = 'C'
-        env['PYTHONCOERCECLOCALE'] = '0'
         code = (
             b'import locale; '
             b'print(ascii("' + undecodable + b'"), '
                 b'locale.getpreferredencoding())')
         p = subprocess.Popen(
-            [sys.executable, "-c", code],
+            [sys.executable,
+             # Disable C locale coercion and UTF-8 Mode to not use UTF-8
+             "-X", "coerce_c_locale=0",
+             "-X", "utf8=0",
+             "-c", code],
             stdout=subprocess.PIPE, stderr=subprocess.STDOUT,
             env=env)
         stdout, stderr = p.communicate()
 
@@ -267,9 +267,6 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
         'malloc_stats': 0,
         'utf8_mode': 0,
 
-        'coerce_c_locale': 0,
-        'coerce_c_locale_warn': 0,
-
         'program_name': './_testembed',
         'argc': 0,
         'argv': '[]',
@@ -290,6 +287,8 @@ class InitConfigTests(EmbeddingTestsMixin, unittest.TestCase):
 
         '_disable_importlib': 0,
         'Py_FrozenFlag': 0,
+        '_coerce_c_locale': 0,
+        '_coerce_c_locale_warn': 0,
     }
 
     def check_config(self, testname, expected):
 
@@ -656,9 +656,8 @@ def test_getfilesystemencoding(self):
 
     def c_locale_get_error_handler(self, locale, isolated=False, encoding=None):
         # Force the POSIX locale
-        env = os.environ.copy()
+        env = dict(os.environ)
         env["LC_ALL"] = locale
-        env["PYTHONCOERCECLOCALE"] = "0"
         code = '\n'.join((
             'import sys',
             'def dump(name):',
@@ -668,7 +667,10 @@ def c_locale_get_error_handler(self, locale, isolated=False, encoding=None):
             'dump("stdout")',
             'dump("stderr")',
         ))
-        args = [sys.executable, "-c", code]
+        args = [sys.executable,
+                "-X", "utf8=0",
+                "-X", "coerce_c_locale=0",
+                "-c", code]
         if isolated:
             args.append("-I")
         if encoding is not None:
 
@@ -27,6 +27,8 @@ def posix_locale(self):
         return (loc in POSIX_LOCALES)
 
     def get_output(self, *args, failure=False, **kw):
+        # Always disable the C locale coercion (PEP 538)
+        args = ('-X', 'coerce_c_locale=0', *args)
         kw = dict(self.DEFAULT_ENV, **kw)
         if failure:
             out = assert_python_failure(*args, **kw)
@@ -116,7 +118,6 @@ def test_filesystemencoding(self):
             # PYTHONLEGACYWINDOWSFSENCODING disables the UTF-8 mode
             # and has the priority over -X utf8 and PYTHONUTF8
             out = self.get_output('-X', 'utf8', '-c', code,
-                                  PYTHONUTF8='strict',
                                   PYTHONLEGACYWINDOWSFSENCODING='1')
             self.assertEqual(out, 'mbcs/replace')
 
 
@@ -0,0 +1,3 @@
+Py_Initialize() and Py_Main() cannot enable the C locale coercion (PEP 538)
+anymore: it is always disabled. It can now only be enabled by the Python
+program ("python3).
Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,3 @@`
	`1`	`+Py_Initialize() and Py_Main() cannot enable the C locale coercion (PEP 538)`
	`2`	`+anymore: it is always disabled. It can now only be enabled by the Python`
	`3`	`+program ("python3).`