diff --git a/Doc/using/cmdline.rst b/Doc/using/cmdline.rst

index 08dc311..c6ec147 100644

--- a/Doc/using/cmdline.rst

+++ b/Doc/using/cmdline.rst

@@ -713,6 +713,40 @@ conflict.

    .. versionadded:: 3.6

+

+.. envvar:: PYTHONCOERCECLOCALE

+

+   If set to the value ``0``, causes the main Python command line application

+   to skip coercing the legacy ASCII-based C locale to a more capable UTF-8

+   based alternative. Note that this setting is checked even when the

+   :option:`-E` or :option:`-I` options are used, as it is handled prior to

+   the processing of command line options.

+

+   If this variable is *not* set, or is set to a value other than ``0``, and

+   the current locale reported for the ``LC_CTYPE`` category is the default

+   ``C`` locale, then the Python CLI will attempt to configure one of the

+   following locales for the given locale categories before loading the

+   interpreter runtime:

+

+   * ``C.UTF-8`` (``LC_ALL``)

+   * ``C.utf8`` (``LC_ALL``)

+   * ``UTF-8`` (``LC_CTYPE``)

+

+   If setting one of these locale categories succeeds, then the matching

+   environment variables will be set (both ``LC_ALL`` and ``LANG`` for the

+   ``LC_ALL`` category, and ``LC_CTYPE`` for the ``LC_CTYPE`` category) in

+   the current process environment before the Python runtime is initialized.

+

+   Configuring one of these locales (either explicitly or via the above

+   implicit locale coercion) will automatically set the error handler for

+   :data:`sys.stdin` and :data:`sys.stdout` to ``surrogateescape``. This

+   behavior can be overridden using :envvar:`PYTHONIOENCODING` as usual.

+

+   Availability: \*nix

+

+   .. versionadded:: 3.7

+      See :pep:`538` for more details.

+

 Debug-mode variables

 ~~~~~~~~~~~~~~~~~~~~

diff --git a/Lib/test/support/script_helper.py b/Lib/test/support/script_helper.py

index 1e74647..b3ac848 100644

--- a/Lib/test/support/script_helper.py

+++ b/Lib/test/support/script_helper.py

@@ -48,8 +48,35 @@ def interpreter_requires_environment():

     return __cached_interp_requires_environment

-_PythonRunResult = collections.namedtuple("_PythonRunResult",

-                                          ("rc", "out", "err"))

+class _PythonRunResult(collections.namedtuple("_PythonRunResult",

+                                          ("rc", "out", "err"))):

+    """Helper for reporting Python subprocess run results"""

+    def fail(self, cmd_line):

+        """Provide helpful details about failed subcommand runs"""

+        # Limit to 80 lines to ASCII characters

+        maxlen = 80 * 100

+        out, err = self.out, self.err

+        if len(out) > maxlen:

+            out = b'(... truncated stdout ...)' + out[-maxlen:]

+        if len(err) > maxlen:

+            err = b'(... truncated stderr ...)' + err[-maxlen:]

+        out = out.decode('ascii', 'replace').rstrip()

+        err = err.decode('ascii', 'replace').rstrip()

+        raise AssertionError("Process return code is %d\n"

+                             "command line: %r\n"

+                             "\n"

+                             "stdout:\n"

+                             "---\n"

+                             "%s\n"

+                             "---\n"

+                             "\n"

+                             "stderr:\n"

+                             "---\n"

+                             "%s\n"

+                             "---"

+                             % (self.rc, cmd_line,

+                                out,

+                                err))

 # Executing the interpreter in a subprocess

@@ -107,30 +134,7 @@ def run_python_until_end(*args, **env_vars):

 def _assert_python(expected_success, *args, **env_vars):

     res, cmd_line = run_python_until_end(*args, **env_vars)

     if (res.rc and expected_success) or (not res.rc and not expected_success):

-        # Limit to 80 lines to ASCII characters

-        maxlen = 80 * 100

-        out, err = res.out, res.err

-        if len(out) > maxlen:

-            out = b'(... truncated stdout ...)' + out[-maxlen:]

-        if len(err) > maxlen:

-            err = b'(... truncated stderr ...)' + err[-maxlen:]

-        out = out.decode('ascii', 'replace').rstrip()

-        err = err.decode('ascii', 'replace').rstrip()

-        raise AssertionError("Process return code is %d\n"

-                             "command line: %r\n"

-                             "\n"

-                             "stdout:\n"

-                             "---\n"

-                             "%s\n"

-                             "---\n"

-                             "\n"

-                             "stderr:\n"

-                             "---\n"

-                             "%s\n"

-                             "---"

-                             % (res.rc, cmd_line,

-                                out,

-                                err))

+        res.fail(cmd_line)

     return res

 def assert_python_ok(*args, **env_vars):

diff --git a/Lib/test/test_c_locale_coercion.py b/Lib/test/test_c_locale_coercion.py

new file mode 100644

index 0000000..ad6ecac

--- /dev/null

+++ b/Lib/test/test_c_locale_coercion.py

@@ -0,0 +1,269 @@

+# Tests the attempted automatic coercion of the C locale to a UTF-8 locale

+

+import unittest

+import os

+import sys

+import sysconfig

+import shutil

+import subprocess

+from collections import namedtuple

+

+import test.support

+from test.support.script_helper import (

+    run_python_until_end,

+    interpreter_requires_environment,

+)

+

+# In order to get the warning messages to match up as expected, the candidate

+# order here must much the target locale order in Python/pylifecycle.c

+_C_UTF8_LOCALES = (

+    # Entries: (Target locale, expected env var updates)

+    ("C.UTF-8", "LC_CTYPE & LANG"),

+    ("C.utf8", "LC_CTYPE & LANG"),

+    ("UTF-8", "LC_CTYPE"),

+)

+

+# There's no reliable cross-platform way of checking locale alias

+# lists, so the only way of knowing which of these locales will work

+# is to try them with locale.setlocale(). We do that in a subprocess

+# to avoid altering the locale of the test runner.

+def _set_locale_in_subprocess(locale_name):

+    cmd_fmt = "import locale; print(locale.setlocale(locale.LC_CTYPE, '{}'))"

+    cmd = cmd_fmt.format(locale_name)

+    result, py_cmd = run_python_until_end("-c", cmd, __isolated=True)

+    return result.rc == 0

+

+_EncodingDetails = namedtuple("EncodingDetails",

+                              "fsencoding stdin_info stdout_info stderr_info")

+

+class EncodingDetails(_EncodingDetails):

+    CHILD_PROCESS_SCRIPT = ";".join([

+        "import sys",

+        "print(sys.getfilesystemencoding())",

+        "print(sys.stdin.encoding + ':' + sys.stdin.errors)",

+        "print(sys.stdout.encoding + ':' + sys.stdout.errors)",

+        "print(sys.stderr.encoding + ':' + sys.stderr.errors)",

+    ])

+

+    @classmethod

+    def get_expected_details(cls, expected_fsencoding):

+        """Returns expected child process details for a given encoding"""

+        _stream = expected_fsencoding + ":{}"

+        # stdin and stdout should use surrogateescape either because the

+        # coercion triggered, or because the C locale was detected

+        stream_info = 2*[_stream.format("surrogateescape")]

+        # stderr should always use backslashreplace

+        stream_info.append(_stream.format("backslashreplace"))

+        return dict(cls(expected_fsencoding, *stream_info)._asdict())

+

+    @staticmethod

+    def _handle_output_variations(data):

+        """Adjust the output to handle platform specific idiosyncrasies

+

+        * Some platforms report ASCII as ANSI_X3.4-1968

+        * Some platforms report ASCII as US-ASCII

+        * Some platforms report UTF-8 instead of utf-8

+        """

+        data = data.replace(b"ANSI_X3.4-1968", b"ascii")

+        data = data.replace(b"US-ASCII", b"ascii")

+        data = data.lower()

+        return data

+

+    @classmethod

+    def get_child_details(cls, env_vars):

+        """Retrieves fsencoding and standard stream details from a child process

+

+        Returns (encoding_details, stderr_lines):

+

+        - encoding_details: EncodingDetails for eager decoding

+        - stderr_lines: result of calling splitlines() on the stderr output

+

+        The child is run in isolated mode if the current interpreter supports

+        that.

+        """

+        result, py_cmd = run_python_until_end(

+            "-c", cls.CHILD_PROCESS_SCRIPT,

+            __isolated=True,

+            **env_vars

+        )

+        if not result.rc == 0:

+            result.fail(py_cmd)

+        # All subprocess outputs in this test case should be pure ASCII

+        adjusted_output = cls._handle_output_variations(result.out)

+        stdout_lines = adjusted_output.decode("ascii").rstrip().splitlines()

+        child_encoding_details = dict(cls(*stdout_lines)._asdict())

+        stderr_lines = result.err.decode("ascii").rstrip().splitlines()

+        return child_encoding_details, stderr_lines

+

+

+class _ChildProcessEncodingTestCase(unittest.TestCase):

+    # Base class to check for expected encoding details in a child process

+

+    def _check_child_encoding_details(self,

+                                      env_vars,

+                                      expected_fsencoding,

+                                      expected_warning):

+        """Check the C locale handling for the given process environment

+

+        Parameters:

+            expected_fsencoding: the encoding the child is expected to report

+            allow_c_locale: setting to use for PYTHONALLOWCLOCALE

+              None: don't set the variable at all

+              str: the value set in the child's environment

+        """

+        result = EncodingDetails.get_child_details(env_vars)

+        encoding_details, stderr_lines = result

+        self.assertEqual(encoding_details,

+                         EncodingDetails.get_expected_details(

+                             expected_fsencoding))

+        self.assertEqual(stderr_lines, expected_warning)

+

+# Details of the shared library warning emitted at runtime

+LIBRARY_C_LOCALE_WARNING = (

+    "Python runtime initialized with LC_CTYPE=C (a locale with default ASCII "

+    "encoding), which may cause Unicode compatibility problems. Using C.UTF-8, "

+    "C.utf8, or UTF-8 (if available) as alternative Unicode-compatible "

+    "locales is recommended."

+)

+

+@unittest.skipUnless(sysconfig.get_config_var("PY_WARN_ON_C_LOCALE"),

+                     "C locale runtime warning disabled at build time")

+class LocaleWarningTests(_ChildProcessEncodingTestCase):

+    # Test warning emitted when running in the C locale

+

+    def test_library_c_locale_warning(self):

+        self.maxDiff = None

+        for locale_to_set in ("C", "POSIX", "invalid.ascii"):

+            var_dict = {

+                "LC_ALL": locale_to_set

+            }

+            with self.subTest(forced_locale=locale_to_set):

+                self._check_child_encoding_details(var_dict,

+                                                   "ascii",

+                                                   [LIBRARY_C_LOCALE_WARNING])

+

+# Details of the CLI locale coercion warning emitted at runtime

+CLI_COERCION_WARNING_FMT = (

+    "Python detected LC_CTYPE=C: {} coerced to {} (set another locale "

+    "or PYTHONCOERCECLOCALE=0 to disable this locale coercion behavior)."

+)

+

+class _LocaleCoercionTargetsTestCase(_ChildProcessEncodingTestCase):

+    # Base class for test cases that rely on coercion targets being defined

+

+    available_targets = []

+    targets_required = True

+

+    @classmethod

+    def setUpClass(cls):

+        first_target_locale = first_env_updates = None

+        available_targets = cls.available_targets

+        # Find the target locales available in the current system

+        for target_locale, env_updates in _C_UTF8_LOCALES:

+            if _set_locale_in_subprocess(target_locale):

+                available_targets.append(target_locale)

+                if first_target_locale is None:

+                    first_target_locale = target_locale

+                    first_env_updates = env_updates

+        if cls.targets_required and not available_targets:

+            raise unittest.SkipTest("No C-with-UTF-8 locale available")

+        # Expect coercion to use the first available locale

+        cls.EXPECTED_COERCION_WARNING = CLI_COERCION_WARNING_FMT.format(

+            first_env_updates, first_target_locale

+        )

+

+

+class LocaleConfigurationTests(_LocaleCoercionTargetsTestCase):

+    # Test explicit external configuration via the process environment

+

+    def test_external_target_locale_configuration(self):

+        # Explicitly setting a target locale should give the same behaviour as

+        # is seen when implicitly coercing to that target locale

+        self.maxDiff = None

+

+        expected_warning = []

+        expected_fsencoding = "utf-8"

+

+        base_var_dict = {

+            "LANG": "",

+            "LC_CTYPE": "",

+            "LC_ALL": "",

+        }

+        for env_var in ("LANG", "LC_CTYPE"):

+            for locale_to_set in self.available_targets:

+                with self.subTest(env_var=env_var,

+                                  configured_locale=locale_to_set):

+                    var_dict = base_var_dict.copy()

+                    var_dict[env_var] = locale_to_set

+                    self._check_child_encoding_details(var_dict,

+                                                       expected_fsencoding,

+                                                       expected_warning)

+

+

+

+@test.support.cpython_only

+@unittest.skipUnless(sysconfig.get_config_var("PY_COERCE_C_LOCALE"),

+                     "C locale coercion disabled at build time")

+class LocaleCoercionTests(_LocaleCoercionTargetsTestCase):

+    # Test implicit reconfiguration of the environment during CLI startup

+

+    def _check_c_locale_coercion(self, expected_fsencoding, coerce_c_locale):

+        """Check the C locale handling for various configurations

+

+        Parameters:

+            expected_fsencoding: the encoding the child is expected to report

+            allow_c_locale: setting to use for PYTHONALLOWCLOCALE

+              None: don't set the variable at all

+              str: the value set in the child's environment

+        """

+

+        # Check for expected warning on stderr if C locale is coerced

+        self.maxDiff = None

+

+        expected_warning = []

+        if coerce_c_locale != "0":

+            expected_warning.append(self.EXPECTED_COERCION_WARNING)

+

+        base_var_dict = {

+            "LANG": "",

+            "LC_CTYPE": "",

+            "LC_ALL": "",

+        }

+        for env_var in ("LANG", "LC_CTYPE"):

+            for locale_to_set in ("", "C", "POSIX", "invalid.ascii"):

+                with self.subTest(env_var=env_var,

+                                  nominal_locale=locale_to_set,

+                                  PYTHONCOERCECLOCALE=coerce_c_locale):

+                    var_dict = base_var_dict.copy()

+                    var_dict[env_var] = locale_to_set

+                    if coerce_c_locale is not None:

+                        var_dict["PYTHONCOERCECLOCALE"] = coerce_c_locale

+                    self._check_child_encoding_details(var_dict,

+                                                       expected_fsencoding,

+                                                       expected_warning)

+

+    def test_test_PYTHONCOERCECLOCALE_not_set(self):

+        # This should coerce to the first available target locale by default

+        self._check_c_locale_coercion("utf-8", coerce_c_locale=None)

+

+    def test_PYTHONCOERCECLOCALE_not_zero(self):

+        # *Any* string other that "0" is considered "set" for our purposes

+        # and hence should result in the locale coercion being enabled

+        for setting in ("", "1", "true", "false"):

+            self._check_c_locale_coercion("utf-8", coerce_c_locale=setting)

+

+    def test_PYTHONCOERCECLOCALE_set_to_zero(self):

+        # The setting "0" should result in the locale coercion being disabled

+        self._check_c_locale_coercion("ascii", coerce_c_locale="0")

+

+

+def test_main():

+    test.support.run_unittest(

+        LocaleConfigurationTests,

+        LocaleCoercionTests,

+        LocaleWarningTests

+    )

+    test.support.reap_children()

+

+if __name__ == "__main__":

+    test_main()

diff --git a/Lib/test/test_capi.py b/Lib/test/test_capi.py

index eb3e2c5..f677d88 100644

--- a/Lib/test/test_capi.py

+++ b/Lib/test/test_capi.py

@@ -369,14 +369,15 @@ def setUp(self):

     def tearDown(self):

         os.chdir(self.oldcwd)

-    def run_embedded_interpreter(self, *args):

+    def run_embedded_interpreter(self, *args, env=None):

         """Runs a test in the embedded interpreter"""

         cmd = [self.test_exe]

         cmd.extend(args)

         p = subprocess.Popen(cmd,

                              stdout=subprocess.PIPE,

                              stderr=subprocess.PIPE,

-                             universal_newlines=True)

+                             universal_newlines=True,

+                             env=env)

         (out, err) = p.communicate()

         self.assertEqual(p.returncode, 0,

                          "bad returncode %d, stderr is %r" %

@@ -386,7 +387,7 @@ def run_embedded_interpreter(self, *args):

     def test_subinterps(self):

         # This is just a "don't crash" test

         out, err = self.run_embedded_interpreter("repeated_init_and_subinterpreters")

-        if support.verbose:

+        if support.verbose > 1:

             print()

             print(out)

             print(err)

@@ -403,13 +404,14 @@ def _get_default_pipe_encoding():

     def test_forced_io_encoding(self):

         # Checks forced configuration of embedded interpreter IO streams

-        out, err = self.run_embedded_interpreter("forced_io_encoding")

-        if support.verbose:

+        env = {"PYTHONIOENCODING": "UTF-8:surrogateescape"}

+        out, err = self.run_embedded_interpreter("forced_io_encoding", env=env)

+        if support.verbose > 1:

             print()

             print(out)

             print(err)

-        expected_errors = sys.__stdout__.errors

-        expected_stdin_encoding = sys.__stdin__.encoding

+        expected_errors = "surrogateescape"

+        expected_stdin_encoding = "UTF-8"

         expected_pipe_encoding = self._get_default_pipe_encoding()

         expected_output = '\n'.join([

         "--- Use defaults ---",

diff --git a/Lib/test/test_cmd_line.py b/Lib/test/test_cmd_line.py

index 958d282..c4c6850 100644

--- a/Lib/test/test_cmd_line.py

+++ b/Lib/test/test_cmd_line.py

@@ -8,8 +8,9 @@

 import subprocess

 import tempfile

 from test.support import script_helper, is_android

-from test.support.script_helper import (spawn_python, kill_python, assert_python_ok,

-    assert_python_failure)

+from test.support.script_helper import (

+    spawn_python, kill_python, assert_python_ok, assert_python_failure

+)

 # XXX (ncoghlan): Move to script_helper and make consistent with run_python

@@ -150,6 +151,7 @@ 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'"), '

diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py

index ed78e2a..3844812 100644

--- a/Lib/test/test_sys.py

+++ b/Lib/test/test_sys.py

@@ -682,6 +682,7 @@ def c_locale_get_error_handler(self, isolated=False, encoding=None):

         # Force the POSIX locale

         env = os.environ.copy()

         env["LC_ALL"] = "C"

+        env["PYTHONCOERCECLOCALE"] = "0"

         code = '\n'.join((

             'import sys',

             'def dump(name):',

diff --git a/Programs/_testembed.c b/Programs/_testembed.c

index a68d4fa..280bf50 100644

--- a/Programs/_testembed.c

+++ b/Programs/_testembed.c

@@ -1,4 +1,5 @@

-#include <Python.h>

+#include "Python.h"

+#include "pyconfig.h"

 #include <stdio.h>

 /*********************************************************

diff --git a/Programs/python.c b/Programs/python.c

index a7afbc7..03f8295 100644

--- a/Programs/python.c

+++ b/Programs/python.c

@@ -15,6 +15,21 @@ wmain(int argc, wchar_t **argv)

 }

 #else

+/* Access private pylifecycle helper API to better handle the legacy C locale

+ *

+ * The legacy C locale assumes ASCII as the default text encoding, which

+ * causes problems not only for the CPython runtime, but also other

+ * components like GNU readline.

+ *

+ * Accordingly, when the CLI detects it, it attempts to coerce it to a

+ * more capable UTF-8 based alternative.

+ *

+ * See the documentation of the PYTHONCOERCECLOCALE setting for more details.

+ *

+ */

+extern int _Py_LegacyLocaleDetected(void);

+extern void _Py_CoerceLegacyLocale(void);

+

 int

 main(int argc, char **argv)

 {

@@ -25,7 +40,11 @@ main(int argc, char **argv)

     char *oldloc;

     /* Force malloc() allocator to bootstrap Python */

+#ifdef Py_DEBUG

+    (void)_PyMem_SetupAllocators("malloc_debug");

+#  else

     (void)_PyMem_SetupAllocators("malloc");

+#  endif

     argv_copy = (wchar_t **)PyMem_RawMalloc(sizeof(wchar_t*) * (argc+1));

     argv_copy2 = (wchar_t **)PyMem_RawMalloc(sizeof(wchar_t*) * (argc+1));

@@ -49,7 +68,21 @@ main(int argc, char **argv)

         return 1;

     }

+#ifdef __ANDROID__

+    /* Passing "" to setlocale() on Android requests the C locale rather

+     * than checking environment variables, so request C.UTF-8 explicitly

+     */

+    setlocale(LC_ALL, "C.UTF-8");

+#else

+    /* Reconfigure the locale to the default for this process */

     setlocale(LC_ALL, "");

+#endif

+

+    if (_Py_LegacyLocaleDetected()) {

+        _Py_CoerceLegacyLocale();

+    }

+

+    /* Convert from char to wchar_t based on the locale settings */

     for (i = 0; i < argc; i++) {

         argv_copy[i] = Py_DecodeLocale(argv[i], NULL);

         if (!argv_copy[i]) {

@@ -70,7 +103,11 @@ main(int argc, char **argv)

     /* Force again malloc() allocator to release memory blocks allocated

        before Py_Main() */

+#ifdef Py_DEBUG

+    (void)_PyMem_SetupAllocators("malloc_debug");

+#  else

     (void)_PyMem_SetupAllocators("malloc");

+#  endif

     for (i = 0; i < argc; i++) {

         PyMem_RawFree(argv_copy2[i]);

diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c

index c0f41b3..278a5af 100644

--- a/Python/pylifecycle.c

+++ b/Python/pylifecycle.c

@@ -167,6 +167,7 @@ Py_SetStandardStreamEncoding(const char *encoding, const char *errors)

     return 0;

 }

+

 /* Global initializations.  Can be undone by Py_FinalizeEx().  Don't

    call this twice without an intervening Py_FinalizeEx() call.  When

    initializations fail, a fatal error is issued and the function does

@@ -302,6 +303,167 @@ import_init(PyInterpreterState *interp, PyObject *sysmod)

 }

+/* Helper functions to better handle the legacy C locale

+ *

+ * The legacy C locale assumes ASCII as the default text encoding, which

+ * causes problems not only for the CPython runtime, but also other

+ * components like GNU readline.

+ *

+ * Accordingly, when the CLI detects it, it attempts to coerce it to a

+ * more capable UTF-8 based alternative as follows:

+ *

+ *     if (_Py_LegacyLocaleDetected()) {

+ *         _Py_CoerceLegacyLocale();

+ *     }

+ *

+ * See the documentation of the PYTHONCOERCECLOCALE setting for more details.

+ *

+ * Locale coercion also impacts the default error handler for the standard

+ * streams: while the usual default is "strict", the default for the legacy

+ * C locale and for any of the coercion target locales is "surrogateescape".

+ */

+

+int

+_Py_LegacyLocaleDetected(void)

+{

+    const char *ctype_loc = setlocale(LC_CTYPE, NULL);

+    return ctype_loc != NULL && strcmp(ctype_loc, "C") == 0;

+}

+

+typedef struct _CandidateLocale {

+    const char *locale_name; /* The locale to try as a coercion target */

+    int set_LANG;            /* Whether to set LANG in addition to LC_CTYPE */

+} _LocaleCoercionTarget;

+

+static _LocaleCoercionTarget _TARGET_LOCALES[] = {

+    { "C.UTF-8", 1 },

+    { "C.utf8",  1},

+    { "UTF-8", 0 },

+    { NULL, 0 }

+};

+

+static char *

+get_default_standard_stream_error_handler(void)

+{

+    const char *ctype_loc = setlocale(LC_CTYPE, NULL);

+    if (ctype_loc != NULL) {

+        /* "surrogateescape" is the default in the legacy C locale */

+        if (strcmp(ctype_loc, "C") == 0) {

+            return "surrogateescape";

+        }

+

+        /* "surrogateescape" is the default in locale coercion target locales */

+        const _LocaleCoercionTarget *target = NULL;

+        for (target = _TARGET_LOCALES; target->locale_name; target++) {

+            if (strcmp(ctype_loc, target->locale_name) == 0) {

+                return "surrogateescape";

+            }

+        }

+   }

+

+   /* Otherwise return NULL to request the typical default error handler */

+   return NULL;

+}

+

+#ifdef PY_COERCE_C_LOCALE

+static const char *_C_LOCALE_COERCION_WARNING =

+    "Python detected LC_CTYPE=C: %.20s coerced to %.20s (set another locale "

+    "or PYTHONCOERCECLOCALE=0 to disable this locale coercion behavior).\n";

+

+static void

+_coerce_default_locale_settings(const _LocaleCoercionTarget *target)

+{

+    const char *env_vars_updated = "LC_CTYPE";

+    const char *newloc = target->locale_name;

+

+    /* Reset locale back to currently configured defaults */

+    setlocale(LC_ALL, "");

+

+    /* Set the relevant locale environment variables */

+    if (setenv("LC_CTYPE", newloc, 1)) {

+        fprintf(stderr,

+                "Error setting LC_CTYPE, skipping C locale coercion\n");

+        return;

+    }

+    if (target->set_LANG) {

+        if (setenv("LANG", newloc, 1) == 0) {

+            env_vars_updated = "LC_CTYPE & LANG";

+        } else {

+            fprintf(stderr,

+                    "Error setting LANG during C locale coercion\n");

+        }

+    }

+    fprintf(stderr, _C_LOCALE_COERCION_WARNING, env_vars_updated, newloc);

+

+    /* Reconfigure with the overridden environment variables */

+    setlocale(LC_ALL, "");

+}

+

+static int

+c_locale_coercion_is_expected(void)

+{

+    /* This may be called prior to Py_Initialize, so we don't call any other

+     * Python APIs, and we ignore the -E and -I flags

+     */

+    const char *coerce_c_locale = getenv("PYTHONCOERCECLOCALE");

+    if (coerce_c_locale == NULL || strncmp(coerce_c_locale, "0", 2) != 0) {

+        return 1;

+    }

+    return 0;

+}

+#endif

+

+void

+_Py_CoerceLegacyLocale(void)

+{

+#ifdef PY_COERCE_C_LOCALE

+    /* We ignore the Python -E and -I flags here, as the CLI needs to sort out

+     * the locale settings *before* we try to do anything with the command

+     * line arguments. For cross-platform debugging purposes, we also need

+     * to give end users a way to force even scripts that are otherwise

+     * isolated from their environment to use the legacy ASCII-centric C

+     * locale.

+    */

+    if (c_locale_coercion_is_expected()) {

+        /* PYTHONCOERCECLOCALE is not set, or is not set to exactly "0" */

+        const char *locale_override = getenv("LC_ALL");

+        if (locale_override == NULL || *locale_override == '\0') {

+            /* LC_ALL is also not set (or is set to an empty string) */

+            const _LocaleCoercionTarget *target = NULL;

+            for (target = _TARGET_LOCALES; target->locale_name; target++) {

+                const char *new_locale = setlocale(LC_CTYPE,

+                                                   target->locale_name);