diff --git a/Doc/c-api/sys.rst b/Doc/c-api/sys.rst
@@ -127,6 +127,9 @@ Operating System Utilities

   .. versionadded:: 3.5


.. c:function:: char* Py_EncodeLocale(const wchar_t *text, size_t *error_pos)

@@ -138,19 +141,25 @@ Operating System Utilities
   to free the memory. Return ``NULL`` on encoding error or memory allocation
   error

-   If error_pos is not ``NULL``, ``*error_pos`` is set to the index of the
-   invalid character on encoding error, or set to ``(size_t)-1`` otherwise.

   Use the :c:func:`Py_DecodeLocale` function to decode the bytes string back
   to a wide character string.

   .. seealso::

      The :c:func:`PyUnicode_EncodeFSDefault` and
      :c:func:`PyUnicode_EncodeLocale` functions.

   .. versionadded:: 3.5


.. _systemfunctions:


diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
@@ -313,6 +313,9 @@ always available.
      has caught :exc:`SystemExit` (such as an error flushing buffered data
      in the standard streams), the exit status is changed to 120.


.. data:: flags

@@ -335,6 +338,7 @@ always available.
   :const:`quiet`                :option:`-q`
   :const:`hash_randomization`   :option:`-R`
   :const:`dev_mode`             :option:`-X` ``dev``
   ============================= =============================

   .. versionchanged:: 3.2
@@ -347,7 +351,8 @@ always available.
      Removed obsolete ``division_warning`` attribute.

   .. versionchanged:: 3.7
-      Added ``dev_mode`` attribute for the new :option:`-X` ``dev`` flag.


.. data:: float_info
@@ -492,6 +497,8 @@ always available.
   :func:`os.fsencode` and :func:`os.fsdecode` should be used to ensure that
   the correct encoding and errors mode are used.

   * On Mac OS X, the encoding is ``'utf-8'``.

   * On Unix, the encoding is the locale encoding.
@@ -506,6 +513,10 @@ always available.
      Windows is no longer guaranteed to return ``'mbcs'``. See :pep:`529`
      and :func:`_enablelegacywindowsfsencoding` for more information.

.. function:: getfilesystemencodeerrors()

   Return the name of the error mode used to convert between Unicode filenames

diff --git a/Doc/using/cmdline.rst b/Doc/using/cmdline.rst
@@ -439,6 +439,9 @@ Miscellaneous options
     * Set the :attr:`~sys.flags.dev_mode` attribute of :attr:`sys.flags` to
       ``True``

   It also allows passing arbitrary values and retrieving them through the
   :data:`sys._xoptions` dictionary.

@@ -455,7 +458,7 @@ Miscellaneous options
      The ``-X showalloccount`` option.

   .. versionadded:: 3.7
-      The ``-X importtime`` and ``-X dev`` options.


Options you shouldn't use
@@ -816,6 +819,14 @@ conflict.

   .. versionadded:: 3.7

Debug-mode variables
~~~~~~~~~~~~~~~~~~~~


diff --git a/Doc/whatsnew/3.7.rst b/Doc/whatsnew/3.7.rst
@@ -185,6 +185,23 @@ resolution on Linux and Windows.
       PEP written and implemented by Victor Stinner


New Development Mode: -X dev
----------------------------

@@ -353,6 +370,10 @@ Added another argument *monetary* in :meth:`format_string` of :mod:`locale`.
If *monetary* is true, the conversion uses monetary thousands separator and
grouping strings. (Contributed by Garvit in :issue:`10379`.)

math
----


diff --git a/Include/fileobject.h b/Include/fileobject.h
@@ -28,6 +28,10 @@ PyAPI_DATA(const char *) Py_FileSystemDefaultEncodeErrors;
#endif
PyAPI_DATA(int) Py_HasFileSystemDefaultEncoding;

/* Internal API

   The std printer acts as a preliminary sys.stderr until the new io

diff --git a/Include/pystate.h b/Include/pystate.h
@@ -38,6 +38,7 @@ typedef struct {
    int show_alloc_count;   /* -X showalloccount */
    int dump_refs;          /* PYTHONDUMPREFS */
    int malloc_stats;       /* PYTHONMALLOCSTATS */
} _PyCoreConfig;

#define _PyCoreConfig_INIT (_PyCoreConfig){.use_hash_seed = -1}

diff --git a/Lib/_bootlocale.py b/Lib/_bootlocale.py
@@ -9,6 +9,8 @@

if sys.platform.startswith("win"):
    def getpreferredencoding(do_setlocale=True):
        return _locale._getdefaultlocale()[1]
else:
    try:
@@ -21,13 +23,17 @@ def getpreferredencoding(do_setlocale=True):
                return 'UTF-8'
        else:
            def getpreferredencoding(do_setlocale=True):
                # This path for legacy systems needs the more complex
                # getdefaultlocale() function, import the full locale module.
                import locale
                return locale.getpreferredencoding(do_setlocale)
    else:
        def getpreferredencoding(do_setlocale=True):
            assert not do_setlocale
            result = _locale.nl_langinfo(_locale.CODESET)
            if not result and sys.platform == 'darwin':
                # nl_langinfo can return an empty string

diff --git a/Lib/encodings/__init__.py b/Lib/encodings/__init__.py
@@ -158,8 +158,9 @@ def search_function(encoding):
if sys.platform == 'win32':
    def _alias_mbcs(encoding):
        try:
-            import _bootlocale
-            if encoding == _bootlocale.getpreferredencoding(False):
                import encodings.mbcs
                return encodings.mbcs.getregentry()
        except ImportError:

diff --git a/Lib/locale.py b/Lib/locale.py
@@ -617,6 +617,8 @@ def resetlocale(category=LC_ALL):
    # On Win32, this will return the ANSI code page
    def getpreferredencoding(do_setlocale = True):
        """Return the charset that the user is likely using."""
        import _bootlocale
        return _bootlocale.getpreferredencoding(False)
else:
@@ -634,6 +636,8 @@ def getpreferredencoding(do_setlocale = True):
            def getpreferredencoding(do_setlocale = True):
                """Return the charset that the user is likely using,
                by looking at environment variables."""
                res = getdefaultlocale()[1]
                if res is None:
                    # LANG not set, default conservatively to ASCII
@@ -643,6 +647,8 @@ def getpreferredencoding(do_setlocale = True):
        def getpreferredencoding(do_setlocale = True):
            """Return the charset that the user is likely using,
            according to the system configuration."""
            import _bootlocale
            if do_setlocale:
                oldloc = setlocale(LC_CTYPE)

diff --git a/Lib/subprocess.py b/Lib/subprocess.py
@@ -280,7 +280,7 @@ def _args_from_interpreter_flags():
    if dev_mode:
        args.extend(('-X', 'dev'))
    for opt in ('faulthandler', 'tracemalloc', 'importtime',
-                'showalloccount', 'showrefcount'):
        if opt in xoptions:
            value = xoptions[opt]
            if value is True:

diff --git a/Lib/test/test_c_locale_coercion.py b/Lib/test/test_c_locale_coercion.py
@@ -130,7 +130,7 @@ def get_child_details(cls, env_vars):
        that.
        """
        result, py_cmd = run_python_until_end(
-            "-c", cls.CHILD_PROCESS_SCRIPT,
            __isolated=True,
            **env_vars
        )

diff --git a/Lib/test/test_codecs.py b/Lib/test/test_codecs.py
@@ -5,6 +5,7 @@
import sys
import unittest
import encodings

from test import support

@@ -3180,16 +3181,9 @@ def test_incremental(self):
    def test_mbcs_alias(self):
        # Check that looking up our 'default' codepage will return
        # mbcs when we don't have a more specific one available
-        import _bootlocale
-        def _get_fake_codepage(*a):
-            return 'cp123'
-        old_getpreferredencoding = _bootlocale.getpreferredencoding
-        _bootlocale.getpreferredencoding = _get_fake_codepage
-        try:
            codec = codecs.lookup('cp123')
            self.assertEqual(codec.name, 'mbcs')
-        finally:
-            _bootlocale.getpreferredencoding = old_getpreferredencoding


class ASCIITest(unittest.TestCase):

diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
@@ -527,14 +527,16 @@ def test_sys_flags(self):
                 "inspect", "interactive", "optimize", "dont_write_bytecode",
                 "no_user_site", "no_site", "ignore_environment", "verbose",
                 "bytes_warning", "quiet", "hash_randomization", "isolated",
-                 "dev_mode")
        for attr in attrs:
            self.assertTrue(hasattr(sys.flags, attr), attr)
            attr_type = bool if attr == "dev_mode" else int
            self.assertEqual(type(getattr(sys.flags, attr)), attr_type, attr)
        self.assertTrue(repr(sys.flags))
        self.assertEqual(len(sys.flags), len(attrs))

    def assert_raise_on_new_sys_type(self, sys_attr):
        # Users are intentionally prevented from creating new instances of
        # sys.flags, sys.version_info, and sys.getwindowsversion.
@@ -710,8 +712,8 @@ def test_c_locale_surrogateescape(self):
        # have no any effect
        out = self.c_locale_get_error_handler(encoding=':')
        self.assertEqual(out,
-                         'stdin: surrogateescape\n'
-                         'stdout: surrogateescape\n'
                         'stderr: backslashreplace\n')
        out = self.c_locale_get_error_handler(encoding='')
        self.assertEqual(out,
-Original file line number
+Diff line change
@@ @@ -185,6 +185,23 @@ resolution on Linux and Windows. @@
            PEP written and implemented by Victor Stinner
     New Development Mode: -X dev
     ----------------------------
@@ Expand Down Expand Up @@
     If *monetary* is true, the conversion uses monetary thousands separator and
     grouping strings. (Contributed by Garvit in :issue:`10379`.)
     math
     ----
@@ Expand Down @@