bpo-29240: PEP 540: Add a new UTF-8 Mode (#855)

* Add -X utf8 command line option, PYTHONUTF8 environment variable
  and a new sys.flags.utf8_mode flag.
* If the LC_CTYPE locale is "C" at startup: enable automatically the
  UTF-8 mode.
* Add _winapi.GetACP(). encodings._alias_mbcs() now calls
  _winapi.GetACP() to get the ANSI code page
* locale.getpreferredencoding() now returns 'UTF-8' in the UTF-8
  mode. As a side effect, open() now uses the UTF-8 encoding by
  default in this mode.
* Py_DecodeLocale() and Py_EncodeLocale() now use the UTF-8 encoding
  in the UTF-8 Mode.
* Update subprocess._args_from_interpreter_flags() to handle -X utf8
* Skip some tests relying on the current locale if the UTF-8 mode is
  enabled.
* Add test_utf8mode.py.
* _Py_DecodeUTF8_surrogateescape() gets a new optional parameter to
  return also the length (number of wide characters).
* pymain_get_global_config() and pymain_set_global_config() now
  always copy flag values, rather than only copying if the new value
  is greater than the old value.

5 files changed, 63 insertions, 4 deletions

diff --git a/Doc/c-api/sys.rst b/Doc/c-api/sys.rst
index 95d9d65..20bc7bd 100644
--- a/Doc/c-api/sys.rst
+++ b/Doc/c-api/sys.rst
@@ -127,6 +127,9 @@ Operating System Utilities
 
    .. versionadded:: 3.5
 
+   .. versionchanged:: 3.7
+      The function now uses the UTF-8 encoding in the UTF-8 mode.
+
 
 .. c:function:: char* Py_EncodeLocale(const wchar_t *text, size_t *error_pos)
 
@@ -138,12 +141,15 @@ 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.
+   If error_pos is not ``NULL``, ``*error_pos`` is set to ``(size_t)-1`` on
+   success,  or set to the index of the invalid character on encoding error.
 
    Use the :c:func:`Py_DecodeLocale` function to decode the bytes string back
    to a wide character string.
 
+   .. versionchanged:: 3.7
+      The function now uses the UTF-8 encoding in the UTF-8 mode.
+
    .. seealso::
 
       The :c:func:`PyUnicode_EncodeFSDefault` and
@@ -151,6 +157,9 @@ Operating System Utilities
 
    .. versionadded:: 3.5
 
+   .. versionchanged:: 3.7
+      The function now supports the UTF-8 mode.
+
 
 .. _systemfunctions:
 
diff --git a/Doc/library/locale.rst b/Doc/library/locale.rst
index e8567a7..7da94a2 100644
--- a/Doc/library/locale.rst
+++ b/Doc/library/locale.rst
@@ -316,6 +316,13 @@ The :mod:`locale` module defines the following exception and functions:
    preferences, so this function is not thread-safe. If invoking setlocale is not
    necessary or desired, *do_setlocale* should be set to ``False``.
 
+   On Android or in the UTF-8 mode (:option:`-X` ``utf8`` option), always
+   return ``'UTF-8'``, the locale and the *do_setlocale* argument are ignored.
+
+   .. versionchanged:: 3.7
+      The function now always returns ``UTF-8`` on Android or if the UTF-8 mode
+      is enabled.
+
 
 .. function:: normalize(localename)
 
diff --git a/Doc/library/sys.rst b/Doc/library/sys.rst
index 9e47681..957d02b 100644
--- 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.
 
+   .. versionchanged:: 3.7
+      Added ``utf8_mode`` attribute for the new :option:`-X` ``utf8`` flag.
+
 
 .. data:: flags
 
@@ -335,6 +338,7 @@ always available.
    :const:`quiet`                :option:`-q`
    :const:`hash_randomization`   :option:`-R`
    :const:`dev_mode`             :option:`-X` ``dev``
+   :const:`utf8_mode`            :option:`-X` ``utf8``
    ============================= =============================
 
    .. 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.
+      Added ``dev_mode`` attribute for the new :option:`-X` ``dev`` flag
+      and ``utf8_mode`` attribute for the new  :option:`-X` ``utf8`` 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.
 
+   * In the UTF-8 mode, the encoding is ``utf-8`` on any platform.
+
    * 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.
 
+   .. versionchanged:: 3.7
+      Return 'utf-8' in the UTF-8 mode.
+
+
 .. 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
index e32f77e..5cb9071 100644
--- 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``
 
+   * ``-X utf8`` enables the UTF-8 mode, whereas ``-X utf8=0`` disables the
+     UTF-8 mode.
+
    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.
+      The ``-X importtime``, ``-X dev`` and ``-X utf8`` options.
 
 
 Options you shouldn't use
@@ -816,6 +819,14 @@ conflict.
 
    .. versionadded:: 3.7
 
+.. envvar:: PYTHONUTF8
+
+   If set to ``1``, enable the UTF-8 mode. If set to ``0``, disable the UTF-8
+   mode. Any other non-empty string cause an error.
+
+   .. versionadded:: 3.7
+
+
 Debug-mode variables
 ~~~~~~~~~~~~~~~~~~~~
 
diff --git a/Doc/whatsnew/3.7.rst b/Doc/whatsnew/3.7.rst
index 58bfaef..81a88a0 100644
--- 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
 
 
+PEP 540: Add a new UTF-8 mode
+-----------------------------
+
+Add a new UTF-8 mode to ignore the locale, use the UTF-8 encoding, and change
+:data:`sys.stdin` and :data:`sys.stdout` error handlers to ``surrogateescape``.
+This mode is enabled by default in the POSIX locale, but otherwise disabled by
+default.
+
+The new :option:`-X` ``utf8`` command line option and :envvar:`PYTHONUTF8`
+environment variable are added to control the UTF-8 mode.
+
+.. seealso::
+
+    :pep:`540` -- Add a new UTF-8 mode
+       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`.)
 
+The :func:`locale.getpreferredencoding` function now always returns ``'UTF-8'``
+on Android or in the UTF-8 mode  (:option:`-X` ``utf8`` option), the locale and
+the *do_setlocale* argument are ignored.
+
 math
 ----