PEP 686: Update (#2470)

Author: methane

pep-0686.rst

@@ -54,22 +54,37 @@ Users can still disable UTF-8 mode by setting ``PYTHONUTF8=0`` or
 ``-X utf8=0``.
 
 
-``locale.get_encoding()``
--------------------------
+``locale.getencoding()``
+------------------------
 
-Currently, ``TextIOWrapper`` uses ``locale.getpreferredencoding(False)``
-when ``encoding="locale"`` option is specified. It is ``"UTF-8"`` in UTF-8 mode.
+Since UTF-8 mode affects ``locale.getpreferredencoding(False)``,
+we need an API to get locale encoding regardless of UTF-8 mode.
 
-This behavior is inconsistent with the :pep:`597` motivation. 
-``TextIOWrapper`` should use locale encoding when ``encoding="locale"`` is
-passed before/after the default encoding is changed to UTF-8.
+``locale.getencoding()`` will be added for this purpose.
+It returns locale encoding too, but ignores UTF-8 mode.
+
+When ``warn_default_encoding`` option is specified,
+``locale.getpreferredencoding()`` will emit ``EncodingWarning`` like
+``open()`` (see also :pep:`597`).
+
+
+Fixing ``encoding="locale"`` option
+-----------------------------------
 
-To fix this inconsistency, we will add ``locale.get_encoding()``.
-It is the same as ``locale.getpreferredencoding(False)`` but it ignores
-the UTF-8 mode.
+:pep:`597` added the ``encoding="locale"`` option to the ``TextIOWrapper``.
+This option is used to specify the locale encoding explicitly.
+``TextIOWrapper`` should use locale encoding when the option is specified,
+regardless of default text encoding.
 
-This change will be released in Python 3.11 so that users can use UTF-8 mode
-that is the same as Python 3.13.
+But ``TextIOWrapper`` uses ``"UTF-8"`` in UTF-8 mode even if
+``encoding="locale"`` is specified for now.
+This behavior is inconsistent with the :pep:`597` motivation.
+It is because we didn't expect making UTF-8 mode default when Python
+changes its default text encoding.
+
+This inconsistency should be fixed before making UTF-8 mode default.
+``TextIOWrapper`` should use locale encoding when ``encoding="locale"`` is
+passed even in UTF-8 mode.
 
 
 Backward Compatibility
@@ -83,16 +98,18 @@ When a Python program depends on the default encoding, this change may cause
 ``UnicodeError``, mojibake, or even silent data corruption.
 So this change should be announced loudly.
 
-To resolve this backward incompatibility, users can do:
+This is the guideline to fix this backward compatibility issue:
+
+1. Disable UTF-8 mode.
+2. Use ``EncodingWarning`` (:pep:`597`) to find every places UTF-8 mode
+   affects.
+
+  * If ``encoding`` option is omitted, consider using ``encoding="utf-8"``
+    or ``encoding="locale"``.
+  * If ``locale.getpreferredencoding()`` is used, consider using
+    ``"utf-8"`` or ``locale.getencoding()``.
 
-* Disable UTF-8 mode.
-* Use ``EncodingWarning`` to find where the default encoding is used and use
-  ``encoding="locale"`` option if locale encoding should be used
-  (as defined in :pep:`597`).
-* Find every occurrence of ``locale.getpreferredencoding(False)`` in the
-  application, and replace it with ``locale.get_locale_encoding()`` if
-  locale encoding should be used.
-* Test the application with UTF-8 mode.
+3. Test the application with UTF-8 mode.
 
 
 Preceding examples
@@ -122,10 +139,31 @@ Additionally, such warnings are not useful for non-cross platform applications
 run on Unix.
 
 So forcing users to specify the ``encoding`` everywhere is too painful.
+Emitting a lot of ``DeprecationWarning`` will lead users ignore warnings.
+
+:pep:`387` requires adding a warning for backward incompatible changes.
+But it doesn't require using ``DeprecationWarning``.
+So using optional ``EncodingWarning`` doesn't violate the :pep:`387`.
 
 Java also rejected this idea in `JEP 400`_.
 
 
+Use ``PYTHONIOENCODING`` for PIPEs
+----------------------------------
+
+To ease backward compatibility issue, using ``PYTHONIOENCODING`` as the
+default encoding of PIPEs in the ``subprocess`` module is considered.
+
+With this idea, users can use legacy encoding for
+``subprocess.Popen(text=True)`` even in UTF-8 mode.
+
+But this idea makes "default encoding" complicated.
+And this idea is also backward incompatible.
+
+So this idea is rejected. Users can disable UTF-8 mode until they replace
+``text=True`` with ``encoding="utf-8"`` or ``encoding="locale"``.
+
+
 How to teach this
 =================