PEP 597: Add io.LOCALE_ENCODING constant (#1585)

pep-0597.rst

@@ -21,6 +21,9 @@ This PEP proposes:
 * Add ``encoding="locale"`` option to ``TextIOWrapper``. It behaves
   like ``encoding=None`` but don't raise a warning.
 
+* Add ``io.LOCALE_ENCODING = "locale"`` constant to avoid confusing
+  ``LookupError``. unknown encoding: locale``.
+
 
 Motivation
 ==========
@@ -92,6 +95,26 @@ This option can be used to use the locale encoding explicitly and
 suppress the ``PendingDeprecationWarning``.
 
 
+``io.LOCALE_ENCODING``
+----------------------
+
+``io`` module has ``io.LOCALE_ENCODING = "locale"`` constant. This
+constant can be used to avoid confusing ``LookupError: unknown
+encoding: locale`` error when the code is run in Python older than
+3.10 accidentally.
+
+The constant can be used to test that ``encoding="locale"`` option
+is supported too.
+
+::
+
+   # Want to suppress the Warning in dev mode but still need support
+   # old Python versions.
+   locale_encoding = getattr(io, "LOCALE_ENCODING", None)
+   with open(filename, encoding=locale_encoding) as f:
+       ...
+
+
 ``io.text_encoding``
 --------------------
 
@@ -121,7 +144,7 @@ it. Pure Python implementation will be like this::
                        "'encoding' option is not specified. The default encoding "
                        "might be changed to 'utf-8' in the future",
                        PendingDeprecationWarning, stacklevel + 2)
-           encoding = "locale"
+           encoding = LOCALE_ENCODING
        return encoding
 
 ``pathlib.Path.read_text()`` can use this function like this::
@@ -139,8 +162,8 @@ subprocess module doesn't warn
 ------------------------------
 
 While the subprocess module uses TextIOWrapper, it doesn't raise
-``PendingDeprecationWarning``.  It uses the "locale" encoding by
-default.
+``PendingDeprecationWarning``.  It uses the ``io.LOCALE_ENCODING``
+by default.
 
 
 Rationale