gh-91156: Fix encoding="locale" in UTF-8 mode

Doc/library/io.rst

@@ -112,7 +112,7 @@ Text Encoding
 -------------
 
 The default encoding of :class:`TextIOWrapper` and :func:`open` is
-locale-specific (:func:`locale.getpreferredencoding(False) <locale.getpreferredencoding>`).
+locale-specific (:func:`locale.getencoding`).
 
 However, many developers forget to specify the encoding when opening text files
 encoded in UTF-8 (e.g. JSON, TOML, Markdown, etc...) since most Unix
@@ -948,8 +948,7 @@ Text I/O
    :class:`TextIOBase`.
 
    *encoding* gives the name of the encoding that the stream will be decoded or
-   encoded with.  It defaults to
-   :func:`locale.getpreferredencoding(False) <locale.getpreferredencoding>`.
+   encoded with.  It defaults to :func:`locale.getencoding()`.
    ``encoding="locale"`` can be used to specify the current locale's encoding
    explicitly. See :ref:`io-text-encoding` for more information.
 

Doc/using/windows.rst

@@ -618,7 +618,7 @@ UTF-8 mode
 
 Windows still uses legacy encodings for the system encoding (the ANSI Code
 Page).  Python uses it for the default encoding of text files (e.g.
-:func:`locale.getpreferredencoding`).
+:func:`locale.getencoding`).
 
 This may cause issues because UTF-8 is widely used on the internet
 and most Unix systems, including WSL (Windows Subsystem for Linux).

Lib/_pyio.py

@@ -1988,7 +1988,7 @@ class TextIOWrapper(TextIOBase):
     r"""Character and line based layer over a BufferedIOBase object, buffer.
 
     encoding gives the name of the encoding that the stream will be
-    decoded or encoded with. It defaults to locale.getpreferredencoding(False).
+    decoded or encoded with. It defaults to locale.getencoding().
 
     errors determines the strictness of encoding and decoding (see the
     codecs.register) and defaults to "strict".
@@ -2021,7 +2021,9 @@ def __init__(self, buffer, encoding=None, errors=None, newline=None,
         self._check_newline(newline)
         encoding = text_encoding(encoding)
 
-        if encoding == "locale":
+        if encoding == "locale" and sys.platform == "win32":
+            # On Unix, os.device_encoding() returns "utf-8" instead of locale encoding
+            # in the UTF-8 mode. So we use os.device_encoding() only on Windows.
             try:
                 encoding = os.device_encoding(buffer.fileno()) or "locale"
             except (AttributeError, UnsupportedOperation):
@@ -2034,7 +2036,7 @@ def __init__(self, buffer, encoding=None, errors=None, newline=None,
                 # Importing locale may fail if Python is being built
                 encoding = "utf-8"
             else:
-                encoding = locale.getpreferredencoding(False)
+                encoding = locale.getencoding()
 
         if not isinstance(encoding, str):
             raise ValueError("invalid encoding: %r" % encoding)

Lib/locale.py

@@ -557,7 +557,7 @@ def getdefaultlocale(envvars=('LC_ALL', 'LC_CTYPE', 'LANG', 'LANGUAGE')):
 
     import warnings
     warnings.warn(
-        "Use setlocale(), getpreferredencoding(False) and getlocale() instead",
+        "Use setlocale(), getencoding() and getlocale() instead",
         DeprecationWarning, stacklevel=2
     )
 

Lib/test/test_io.py

@@ -2737,6 +2737,7 @@ def test_default_encoding(self):
             os.environ.update(old_environ)
 
     @support.cpython_only
+    @unittest.skipIf(sys.platform != "win32", "Windows-only test")
     @unittest.skipIf(sys.flags.utf8_mode, "utf-8 mode is enabled")
     def test_device_encoding(self):
         # Issue 15989

Misc/NEWS.d/next/Library/2022-04-10-17-50-18.bpo-47000.JlQkFx.rst

@@ -0,0 +1,2 @@
+Make :class:`TextIOWrapper` uses locale encoding when ``encoding="locale"``
+is specified even in UTF-8 mode.

Modules/_io/_iomodule.c

@@ -92,9 +92,9 @@ it already exists), 'x' for creating and writing to a new file, and
 'a' for appending (which on some Unix systems, means that all writes
 append to the end of the file regardless of the current seek position).
 In text mode, if encoding is not specified the encoding used is platform
-dependent: locale.getpreferredencoding(False) is called to get the
-current locale encoding. (For reading and writing raw bytes use binary
-mode and leave encoding unspecified.) The available modes are:
+dependent: locale.getencoding() is called to get the current locale encoding.
+(For reading and writing raw bytes use binary mode and leave encoding
+unspecified.) The available modes are:
 
 ========= ===============================================================
 Character Meaning
@@ -196,7 +196,7 @@ static PyObject *
 _io_open_impl(PyObject *module, PyObject *file, const char *mode,
               int buffering, const char *encoding, const char *errors,
               const char *newline, int closefd, PyObject *opener)
-/*[clinic end generated code: output=aefafc4ce2b46dc0 input=1543f4511d2356a5]*/
+/*[clinic end generated code: output=aefafc4ce2b46dc0 input=5bb37f174cb2fb11]*/
 {
     unsigned i;
 

Modules/_io/textio.c

@@ -1023,7 +1023,7 @@ _io.TextIOWrapper.__init__
 Character and line based layer over a BufferedIOBase object, buffer.
 
 encoding gives the name of the encoding that the stream will be
-decoded or encoded with. It defaults to locale.getpreferredencoding(False).
+decoded or encoded with. It defaults to locale.getencoding().
 
 errors determines the strictness of encoding and decoding (see
 help(codecs.Codec) or the documentation for codecs.register) and
@@ -1055,12 +1055,12 @@ _io_TextIOWrapper___init___impl(textio *self, PyObject *buffer,
                                 const char *encoding, PyObject *errors,
                                 const char *newline, int line_buffering,
                                 int write_through)
-/*[clinic end generated code: output=72267c0c01032ed2 input=77d8696d1a1f460b]*/
+/*[clinic end generated code: output=72267c0c01032ed2 input=72590963698f289b]*/
 {
     PyObject *raw, *codec_info = NULL;
-    _PyIO_State *state = NULL;
     PyObject *res;
     int r;
+    int use_locale_encoding = 0; // Use locale encoding even in UTF-8 mode.
 
     self->ok = 0;
     self->detached = 0;
@@ -1076,6 +1076,7 @@ _io_TextIOWrapper___init___impl(textio *self, PyObject *buffer,
     }
     else if (strcmp(encoding, "locale") == 0) {
         encoding = NULL;
+        use_locale_encoding = 1;
     }
 
     if (errors == Py_None) {
@@ -1113,10 +1114,15 @@ _io_TextIOWrapper___init___impl(textio *self, PyObject *buffer,
     self->encodefunc = NULL;
     self->b2cratio = 0.0;
 
+#ifdef MS_WINDOWS
+    // os.device_encoding() on Unix is the locale encoding or UTF-8
+    // according to UTF-8 Mode.
+    // Since UTF-8 mode shouldn't affect `encoding="locale"`, we call
+    // os.device_encoding() only on Windows.
     if (encoding == NULL) {
         /* Try os.device_encoding(fileno) */
         PyObject *fileno;
-        state = IO_STATE();
+        _PyIO_State *state = IO_STATE();
         if (state == NULL)
             goto error;
         fileno = PyObject_CallMethodNoArgs(buffer, &_Py_ID(fileno));
@@ -1144,8 +1150,10 @@ _io_TextIOWrapper___init___impl(textio *self, PyObject *buffer,
                 Py_CLEAR(self->encoding);
         }
     }
+#endif
+
     if (encoding == NULL && self->encoding == NULL) {
-        if (_PyRuntime.preconfig.utf8_mode) {
+        if (_PyRuntime.preconfig.utf8_mode && !use_locale_encoding) {
             _Py_DECLARE_STR(utf_8, "utf-8");
             self->encoding = Py_NewRef(&_Py_STR(utf_8));
         }

Tools/c-analyzer/TODO

@@ -251,7 +251,6 @@ Modules/_io/textio.c:PyId_close                                  _Py_IDENTIFIER(
 Modules/_io/textio.c:PyId_decode                                 _Py_IDENTIFIER(decode)
 Modules/_io/textio.c:PyId_fileno                                 _Py_IDENTIFIER(fileno)
 Modules/_io/textio.c:PyId_flush                                  _Py_IDENTIFIER(flush)
-Modules/_io/textio.c:PyId_getpreferredencoding                   _Py_IDENTIFIER(getpreferredencoding)
 Modules/_io/textio.c:PyId_isatty                                 _Py_IDENTIFIER(isatty)
 Modules/_io/textio.c:PyId_mode                                   _Py_IDENTIFIER(mode)
 Modules/_io/textio.c:PyId_name                                   _Py_IDENTIFIER(name)