gh-91156: Fix `encoding="locale"` in UTF-8 mode (GH-70056)

11 files changed, 35 insertions, 24 deletions

diff --git a/Doc/library/io.rst b/Doc/library/io.rst
index 80107d5..53dad99 100644
--- a/Doc/library/io.rst
+++ b/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.
 
diff --git a/Doc/using/windows.rst b/Doc/using/windows.rst
index 83eee28..88dcb00 100644
--- a/Doc/using/windows.rst
+++ b/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).
diff --git a/Lib/_pyio.py b/Lib/_pyio.py
index e3ff59e..0f33ed5 100644
--- a/Lib/_pyio.py
+++ b/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 @@ class TextIOWrapper(TextIOBase):
         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 @@ class TextIOWrapper(TextIOBase):
                 # 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)
diff --git a/Lib/locale.py b/Lib/locale.py
index 496cc80..170e5ee 100644
--- a/Lib/locale.py
+++ b/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
     )
 
diff --git a/Lib/test/test_io.py b/Lib/test/test_io.py
index 29fe287..c86251d 100644
--- a/Lib/test/test_io.py
+++ b/Lib/test/test_io.py
@@ -2737,6 +2737,7 @@ class TextIOWrapperTest(unittest.TestCase):
             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
diff --git a/Misc/NEWS.d/next/Library/2022-04-10-17-50-18.bpo-47000.JlQkFx.rst b/Misc/NEWS.d/next/Library/2022-04-10-17-50-18.bpo-47000.JlQkFx.rst
new file mode 100644
index 0000000..77d5b8f
--- /dev/null
+++ b/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.
diff --git a/Modules/_io/_iomodule.c b/Modules/_io/_iomodule.c
index 065f5e2..38ef246 100644
--- a/Modules/_io/_iomodule.c
+++ b/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;
 
diff --git a/Modules/_io/clinic/_iomodule.c.h b/Modules/_io/clinic/_iomodule.c.h
index e4a6b8c..1fdbe68 100644
--- a/Modules/_io/clinic/_iomodule.c.h
+++ b/Modules/_io/clinic/_iomodule.c.h
@@ -22,9 +22,9 @@ PyDoc_STRVAR(_io_open__doc__,
 "\'a\' for appending (which on some Unix systems, means that all writes\n"
 "append to the end of the file regardless of the current seek position).\n"
 "In text mode, if encoding is not specified the encoding used is platform\n"
-"dependent: locale.getpreferredencoding(False) is called to get the\n"
-"current locale encoding. (For reading and writing raw bytes use binary\n"
-"mode and leave encoding unspecified.) The available modes are:\n"
+"dependent: locale.getencoding() is called to get the current locale encoding.\n"
+"(For reading and writing raw bytes use binary mode and leave encoding\n"
+"unspecified.) The available modes are:\n"
 "\n"
 "========= ===============================================================\n"
 "Character Meaning\n"
@@ -355,4 +355,4 @@ _io_open_code(PyObject *module, PyObject *const *args, Py_ssize_t nargs, PyObjec
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=1a7fd7755c9a9609 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=e562f29e3c2533a6 input=a9049054013a1b77]*/
diff --git a/Modules/_io/clinic/textio.c.h b/Modules/_io/clinic/textio.c.h
index 0b047ac..7e81eb3 100644
--- a/Modules/_io/clinic/textio.c.h
+++ b/Modules/_io/clinic/textio.c.h
@@ -146,7 +146,7 @@ PyDoc_STRVAR(_io_TextIOWrapper___init____doc__,
 "Character and line based layer over a BufferedIOBase object, buffer.\n"
 "\n"
 "encoding gives the name of the encoding that the stream will be\n"
-"decoded or encoded with. It defaults to locale.getpreferredencoding(False).\n"
+"decoded or encoded with. It defaults to locale.getencoding().\n"
 "\n"
 "errors determines the strictness of encoding and decoding (see\n"
 "help(codecs.Codec) or the documentation for codecs.register) and\n"
@@ -671,4 +671,4 @@ _io_TextIOWrapper_close(textio *self, PyObject *Py_UNUSED(ignored))
 {
     return _io_TextIOWrapper_close_impl(self);
 }
-/*[clinic end generated code: output=2604c8f3a45b9a03 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=e88abad34e31c0cb input=a9049054013a1b77]*/
diff --git a/Modules/_io/textio.c b/Modules/_io/textio.c
index 0e20741..6ba7393 100644
--- a/Modules/_io/textio.c
+++ b/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));
         }
diff --git a/Tools/c-analyzer/TODO b/Tools/c-analyzer/TODO
index 55338eb..6683df5 100644
--- a/Tools/c-analyzer/TODO
+++ b/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)