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

3 files changed, 7 insertions, 4 deletions

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