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

1 files changed, 5 insertions, 3 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)