Currently,TextIOWrapper.__init__ callsos.device_encoding(file.fileno()) when encoding is not specified and fileno is 0-2 (e.g.open(0)).

However,sys.stdin,stdout, andstderr don't use it even whenPYTHONLEGACYWINDOWSSTDIO=1.
config->stdio_encoding is initialized byGetACP(), andcreate_stdio() passesconfig->stdio_encoding toTextIOWrapper.

How about removingos.device_encoding(file.fileno()) fromTextIOWrapper.__init__()?

• Thanks to WindowsConsoleIO, most user don't use the console encoding at all.
• WindowsConsoleIO require UTF-8. SoTextIOWrapper(sys.stdout) cause mojibake when sys.stdout is WindowsConsoleIO
• The only use case ofPYTHONLEGACYWINDOWSSTDIO I know isthis. But this use case don't create TextIOWrapper for fd=0,1,2.
• This TextIOWrapper behavior is never documented. It makesTextIOWrapper.__init__ complicated and inconsistent.
• If we want to use console encoding for stdio when PYTHONLEGACYWINDOWSSTDIO is set, we can fix it increate_stdio().create_stdio() has special case for WindowsConsoleIO already. (here)

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