bpo-42236: os.device_encoding() respects UTF-8 Mode (GH-23119)

On Unix, the os.device_encoding() function now returns 'UTF-8' rather
than the device encoding if the Python UTF-8 Mode is enabled.

Lib/test/test_utf8_mode.py

@@ -3,11 +3,13 @@ Test the implementation of the PEP 540: the UTF-8 Mode.
 """
 
 import locale
+import subprocess
 import sys
 import textwrap
 import unittest
 from test import support
 from test.support.script_helper import assert_python_ok, assert_python_failure
+from test.support import os_helper
 
 
 MS_WINDOWS = (sys.platform == 'win32')
@@ -250,6 +252,31 @@ class UTF8ModeTests(unittest.TestCase):
         out = self.get_output('-X', 'utf8', '-E', '-c', code)
         self.assertEqual(out, '1')
 
+    @unittest.skipIf(MS_WINDOWS,
+                     "os.device_encoding() doesn't implement "
+                     "the UTF-8 Mode on Windows")
+    def test_device_encoding(self):
+        # Use stdout as TTY
+        if not sys.stdout.isatty():
+            self.skipTest("sys.stdout is not a TTY")
+
+        filename = 'out.txt'
+        self.addCleanup(os_helper.unlink, filename)
+
+        code = (f'import os, sys; fd = sys.stdout.fileno(); '
+                f'out = open({filename!r}, "w", encoding="utf-8"); '
+                f'print(os.isatty(fd), os.device_encoding(fd), file=out); '
+                f'out.close()')
+        cmd = [sys.executable, '-X', 'utf8', '-c', code]
+        # The stdout TTY is inherited to the child process
+        proc = subprocess.run(cmd, text=True)
+        self.assertEqual(proc.returncode, 0, proc)
+
+        # In UTF-8 Mode, device_encoding(fd) returns "UTF-8" if fd is a TTY
+        with open(filename, encoding="utf8") as fp:
+            out = fp.read().rstrip()
+        self.assertEqual(out, 'True UTF-8')
+
 
 if __name__ == "__main__":
     unittest.main()