Close #11022: TextIOWrapper doesn't call locale.setlocale() anymore

open() and io.TextIOWrapper are now calling locale.getpreferredencoding(False)
instead of locale.getpreferredencoding() in text mode if the encoding is not
specified. Don't change temporary the locale encoding using locale.setlocale(),
use the current locale encoding instead of the user preferred encoding.

Explain also in open() documentation that locale.getpreferredencoding(False) is
called if the encoding is not specified.

Lib/test/test_io.py

@@ -19,20 +19,21 @@
 # test both implementations. This file has lots of examples.
 ################################################################################
 
+import abc
+import array
+import errno
+import locale
 import os
+import pickle
+import random
+import signal
 import sys
 import time
-import array
-import random
 import unittest
-import weakref
-import abc
-import signal
-import errno
 import warnings
-import pickle
-from itertools import cycle, count
+import weakref
 from collections import deque
+from itertools import cycle, count
 from test import support
 
 import codecs
@@ -1881,6 +1882,24 @@ class TextIOWrapperTest(unittest.TestCase):
         t.write("A\rB")
         self.assertEqual(r.getvalue(), b"XY\nZA\rB")
 
+    def test_default_encoding(self):
+        old_environ = dict(os.environ)
+        try:
+            # try to get a user preferred encoding different than the current
+            # locale encoding to check that TextIOWrapper() uses the current
+            # locale encoding and not the user preferred encoding
+            for key in ('LC_ALL', 'LANG', 'LC_CTYPE'):
+                if key in os.environ:
+                    del os.environ[key]
+
+            current_locale_encoding = locale.getpreferredencoding(False)
+            b = self.BytesIO()
+            t = self.TextIOWrapper(b)
+            self.assertEqual(t.encoding, current_locale_encoding)
+        finally:
+            os.environ.clear()
+            os.environ.update(old_environ)
+
     def test_encoding(self):
         # Check the encoding attribute is always set, and valid
         b = self.BytesIO()