diff --git a/Lib/idlelib/idle_test/test_run.py b/Lib/idlelib/idle_test/test_run.py
index cad0b4d..9995dbe 100644
--- a/Lib/idlelib/idle_test/test_run.py
+++ b/Lib/idlelib/idle_test/test_run.py
@@ -36,7 +36,7 @@
         self.assertIn('UnhashableException: ex1', tb[10])
 
 
-# PseudoFile tests.
+# StdioFile tests.
 
 class S(str):
     def __str__(self):
@@ -68,14 +68,14 @@
         self.lines = list(lines)[::-1]
 
 
-class PseudeInputFilesTest(unittest.TestCase):
+class StdInputFilesTest(unittest.TestCase):
 
     def test_misc(self):
         shell = MockShell()
-        f = run.PseudoInputFile(shell, 'stdin', 'utf-8')
+        f = run.StdInputFile(shell, 'stdin')
         self.assertIsInstance(f, io.TextIOBase)
         self.assertEqual(f.encoding, 'utf-8')
-        self.assertIsNone(f.errors)
+        self.assertEqual(f.errors, 'strict')
         self.assertIsNone(f.newlines)
         self.assertEqual(f.name, '<stdin>')
         self.assertFalse(f.closed)
@@ -86,7 +86,7 @@
 
     def test_unsupported(self):
         shell = MockShell()
-        f = run.PseudoInputFile(shell, 'stdin', 'utf-8')
+        f = run.StdInputFile(shell, 'stdin')
         self.assertRaises(OSError, f.fileno)
         self.assertRaises(OSError, f.tell)
         self.assertRaises(OSError, f.seek, 0)
@@ -95,7 +95,7 @@
 
     def test_read(self):
         shell = MockShell()
-        f = run.PseudoInputFile(shell, 'stdin', 'utf-8')
+        f = run.StdInputFile(shell, 'stdin')
         shell.push(['one\n', 'two\n', ''])
         self.assertEqual(f.read(), 'one\ntwo\n')
         shell.push(['one\n', 'two\n', ''])
@@ -115,7 +115,7 @@
 
     def test_readline(self):
         shell = MockShell()
-        f = run.PseudoInputFile(shell, 'stdin', 'utf-8')
+        f = run.StdInputFile(shell, 'stdin')
         shell.push(['one\n', 'two\n', 'three\n', 'four\n'])
         self.assertEqual(f.readline(), 'one\n')
         self.assertEqual(f.readline(-1), 'two\n')
@@ -140,7 +140,7 @@
 
     def test_readlines(self):
         shell = MockShell()
-        f = run.PseudoInputFile(shell, 'stdin', 'utf-8')
+        f = run.StdInputFile(shell, 'stdin')
         shell.push(['one\n', 'two\n', ''])
         self.assertEqual(f.readlines(), ['one\n', 'two\n'])
         shell.push(['one\n', 'two\n', ''])
@@ -161,7 +161,7 @@
 
     def test_close(self):
         shell = MockShell()
-        f = run.PseudoInputFile(shell, 'stdin', 'utf-8')
+        f = run.StdInputFile(shell, 'stdin')
         shell.push(['one\n', 'two\n', ''])
         self.assertFalse(f.closed)
         self.assertEqual(f.readline(), 'one\n')
@@ -171,14 +171,14 @@
         self.assertRaises(TypeError, f.close, 1)
 
 
-class PseudeOutputFilesTest(unittest.TestCase):
+class StdOutputFilesTest(unittest.TestCase):
 
     def test_misc(self):
         shell = MockShell()
-        f = run.PseudoOutputFile(shell, 'stdout', 'utf-8')
+        f = run.StdOutputFile(shell, 'stdout')
         self.assertIsInstance(f, io.TextIOBase)
         self.assertEqual(f.encoding, 'utf-8')
-        self.assertIsNone(f.errors)
+        self.assertEqual(f.errors, 'strict')
         self.assertIsNone(f.newlines)
         self.assertEqual(f.name, '<stdout>')
         self.assertFalse(f.closed)
@@ -189,7 +189,7 @@
 
     def test_unsupported(self):
         shell = MockShell()
-        f = run.PseudoOutputFile(shell, 'stdout', 'utf-8')
+        f = run.StdOutputFile(shell, 'stdout')
         self.assertRaises(OSError, f.fileno)
         self.assertRaises(OSError, f.tell)
         self.assertRaises(OSError, f.seek, 0)
@@ -198,16 +198,36 @@
 
     def test_write(self):
         shell = MockShell()
-        f = run.PseudoOutputFile(shell, 'stdout', 'utf-8')
+        f = run.StdOutputFile(shell, 'stdout')
         f.write('test')
         self.assertEqual(shell.written, [('test', 'stdout')])
         shell.reset()
-        f.write('t\xe8st')
-        self.assertEqual(shell.written, [('t\xe8st', 'stdout')])
+        f.write('t\xe8\u015b\U0001d599')
+        self.assertEqual(shell.written, [('t\xe8\u015b\U0001d599', 'stdout')])
         shell.reset()
 
-        f.write(S('t\xe8st'))
-        self.assertEqual(shell.written, [('t\xe8st', 'stdout')])
+        f.write(S('t\xe8\u015b\U0001d599'))
+        self.assertEqual(shell.written, [('t\xe8\u015b\U0001d599', 'stdout')])
+        self.assertEqual(type(shell.written[0][0]), str)
+        shell.reset()
+
+        self.assertRaises(TypeError, f.write)
+        self.assertEqual(shell.written, [])
+        self.assertRaises(TypeError, f.write, b'test')
+        self.assertRaises(TypeError, f.write, 123)
+        self.assertEqual(shell.written, [])
+        self.assertRaises(TypeError, f.write, 'test', 'spam')
+        self.assertEqual(shell.written, [])
+
+    def test_write_stderr_nonencodable(self):
+        shell = MockShell()
+        f = run.StdOutputFile(shell, 'stderr', 'iso-8859-15', 'backslashreplace')
+        f.write('t\xe8\u015b\U0001d599\xa4')
+        self.assertEqual(shell.written, [('t\xe8\\u015b\\U0001d599\\xa4', 'stderr')])
+        shell.reset()
+
+        f.write(S('t\xe8\u015b\U0001d599\xa4'))
+        self.assertEqual(shell.written, [('t\xe8\\u015b\\U0001d599\\xa4', 'stderr')])
         self.assertEqual(type(shell.written[0][0]), str)
         shell.reset()
 
@@ -221,7 +241,7 @@
 
     def test_writelines(self):
         shell = MockShell()
-        f = run.PseudoOutputFile(shell, 'stdout', 'utf-8')
+        f = run.StdOutputFile(shell, 'stdout')
         f.writelines([])
         self.assertEqual(shell.written, [])
         shell.reset()
@@ -251,7 +271,7 @@
 
     def test_close(self):
         shell = MockShell()
-        f = run.PseudoOutputFile(shell, 'stdout', 'utf-8')
+        f = run.StdOutputFile(shell, 'stdout')
         self.assertFalse(f.closed)
         f.write('test')
         f.close()
diff --git a/Lib/idlelib/iomenu.py b/Lib/idlelib/iomenu.py
index b9e813b..b5533be 100644
--- a/Lib/idlelib/iomenu.py
+++ b/Lib/idlelib/iomenu.py
@@ -15,6 +15,7 @@
 
 if idlelib.testing:  # Set True by test.test_idle to avoid setlocale.
     encoding = 'utf-8'
+    errors = 'surrogateescape'
 else:
     # Try setting the locale, so that we can find out
     # what encoding to use
@@ -24,15 +25,9 @@
     except (ImportError, locale.Error):
         pass
 
-    locale_decode = 'ascii'
     if sys.platform == 'win32':
-        # On Windows, we could use "mbcs". However, to give the user
-        # a portable encoding name, we need to find the code page
-        try:
-            locale_encoding = locale.getdefaultlocale()[1]
-            codecs.lookup(locale_encoding)
-        except LookupError:
-            pass
+        encoding = 'utf-8'
+        errors = 'surrogateescape'
     else:
         try:
             # Different things can fail here: the locale module may not be
@@ -40,30 +35,30 @@
             # resulting codeset may be unknown to Python. We ignore all
             # these problems, falling back to ASCII
             locale_encoding = locale.nl_langinfo(locale.CODESET)
-            if locale_encoding is None or locale_encoding == '':
-                # situation occurs on macOS
-                locale_encoding = 'ascii'
-            codecs.lookup(locale_encoding)
+            if locale_encoding:
+                codecs.lookup(locale_encoding)
         except (NameError, AttributeError, LookupError):
             # Try getdefaultlocale: it parses environment variables,
             # which may give a clue. Unfortunately, getdefaultlocale has
             # bugs that can cause ValueError.
             try:
                 locale_encoding = locale.getdefaultlocale()[1]
-                if locale_encoding is None or locale_encoding == '':
-                    # situation occurs on macOS
-                    locale_encoding = 'ascii'
-                codecs.lookup(locale_encoding)
+                if locale_encoding:
+                    codecs.lookup(locale_encoding)
             except (ValueError, LookupError):
                 pass
 
-    locale_encoding = locale_encoding.lower()
-
-    encoding = locale_encoding
-    # Encoding is used in multiple files; locale_encoding nowhere.
-    # The only use of 'encoding' below is in _decode as initial value
-    # of deprecated block asking user for encoding.
-    # Perhaps use elsewhere should be reviewed.
+        if locale_encoding:
+            encoding = locale_encoding.lower()
+            errors = 'strict'
+        else:
+            # POSIX locale or macOS
+            encoding = 'ascii'
+            errors = 'surrogateescape'
+        # Encoding is used in multiple files; locale_encoding nowhere.
+        # The only use of 'encoding' below is in _decode as initial value
+        # of deprecated block asking user for encoding.
+        # Perhaps use elsewhere should be reviewed.
 
 coding_re = re.compile(r'^[ \t\f]*#.*?coding[:=][ \t]*([-\w.]+)', re.ASCII)
 blank_re = re.compile(r'^[ \t\f]*(?:[#\r\n]|$)', re.ASCII)
diff --git a/Lib/idlelib/pyshell.py b/Lib/idlelib/pyshell.py
index bc87d24..065122d 100755
--- a/Lib/idlelib/pyshell.py
+++ b/Lib/idlelib/pyshell.py
@@ -54,7 +54,7 @@
 from idlelib.filelist import FileList
 from idlelib.outwin import OutputWindow
 from idlelib import rpc
-from idlelib.run import idle_formatwarning, PseudoInputFile, PseudoOutputFile
+from idlelib.run import idle_formatwarning, StdInputFile, StdOutputFile
 from idlelib.undo import UndoDelegator
 
 HOST = '127.0.0.1' # python execution server on localhost loopback
@@ -902,10 +902,14 @@
         self.save_stderr = sys.stderr
         self.save_stdin = sys.stdin
         from idlelib import iomenu
-        self.stdin = PseudoInputFile(self, "stdin", iomenu.encoding)
-        self.stdout = PseudoOutputFile(self, "stdout", iomenu.encoding)
-        self.stderr = PseudoOutputFile(self, "stderr", iomenu.encoding)
-        self.console = PseudoOutputFile(self, "console", iomenu.encoding)
+        self.stdin = StdInputFile(self, "stdin",
+                                  iomenu.encoding, iomenu.errors)
+        self.stdout = StdOutputFile(self, "stdout",
+                                    iomenu.encoding, iomenu.errors)
+        self.stderr = StdOutputFile(self, "stderr",
+                                    iomenu.encoding, "backslashreplace")
+        self.console = StdOutputFile(self, "console",
+                                     iomenu.encoding, iomenu.errors)
         if not use_subprocess:
             sys.stdout = self.stdout
             sys.stderr = self.stderr
diff --git a/Lib/idlelib/run.py b/Lib/idlelib/run.py
index 41e0ded..5bd84aa 100644
--- a/Lib/idlelib/run.py
+++ b/Lib/idlelib/run.py
@@ -401,18 +401,23 @@
 
 # Pseudofiles for shell-remote communication (also used in pyshell)
 
-class PseudoFile(io.TextIOBase):
+class StdioFile(io.TextIOBase):
 
-    def __init__(self, shell, tags, encoding=None):
+    def __init__(self, shell, tags, encoding='utf-8', errors='strict'):
         self.shell = shell
         self.tags = tags
         self._encoding = encoding
+        self._errors = errors
 
     @property
     def encoding(self):
         return self._encoding
 
     @property
+    def errors(self):
+        return self._errors
+
+    @property
     def name(self):
         return '<%s>' % self.tags
 
@@ -420,7 +425,7 @@
         return True
 
 
-class PseudoOutputFile(PseudoFile):
+class StdOutputFile(StdioFile):
 
     def writable(self):
         return True
@@ -428,19 +433,12 @@
     def write(self, s):
         if self.closed:
             raise ValueError("write to closed file")
-        if type(s) is not str:
-            if not isinstance(s, str):
-                raise TypeError('must be str, not ' + type(s).__name__)
-            # See issue #19481
-            s = str.__str__(s)
+        s = str.encode(s, self.encoding, self.errors).decode(self.encoding, self.errors)
         return self.shell.write(s, self.tags)
 
 
-class PseudoInputFile(PseudoFile):
-
-    def __init__(self, shell, tags, encoding=None):
-        PseudoFile.__init__(self, shell, tags, encoding)
-        self._line_buffer = ''
+class StdInputFile(StdioFile):
+    _line_buffer = ''
 
     def readable(self):
         return True
@@ -495,12 +493,12 @@
         executive = Executive(self)
         self.register("exec", executive)
         self.console = self.get_remote_proxy("console")
-        sys.stdin = PseudoInputFile(self.console, "stdin",
-                iomenu.encoding)
-        sys.stdout = PseudoOutputFile(self.console, "stdout",
-                iomenu.encoding)
-        sys.stderr = PseudoOutputFile(self.console, "stderr",
-                iomenu.encoding)
+        sys.stdin = StdInputFile(self.console, "stdin",
+                                 iomenu.encoding, iomenu.errors)
+        sys.stdout = StdOutputFile(self.console, "stdout",
+                                   iomenu.encoding, iomenu.errors)
+        sys.stderr = StdOutputFile(self.console, "stderr",
+                                   iomenu.encoding, "backslashreplace")
 
         sys.displayhook = rpc.displayhook
         # page help() text to shell.