diff --git a/Lib/hmac.py b/Lib/hmac.py
index 8b4f920db95..8b4eb2fe741 100644
--- a/Lib/hmac.py
+++ b/Lib/hmac.py
@@ -53,7 +53,7 @@ def __init__(self, key, msg=None, digestmod=''):
             raise TypeError("key: expected bytes or bytearray, but got %r" % type(key).__name__)
 
         if not digestmod:
-            raise TypeError("Missing required parameter 'digestmod'.")
+            raise TypeError("Missing required argument 'digestmod'.")
 
         if _hashopenssl and isinstance(digestmod, (str, _functype)):
             try:
diff --git a/Lib/mailbox.py b/Lib/mailbox.py
index 70da07ed2e9..b00d9e8634c 100644
--- a/Lib/mailbox.py
+++ b/Lib/mailbox.py
@@ -395,6 +395,56 @@ def get_file(self, key):
         f = open(os.path.join(self._path, self._lookup(key)), 'rb')
         return _ProxyFile(f)
 
+    def get_info(self, key):
+        """Get the keyed message's "info" as a string."""
+        subpath = self._lookup(key)
+        if self.colon in subpath:
+            return subpath.split(self.colon)[-1]
+        return ''
+
+    def set_info(self, key, info: str):
+        """Set the keyed message's "info" string."""
+        if not isinstance(info, str):
+            raise TypeError(f'info must be a string: {type(info)}')
+        old_subpath = self._lookup(key)
+        new_subpath = old_subpath.split(self.colon)[0]
+        if info:
+            new_subpath += self.colon + info
+        if new_subpath == old_subpath:
+            return
+        old_path = os.path.join(self._path, old_subpath)
+        new_path = os.path.join(self._path, new_subpath)
+        os.rename(old_path, new_path)
+        self._toc[key] = new_subpath
+
+    def get_flags(self, key):
+        """Return as a string the standard flags that are set on the keyed message."""
+        info = self.get_info(key)
+        if info.startswith('2,'):
+            return info[2:]
+        return ''
+
+    def set_flags(self, key, flags: str):
+        """Set the given flags and unset all others on the keyed message."""
+        if not isinstance(flags, str):
+            raise TypeError(f'flags must be a string: {type(flags)}')
+        # TODO: check if flags are valid standard flag characters?
+        self.set_info(key, '2,' + ''.join(sorted(set(flags))))
+
+    def add_flag(self, key, flag: str):
+        """Set the given flag(s) without changing others on the keyed message."""
+        if not isinstance(flag, str):
+            raise TypeError(f'flag must be a string: {type(flag)}')
+        # TODO: check that flag is a valid standard flag character?
+        self.set_flags(key, ''.join(set(self.get_flags(key)) | set(flag)))
+
+    def remove_flag(self, key, flag: str):
+        """Unset the given string flag(s) without changing others on the keyed message."""
+        if not isinstance(flag, str):
+            raise TypeError(f'flag must be a string: {type(flag)}')
+        if self.get_flags(key):
+            self.set_flags(key, ''.join(set(self.get_flags(key)) - set(flag)))
+
     def iterkeys(self):
         """Return an iterator over keys."""
         self._refresh()
@@ -540,6 +590,8 @@ def _refresh(self):
         for subdir in self._toc_mtimes:
             path = self._paths[subdir]
             for entry in os.listdir(path):
+                if entry.startswith('.'):
+                    continue
                 p = os.path.join(path, entry)
                 if os.path.isdir(p):
                     continue
@@ -698,9 +750,13 @@ def flush(self):
         _sync_close(new_file)
         # self._file is about to get replaced, so no need to sync.
         self._file.close()
-        # Make sure the new file's mode is the same as the old file's
-        mode = os.stat(self._path).st_mode
-        os.chmod(new_file.name, mode)
+        # Make sure the new file's mode and owner are the same as the old file's
+        info = os.stat(self._path)
+        os.chmod(new_file.name, info.st_mode)
+        try:
+            os.chown(new_file.name, info.st_uid, info.st_gid)
+        except (AttributeError, OSError):
+            pass
         try:
             os.rename(new_file.name, self._path)
         except FileExistsError:
@@ -778,10 +834,11 @@ def get_message(self, key):
         """Return a Message representation or raise a KeyError."""
         start, stop = self._lookup(key)
         self._file.seek(start)
-        from_line = self._file.readline().replace(linesep, b'')
+        from_line = self._file.readline().replace(linesep, b'').decode('ascii')
         string = self._file.read(stop - self._file.tell())
         msg = self._message_factory(string.replace(linesep, b'\n'))
-        msg.set_from(from_line[5:].decode('ascii'))
+        msg.set_unixfrom(from_line)
+        msg.set_from(from_line[5:])
         return msg
 
     def get_string(self, key, from_=False):
@@ -1089,10 +1146,24 @@ def __len__(self):
         """Return a count of messages in the mailbox."""
         return len(list(self.iterkeys()))
 
+    def _open_mh_sequences_file(self, text):
+        mode = '' if text else 'b'
+        kwargs = {'encoding': 'ASCII'} if text else {}
+        path = os.path.join(self._path, '.mh_sequences')
+        while True:
+            try:
+                return open(path, 'r+' + mode, **kwargs)
+            except FileNotFoundError:
+                pass
+            try:
+                return open(path, 'x+' + mode, **kwargs)
+            except FileExistsError:
+                pass
+
     def lock(self):
         """Lock the mailbox."""
         if not self._locked:
-            self._file = open(os.path.join(self._path, '.mh_sequences'), 'rb+')
+            self._file = self._open_mh_sequences_file(text=False)
             _lock_file(self._file)
             self._locked = True
 
@@ -1146,7 +1217,11 @@ def remove_folder(self, folder):
     def get_sequences(self):
         """Return a name-to-key-list dictionary to define each sequence."""
         results = {}
-        with open(os.path.join(self._path, '.mh_sequences'), 'r', encoding='ASCII') as f:
+        try:
+            f = open(os.path.join(self._path, '.mh_sequences'), 'r', encoding='ASCII')
+        except FileNotFoundError:
+            return results
+        with f:
             all_keys = set(self.keys())
             for line in f:
                 try:
@@ -1169,7 +1244,7 @@ def get_sequences(self):
 
     def set_sequences(self, sequences):
         """Set sequences using the given name-to-key-list dictionary."""
-        f = open(os.path.join(self._path, '.mh_sequences'), 'r+', encoding='ASCII')
+        f = self._open_mh_sequences_file(text=True)
         try:
             os.close(os.open(f.name, os.O_WRONLY | os.O_TRUNC))
             for name, keys in sequences.items():
@@ -1956,10 +2031,7 @@ def readlines(self, sizehint=None):
 
     def __iter__(self):
         """Iterate over lines."""
-        while True:
-            line = self.readline()
-            if not line:
-                return
+        while line := self.readline():
             yield line
 
     def tell(self):
diff --git a/Lib/test/test_hmac.py b/Lib/test/test_hmac.py
index 74ebcb2fe70..1726975e864 100644
--- a/Lib/test/test_hmac.py
+++ b/Lib/test/test_hmac.py
@@ -505,6 +505,14 @@ def test_exercise_all_methods(self):
             self.fail("Exception raised during normal usage of HMAC class.")
 
 
+class UpdateTestCase(unittest.TestCase):
+    @hashlib_helper.requires_hashdigest('sha256')
+    def test_with_str_update(self):
+        with self.assertRaises(TypeError):
+            h = hmac.new(b"key", digestmod='sha256')
+            h.update("invalid update")
+
+
 class CopyTestCase(unittest.TestCase):
 
     @hashlib_helper.requires_hashdigest('sha256')
diff --git a/Lib/test/test_mailbox.py b/Lib/test/test_mailbox.py
new file mode 100644
index 00000000000..940baf39415
--- /dev/null
+++ b/Lib/test/test_mailbox.py
@@ -0,0 +1,2495 @@
+import os
+import sys
+import time
+import stat
+import socket
+import email
+import email.message
+import re
+import io
+import tempfile
+from test import support
+from test.support import import_helper
+from test.support import os_helper
+from test.support import refleak_helper
+from test.support import socket_helper
+from test.support.testcase import ExtraAssertions
+import unittest
+import textwrap
+import mailbox
+import glob
+
+
+if not socket_helper.has_gethostname:
+    raise unittest.SkipTest("test requires gethostname()")
+
+
+class TestBase:
+
+    all_mailbox_types = (mailbox.Message, mailbox.MaildirMessage,
+                         mailbox.mboxMessage, mailbox.MHMessage,
+                         mailbox.BabylMessage, mailbox.MMDFMessage)
+
+    def _check_sample(self, msg):
+        # Inspect a mailbox.Message representation of the sample message
+        self.assertIsInstance(msg, email.message.Message)
+        self.assertIsInstance(msg, mailbox.Message)
+        for key, value in _sample_headers:
+            self.assertIn(value, msg.get_all(key))
+        self.assertTrue(msg.is_multipart())
+        self.assertEqual(len(msg.get_payload()), len(_sample_payloads))
+        for i, payload in enumerate(_sample_payloads):
+            part = msg.get_payload(i)
+            self.assertIsInstance(part, email.message.Message)
+            self.assertNotIsInstance(part, mailbox.Message)
+            self.assertEqual(part.get_payload(), payload)
+
+    def _delete_recursively(self, target):
+        # Delete a file or delete a directory recursively
+        if os.path.isdir(target):
+            os_helper.rmtree(target)
+        elif os.path.exists(target):
+            os_helper.unlink(target)
+
+
+class TestMailbox(TestBase):
+
+    maxDiff = None
+
+    _factory = None     # Overridden by subclasses to reuse tests
+    _template = 'From: foo\n\n%s\n'
+
+    def setUp(self):
+        self._path = os_helper.TESTFN
+        self._delete_recursively(self._path)
+        self._box = self._factory(self._path)
+
+    def tearDown(self):
+        self._box.close()
+        self._delete_recursively(self._path)
+
+    def test_add(self):
+        # Add copies of a sample message
+        keys = []
+        keys.append(self._box.add(self._template % 0))
+        self.assertEqual(len(self._box), 1)
+        keys.append(self._box.add(mailbox.Message(_sample_message)))
+        self.assertEqual(len(self._box), 2)
+        keys.append(self._box.add(email.message_from_string(_sample_message)))
+        self.assertEqual(len(self._box), 3)
+        keys.append(self._box.add(io.BytesIO(_bytes_sample_message)))
+        self.assertEqual(len(self._box), 4)
+        keys.append(self._box.add(_sample_message))
+        self.assertEqual(len(self._box), 5)
+        keys.append(self._box.add(_bytes_sample_message))
+        self.assertEqual(len(self._box), 6)
+        with self.assertWarns(DeprecationWarning):
+            keys.append(self._box.add(
+                io.TextIOWrapper(io.BytesIO(_bytes_sample_message), encoding="utf-8")))
+        self.assertEqual(len(self._box), 7)
+        self.assertEqual(self._box.get_string(keys[0]), self._template % 0)
+        for i in (1, 2, 3, 4, 5, 6):
+            self._check_sample(self._box[keys[i]])
+
+    _nonascii_msg = textwrap.dedent("""\
+            From: foo
+            Subject: Falinaptár házhozszállítással. Már rendeltél?
+
+            0
+            """)
+
+    def test_add_invalid_8bit_bytes_header(self):
+        key = self._box.add(self._nonascii_msg.encode('latin-1'))
+        self.assertEqual(len(self._box), 1)
+        self.assertEqual(self._box.get_bytes(key),
+            self._nonascii_msg.encode('latin-1'))
+
+    def test_invalid_nonascii_header_as_string(self):
+        subj = self._nonascii_msg.splitlines()[1]
+        key = self._box.add(subj.encode('latin-1'))
+        self.assertEqual(self._box.get_string(key),
+            'Subject: =?unknown-8bit?b?RmFsaW5hcHThciBo4Xpob3pzeuFsbO104XNz'
+            'YWwuIE3hciByZW5kZWx06Ww/?=\n\n')
+
+    def test_add_nonascii_string_header_raises(self):
+        with self.assertRaisesRegex(ValueError, "ASCII-only"):
+            self._box.add(self._nonascii_msg)
+        self._box.flush()
+        self.assertEqual(len(self._box), 0)
+        self.assertMailboxEmpty()
+
+    def test_add_that_raises_leaves_mailbox_empty(self):
+        class CustomError(Exception): ...
+        exc_msg = "a fake error"
+
+        def raiser(*args, **kw):
+            raise CustomError(exc_msg)
+        support.patch(self, email.generator.BytesGenerator, 'flatten', raiser)
+        with self.assertRaisesRegex(CustomError, exc_msg):
+            self._box.add(email.message_from_string("From: Alphöso"))
+        self.assertEqual(len(self._box), 0)
+        self._box.close()
+        self.assertMailboxEmpty()
+
+    _non_latin_bin_msg = textwrap.dedent("""\
+        From: foo@bar.com
+        To: báz
+        Subject: Maintenant je vous présente mon collègue, le pouf célèbre
+        \tJean de Baddie
+        Mime-Version: 1.0
+        Content-Type: text/plain; charset="utf-8"
+        Content-Transfer-Encoding: 8bit
+
+        Да, они летят.
+        """).encode('utf-8')
+
+    def test_add_8bit_body(self):
+        key = self._box.add(self._non_latin_bin_msg)
+        self.assertEqual(self._box.get_bytes(key),
+                         self._non_latin_bin_msg)
+        with self._box.get_file(key) as f:
+            self.assertEqual(f.read(),
+                             self._non_latin_bin_msg.replace(b'\n',
+                                os.linesep.encode()))
+        self.assertEqual(self._box[key].get_payload(),
+                        "Да, они летят.\n")
+
+    def test_add_binary_file(self):
+        with tempfile.TemporaryFile('wb+') as f:
+            f.write(_bytes_sample_message)
+            f.seek(0)
+            key = self._box.add(f)
+        self.assertEqual(self._box.get_bytes(key).split(b'\n'),
+            _bytes_sample_message.split(b'\n'))
+
+    def test_add_binary_nonascii_file(self):
+        with tempfile.TemporaryFile('wb+') as f:
+            f.write(self._non_latin_bin_msg)
+            f.seek(0)
+            key = self._box.add(f)
+        self.assertEqual(self._box.get_bytes(key).split(b'\n'),
+            self._non_latin_bin_msg.split(b'\n'))
+
+    def test_add_text_file_warns(self):
+        with tempfile.TemporaryFile('w+', encoding='utf-8') as f:
+            f.write(_sample_message)
+            f.seek(0)
+            with self.assertWarns(DeprecationWarning):
+                key = self._box.add(f)
+        self.assertEqual(self._box.get_bytes(key).split(b'\n'),
+            _bytes_sample_message.split(b'\n'))
+
+    def test_add_StringIO_warns(self):
+        with self.assertWarns(DeprecationWarning):
+            key = self._box.add(io.StringIO(self._template % "0"))
+        self.assertEqual(self._box.get_string(key), self._template % "0")
+
+    def test_add_nonascii_StringIO_raises(self):
+        with self.assertWarns(DeprecationWarning):
+            with self.assertRaisesRegex(ValueError, "ASCII-only"):
+                self._box.add(io.StringIO(self._nonascii_msg))
+        self.assertEqual(len(self._box), 0)
+        self._box.close()
+        self.assertMailboxEmpty()
+
+    def test_remove(self):
+        # Remove messages using remove()
+        self._test_remove_or_delitem(self._box.remove)
+
+    def test_delitem(self):
+        # Remove messages using __delitem__()
+        self._test_remove_or_delitem(self._box.__delitem__)
+
+    def _test_remove_or_delitem(self, method):
+        # (Used by test_remove() and test_delitem().)
+        key0 = self._box.add(self._template % 0)
+        key1 = self._box.add(self._template % 1)
+        self.assertEqual(len(self._box), 2)
+        method(key0)
+        self.assertEqual(len(self._box), 1)
+        self.assertRaises(KeyError, lambda: self._box[key0])
+        self.assertRaises(KeyError, lambda: method(key0))
+        self.assertEqual(self._box.get_string(key1), self._template % 1)
+        key2 = self._box.add(self._template % 2)
+        self.assertEqual(len(self._box), 2)
+        method(key2)
+        self.assertEqual(len(self._box), 1)
+        self.assertRaises(KeyError, lambda: self._box[key2])
+        self.assertRaises(KeyError, lambda: method(key2))
+        self.assertEqual(self._box.get_string(key1), self._template % 1)
+        method(key1)
+        self.assertEqual(len(self._box), 0)
+        self.assertRaises(KeyError, lambda: self._box[key1])
+        self.assertRaises(KeyError, lambda: method(key1))
+
+    def test_discard(self, repetitions=10):
+        # Discard messages
+        key0 = self._box.add(self._template % 0)
+        key1 = self._box.add(self._template % 1)
+        self.assertEqual(len(self._box), 2)
+        self._box.discard(key0)
+        self.assertEqual(len(self._box), 1)
+        self.assertRaises(KeyError, lambda: self._box[key0])
+        self._box.discard(key0)
+        self.assertEqual(len(self._box), 1)
+        self.assertRaises(KeyError, lambda: self._box[key0])
+
+    def test_get(self):
+        # Retrieve messages using get()
+        key0 = self._box.add(self._template % 0)
+        msg = self._box.get(key0)
+        self.assertEqual(msg['from'], 'foo')
+        self.assertEqual(msg.get_payload(), '0\n')
+        self.assertIsNone(self._box.get('foo'))
+        self.assertIs(self._box.get('foo', False), False)
+        self._box.close()
+        self._box = self._factory(self._path)
+        key1 = self._box.add(self._template % 1)
+        msg = self._box.get(key1)
+        self.assertEqual(msg['from'], 'foo')
+        self.assertEqual(msg.get_payload(), '1\n')
+
+    def test_getitem(self):
+        # Retrieve message using __getitem__()
+        key0 = self._box.add(self._template % 0)
+        msg = self._box[key0]
+        self.assertEqual(msg['from'], 'foo')
+        self.assertEqual(msg.get_payload(), '0\n')
+        self.assertRaises(KeyError, lambda: self._box['foo'])
+        self._box.discard(key0)
+        self.assertRaises(KeyError, lambda: self._box[key0])
+
+    def test_get_message(self):
+        # Get Message representations of messages
+        key0 = self._box.add(self._template % 0)
+        key1 = self._box.add(_sample_message)
+        msg0 = self._box.get_message(key0)
+        self.assertIsInstance(msg0, mailbox.Message)
+        self.assertEqual(msg0['from'], 'foo')
+        self.assertEqual(msg0.get_payload(), '0\n')
+        self._check_sample(self._box.get_message(key1))
+
+    def test_get_bytes(self):
+        # Get bytes representations of messages
+        key0 = self._box.add(self._template % 0)
+        key1 = self._box.add(_sample_message)
+        self.assertEqual(self._box.get_bytes(key0),
+            (self._template % 0).encode('ascii'))
+        self.assertEqual(self._box.get_bytes(key1), _bytes_sample_message)
+
+    def test_get_string(self):
+        # Get string representations of messages
+        key0 = self._box.add(self._template % 0)
+        key1 = self._box.add(_sample_message)
+        self.assertEqual(self._box.get_string(key0), self._template % 0)
+        self.assertEqual(self._box.get_string(key1).split('\n'),
+                         _sample_message.split('\n'))
+
+    def test_get_file(self):
+        # Get file representations of messages
+        key0 = self._box.add(self._template % 0)
+        key1 = self._box.add(_sample_message)
+        with self._box.get_file(key0) as file:
+            data0 = file.read()
+        with self._box.get_file(key1) as file:
+            data1 = file.read()
+        self.assertEqual(data0.decode('ascii').replace(os.linesep, '\n'),
+                         self._template % 0)
+        self.assertEqual(data1.decode('ascii').replace(os.linesep, '\n'),
+                         _sample_message)
+
+    def test_get_file_can_be_closed_twice(self):
+        # Issue 11700
+        key = self._box.add(_sample_message)
+        f = self._box.get_file(key)
+        f.close()
+        f.close()
+
+    def test_iterkeys(self):
+        # Get keys using iterkeys()
+        self._check_iteration(self._box.iterkeys, do_keys=True, do_values=False)
+
+    def test_keys(self):
+        # Get keys using keys()
+        self._check_iteration(self._box.keys, do_keys=True, do_values=False)
+
+    def test_itervalues(self):
+        # Get values using itervalues()
+        self._check_iteration(self._box.itervalues, do_keys=False,
+                              do_values=True)
+
+    def test_iter(self):
+        # Get values using __iter__()
+        self._check_iteration(self._box.__iter__, do_keys=False,
+                              do_values=True)
+
+    def test_values(self):
+        # Get values using values()
+        self._check_iteration(self._box.values, do_keys=False, do_values=True)
+
+    def test_iteritems(self):
+        # Get keys and values using iteritems()
+        self._check_iteration(self._box.iteritems, do_keys=True,
+                              do_values=True)
+
+    def test_items(self):
+        # Get keys and values using items()
+        self._check_iteration(self._box.items, do_keys=True, do_values=True)
+
+    def _check_iteration(self, method, do_keys, do_values, repetitions=10):
+        for value in method():
+            self.fail("Not empty")
+        keys, values = [], []
+        for i in range(repetitions):
+            keys.append(self._box.add(self._template % i))
+            values.append(self._template % i)
+        if do_keys and not do_values:
+            returned_keys = list(method())
+        elif do_values and not do_keys:
+            returned_values = list(method())
+        else:
+            returned_keys, returned_values = [], []
+            for key, value in method():
+                returned_keys.append(key)
+                returned_values.append(value)
+        if do_keys:
+            self.assertEqual(len(keys), len(returned_keys))
+            self.assertEqual(set(keys), set(returned_keys))
+        if do_values:
+            count = 0
+            for value in returned_values:
+                self.assertEqual(value['from'], 'foo')
+                self.assertLess(int(value.get_payload()), repetitions)
+                count += 1
+            self.assertEqual(len(values), count)
+
+    def test_contains(self):
+        # Check existence of keys using __contains__()
+        self.assertNotIn('foo', self._box)
+        key0 = self._box.add(self._template % 0)
+        self.assertIn(key0, self._box)
+        self.assertNotIn('foo', self._box)
+        key1 = self._box.add(self._template % 1)
+        self.assertIn(key1, self._box)
+        self.assertIn(key0, self._box)
+        self.assertNotIn('foo', self._box)
+        self._box.remove(key0)
+        self.assertNotIn(key0, self._box)
+        self.assertIn(key1, self._box)
+        self.assertNotIn('foo', self._box)
+        self._box.remove(key1)
+        self.assertNotIn(key1, self._box)
+        self.assertNotIn(key0, self._box)
+        self.assertNotIn('foo', self._box)
+
+    def test_len(self, repetitions=10):
+        # Get message count
+        keys = []
+        for i in range(repetitions):
+            self.assertEqual(len(self._box), i)
+            keys.append(self._box.add(self._template % i))
+            self.assertEqual(len(self._box), i + 1)
+        for i in range(repetitions):
+            self.assertEqual(len(self._box), repetitions - i)
+            self._box.remove(keys[i])
+            self.assertEqual(len(self._box), repetitions - i - 1)
+
+    def test_set_item(self):
+        # Modify messages using __setitem__()
+        key0 = self._box.add(self._template % 'original 0')
+        self.assertEqual(self._box.get_string(key0),
+                         self._template % 'original 0')
+        key1 = self._box.add(self._template % 'original 1')
+        self.assertEqual(self._box.get_string(key1),
+                         self._template % 'original 1')
+        self._box[key0] = self._template % 'changed 0'
+        self.assertEqual(self._box.get_string(key0),
+                         self._template % 'changed 0')
+        self._box[key1] = self._template % 'changed 1'
+        self.assertEqual(self._box.get_string(key1),
+                         self._template % 'changed 1')
+        self._box[key0] = _sample_message
+        self._check_sample(self._box[key0])
+        self._box[key1] = self._box[key0]
+        self._check_sample(self._box[key1])
+        self._box[key0] = self._template % 'original 0'
+        self.assertEqual(self._box.get_string(key0),
+                     self._template % 'original 0')
+        self._check_sample(self._box[key1])
+        self.assertRaises(KeyError,
+                          lambda: self._box.__setitem__('foo', 'bar'))
+        self.assertRaises(KeyError, lambda: self._box['foo'])
+        self.assertEqual(len(self._box), 2)
+
+    def test_clear(self, iterations=10):
+        # Remove all messages using clear()
+        keys = []
+        for i in range(iterations):
+            self._box.add(self._template % i)
+        for i, key in enumerate(keys):
+            self.assertEqual(self._box.get_string(key), self._template % i)
+        self._box.clear()
+        self.assertEqual(len(self._box), 0)
+        for i, key in enumerate(keys):
+            self.assertRaises(KeyError, lambda: self._box.get_string(key))
+
+    def test_pop(self):
+        # Get and remove a message using pop()
+        key0 = self._box.add(self._template % 0)
+        self.assertIn(key0, self._box)
+        key1 = self._box.add(self._template % 1)
+        self.assertIn(key1, self._box)
+        self.assertEqual(self._box.pop(key0).get_payload(), '0\n')
+        self.assertNotIn(key0, self._box)
+        self.assertIn(key1, self._box)
+        key2 = self._box.add(self._template % 2)
+        self.assertIn(key2, self._box)
+        self.assertEqual(self._box.pop(key2).get_payload(), '2\n')
+        self.assertNotIn(key2, self._box)
+        self.assertIn(key1, self._box)
+        self.assertEqual(self._box.pop(key1).get_payload(), '1\n')
+        self.assertNotIn(key1, self._box)
+        self.assertEqual(len(self._box), 0)
+
+    def test_popitem(self, iterations=10):
+        # Get and remove an arbitrary (key, message) using popitem()
+        keys = []
+        for i in range(10):
+            keys.append(self._box.add(self._template % i))
+        seen = []
+        for i in range(10):
+            key, msg = self._box.popitem()
+            self.assertIn(key, keys)
+            self.assertNotIn(key, seen)
+            seen.append(key)
+            self.assertEqual(int(msg.get_payload()), keys.index(key))
+        self.assertEqual(len(self._box), 0)
+        for key in keys:
+            self.assertRaises(KeyError, lambda: self._box[key])
+
+    def test_update(self):
+        # Modify multiple messages using update()
+        key0 = self._box.add(self._template % 'original 0')
+        key1 = self._box.add(self._template % 'original 1')
+        key2 = self._box.add(self._template % 'original 2')
+        self._box.update({key0: self._template % 'changed 0',
+                          key2: _sample_message})
+        self.assertEqual(len(self._box), 3)
+        self.assertEqual(self._box.get_string(key0),
+                     self._template % 'changed 0')
+        self.assertEqual(self._box.get_string(key1),
+                     self._template % 'original 1')
+        self._check_sample(self._box[key2])
+        self._box.update([(key2, self._template % 'changed 2'),
+                    (key1, self._template % 'changed 1'),
+                    (key0, self._template % 'original 0')])
+        self.assertEqual(len(self._box), 3)
+        self.assertEqual(self._box.get_string(key0),
+                     self._template % 'original 0')
+        self.assertEqual(self._box.get_string(key1),
+                     self._template % 'changed 1')
+        self.assertEqual(self._box.get_string(key2),
+                     self._template % 'changed 2')
+        self.assertRaises(KeyError,
+                          lambda: self._box.update({'foo': 'bar',
+                                          key0: self._template % "changed 0"}))
+        self.assertEqual(len(self._box), 3)
+        self.assertEqual(self._box.get_string(key0),
+                     self._template % "changed 0")
+        self.assertEqual(self._box.get_string(key1),
+                     self._template % "changed 1")
+        self.assertEqual(self._box.get_string(key2),
+                     self._template % "changed 2")
+
+    def test_flush(self):
+        # Write changes to disk
+        self._test_flush_or_close(self._box.flush, True)
+
+    def test_popitem_and_flush_twice(self):
+        # See #15036.
+        self._box.add(self._template % 0)
+        self._box.add(self._template % 1)
+        self._box.flush()
+
+        self._box.popitem()
+        self._box.flush()
+        self._box.popitem()
+        self._box.flush()
+
+    def test_lock_unlock(self):
+        # Lock and unlock the mailbox
+        self.assertFalse(os.path.exists(self._get_lock_path()))
+        self._box.lock()
+        self.assertTrue(os.path.exists(self._get_lock_path()))
+        self._box.unlock()
+        self.assertFalse(os.path.exists(self._get_lock_path()))
+
+    def test_close(self):
+        # Close mailbox and flush changes to disk
+        self._test_flush_or_close(self._box.close, False)
+
+    def _test_flush_or_close(self, method, should_call_close):
+        contents = [self._template % i for i in range(3)]
+        self._box.add(contents[0])
+        self._box.add(contents[1])
+        self._box.add(contents[2])
+        oldbox = self._box
+        method()
+        if should_call_close:
+            self._box.close()
+        self._box = self._factory(self._path)
+        keys = self._box.keys()
+        self.assertEqual(len(keys), 3)
+        for key in keys:
+            self.assertIn(self._box.get_string(key), contents)
+        oldbox.close()
+
+    def test_dump_message(self):
+        # Write message representations to disk
+        for input in (email.message_from_string(_sample_message),
+                      _sample_message, io.BytesIO(_bytes_sample_message)):
+            output = io.BytesIO()
+            self._box._dump_message(input, output)
+            self.assertEqual(output.getvalue(),
+                _bytes_sample_message.replace(b'\n', os.linesep.encode()))
+        output = io.BytesIO()
+        self.assertRaises(TypeError,
+                          lambda: self._box._dump_message(None, output))
+
+    def _get_lock_path(self):
+        # Return the path of the dot lock file. May be overridden.
+        return self._path + '.lock'
+
+
+class TestMailboxSuperclass(TestBase, unittest.TestCase):
+
+    def test_notimplemented(self):
+        # Test that all Mailbox methods raise NotImplementedException.
+        box = mailbox.Mailbox('path')
+        self.assertRaises(NotImplementedError, lambda: box.add(''))
+        self.assertRaises(NotImplementedError, lambda: box.remove(''))
+        self.assertRaises(NotImplementedError, lambda: box.__delitem__(''))
+        self.assertRaises(NotImplementedError, lambda: box.discard(''))
+        self.assertRaises(NotImplementedError, lambda: box.__setitem__('', ''))
+        self.assertRaises(NotImplementedError, lambda: box.iterkeys())
+        self.assertRaises(NotImplementedError, lambda: box.keys())
+        self.assertRaises(NotImplementedError, lambda: box.itervalues().__next__())
+        self.assertRaises(NotImplementedError, lambda: box.__iter__().__next__())
+        self.assertRaises(NotImplementedError, lambda: box.values())
+        self.assertRaises(NotImplementedError, lambda: box.iteritems().__next__())
+        self.assertRaises(NotImplementedError, lambda: box.items())
+        self.assertRaises(NotImplementedError, lambda: box.get(''))
+        self.assertRaises(NotImplementedError, lambda: box.__getitem__(''))
+        self.assertRaises(NotImplementedError, lambda: box.get_message(''))
+        self.assertRaises(NotImplementedError, lambda: box.get_string(''))
+        self.assertRaises(NotImplementedError, lambda: box.get_bytes(''))
+        self.assertRaises(NotImplementedError, lambda: box.get_file(''))
+        self.assertRaises(NotImplementedError, lambda: '' in box)
+        self.assertRaises(NotImplementedError, lambda: box.__contains__(''))
+        self.assertRaises(NotImplementedError, lambda: box.__len__())
+        self.assertRaises(NotImplementedError, lambda: box.clear())
+        self.assertRaises(NotImplementedError, lambda: box.pop(''))
+        self.assertRaises(NotImplementedError, lambda: box.popitem())
+        self.assertRaises(NotImplementedError, lambda: box.update((('', ''),)))
+        self.assertRaises(NotImplementedError, lambda: box.flush())
+        self.assertRaises(NotImplementedError, lambda: box.lock())
+        self.assertRaises(NotImplementedError, lambda: box.unlock())
+        self.assertRaises(NotImplementedError, lambda: box.close())
+
+
+class TestMaildir(TestMailbox, unittest.TestCase):
+
+    _factory = lambda self, path, factory=None: mailbox.Maildir(path, factory)
+
+    def setUp(self):
+        TestMailbox.setUp(self)
+        if (os.name == 'nt') or (sys.platform == 'cygwin'):
+            self._box.colon = '!'
+
+    def assertMailboxEmpty(self):
+        self.assertEqual(os.listdir(os.path.join(self._path, 'tmp')), [])
+
+    def test_add_MM(self):
+        # Add a MaildirMessage instance
+        msg = mailbox.MaildirMessage(self._template % 0)
+        msg.set_subdir('cur')
+        msg.set_info('foo')
+        key = self._box.add(msg)
+        self.assertTrue(os.path.exists(os.path.join(self._path, 'cur', '%s%sfoo' %
+                                                 (key, self._box.colon))))
+
+    def test_get_MM(self):
+        # Get a MaildirMessage instance
+        msg = mailbox.MaildirMessage(self._template % 0)
+        msg.set_subdir('cur')
+        msg.set_flags('RF')
+        key = self._box.add(msg)
+        msg_returned = self._box.get_message(key)
+        self.assertIsInstance(msg_returned, mailbox.MaildirMessage)
+        self.assertEqual(msg_returned.get_subdir(), 'cur')
+        self.assertEqual(msg_returned.get_flags(), 'FR')
+
+    def test_set_MM(self):
+        # Set with a MaildirMessage instance
+        msg0 = mailbox.MaildirMessage(self._template % 0)
+        msg0.set_flags('TP')
+        key = self._box.add(msg0)
+        msg_returned = self._box.get_message(key)
+        self.assertEqual(msg_returned.get_subdir(), 'new')
+        self.assertEqual(msg_returned.get_flags(), 'PT')
+        msg1 = mailbox.MaildirMessage(self._template % 1)
+        self._box[key] = msg1
+        msg_returned = self._box.get_message(key)
+        self.assertEqual(msg_returned.get_subdir(), 'new')
+        self.assertEqual(msg_returned.get_flags(), '')
+        self.assertEqual(msg_returned.get_payload(), '1\n')
+        msg2 = mailbox.MaildirMessage(self._template % 2)
+        msg2.set_info('2,S')
+        self._box[key] = msg2
+        self._box[key] = self._template % 3
+        msg_returned = self._box.get_message(key)
+        self.assertEqual(msg_returned.get_subdir(), 'new')
+        self.assertEqual(msg_returned.get_flags(), 'S')
+        self.assertEqual(msg_returned.get_payload(), '3\n')
+
+    def test_consistent_factory(self):
+        # Add a message.
+        msg = mailbox.MaildirMessage(self._template % 0)
+        msg.set_subdir('cur')
+        msg.set_flags('RF')
+        key = self._box.add(msg)
+
+        # Create new mailbox with
+        class FakeMessage(mailbox.MaildirMessage):
+            pass
+        box = mailbox.Maildir(self._path, factory=FakeMessage)
+        box.colon = self._box.colon
+        msg2 = box.get_message(key)
+        self.assertIsInstance(msg2, FakeMessage)
+
+    def test_initialize_new(self):
+        # Initialize a non-existent mailbox
+        self.tearDown()
+        self._box = mailbox.Maildir(self._path)
+        self._check_basics()
+        self._delete_recursively(self._path)
+        self._box = self._factory(self._path, factory=None)
+        self._check_basics()
+
+    def test_initialize_existing(self):
+        # Initialize an existing mailbox
+        self.tearDown()
+        for subdir in '', 'tmp', 'new', 'cur':
+            os.mkdir(os.path.normpath(os.path.join(self._path, subdir)))
+        self._box = mailbox.Maildir(self._path)
+        self._check_basics()
+
+    def test_filename_leading_dot(self):
+        self.tearDown()
+        for subdir in '', 'tmp', 'new', 'cur':
+            os.mkdir(os.path.normpath(os.path.join(self._path, subdir)))
+        for subdir in 'tmp', 'new', 'cur':
+            fname = os.path.join(self._path, subdir, '.foo' + subdir)
+            with open(fname, 'wb') as f:
+                f.write(b"@")
+        self._box = mailbox.Maildir(self._path)
+        self.assertNotIn('.footmp', self._box)
+        self.assertNotIn('.foonew', self._box)
+        self.assertNotIn('.foocur', self._box)
+        self.assertEqual(list(self._box.iterkeys()), [])
+
+    def _check_basics(self, factory=None):
+        # (Used by test_open_new() and test_open_existing().)
+        self.assertEqual(self._box._path, os.path.abspath(self._path))
+        self.assertEqual(self._box._factory, factory)
+        for subdir in '', 'tmp', 'new', 'cur':
+            path = os.path.join(self._path, subdir)
+            self.assertTrue(os.path.isdir(path), f"Not a directory: {path!r}")
+
+    def test_list_folders(self):
+        # List folders
+        self._box.add_folder('one')
+        self._box.add_folder('two')
+        self._box.add_folder('three')
+        self.assertEqual(len(self._box.list_folders()), 3)
+        self.assertEqual(set(self._box.list_folders()),
+                     set(('one', 'two', 'three')))
+
+    def test_get_folder(self):
+        # Open folders
+        self._box.add_folder('foo.bar')
+        folder0 = self._box.get_folder('foo.bar')
+        folder0.add(self._template % 'bar')
+        self.assertTrue(os.path.isdir(os.path.join(self._path, '.foo.bar')))
+        folder1 = self._box.get_folder('foo.bar')
+        self.assertEqual(folder1.get_string(folder1.keys()[0]),
+                         self._template % 'bar')
+
+    def test_add_and_remove_folders(self):
+        # Delete folders
+        self._box.add_folder('one')
+        self._box.add_folder('two')
+        self.assertEqual(len(self._box.list_folders()), 2)
+        self.assertEqual(set(self._box.list_folders()), set(('one', 'two')))
+        self._box.remove_folder('one')
+        self.assertEqual(len(self._box.list_folders()), 1)
+        self.assertEqual(set(self._box.list_folders()), set(('two',)))
+        self._box.add_folder('three')
+        self.assertEqual(len(self._box.list_folders()), 2)
+        self.assertEqual(set(self._box.list_folders()), set(('two', 'three')))
+        self._box.remove_folder('three')
+        self.assertEqual(len(self._box.list_folders()), 1)
+        self.assertEqual(set(self._box.list_folders()), set(('two',)))
+        self._box.remove_folder('two')
+        self.assertEqual(len(self._box.list_folders()), 0)
+        self.assertEqual(self._box.list_folders(), [])
+
+    def test_clean(self):
+        # Remove old files from 'tmp'
+        foo_path = os.path.join(self._path, 'tmp', 'foo')
+        bar_path = os.path.join(self._path, 'tmp', 'bar')
+        with open(foo_path, 'w', encoding='utf-8') as f:
+            f.write("@")
+        with open(bar_path, 'w', encoding='utf-8') as f:
+            f.write("@")
+        self._box.clean()
+        self.assertTrue(os.path.exists(foo_path))
+        self.assertTrue(os.path.exists(bar_path))
+        foo_stat = os.stat(foo_path)
+        os.utime(foo_path, (time.time() - 129600 - 2,
+                            foo_stat.st_mtime))
+        self._box.clean()
+        self.assertFalse(os.path.exists(foo_path))
+        self.assertTrue(os.path.exists(bar_path))
+
+    def test_create_tmp(self, repetitions=10):
+        # Create files in tmp directory
+        hostname = socket.gethostname()
+        if '/' in hostname:
+            hostname = hostname.replace('/', r'\057')
+        if ':' in hostname:
+            hostname = hostname.replace(':', r'\072')
+        pid = os.getpid()
+        pattern = re.compile(r"(?P<time>\d+)\.M(?P<M>\d{1,6})P(?P<P>\d+)"
+                             r"Q(?P<Q>\d+)\.(?P<host>[^:/]*)")
+        previous_groups = None
+        for x in range(repetitions):
+            tmp_file = self._box._create_tmp()
+            head, tail = os.path.split(tmp_file.name)
+            self.assertEqual(head, os.path.abspath(os.path.join(self._path,
+                                                                "tmp")),
+                             "File in wrong location: '%s'" % head)
+            match = pattern.match(tail)
+            self.assertIsNotNone(match, "Invalid file name: '%s'" % tail)
+            groups = match.groups()
+            if previous_groups is not None:
+                self.assertGreaterEqual(int(groups[0]), int(previous_groups[0]),
+                             "Non-monotonic seconds: '%s' before '%s'" %
+                             (previous_groups[0], groups[0]))
+                if int(groups[0]) == int(previous_groups[0]):
+                    self.assertGreaterEqual(int(groups[1]), int(previous_groups[1]),
+                                "Non-monotonic milliseconds: '%s' before '%s'" %
+                                (previous_groups[1], groups[1]))
+                self.assertEqual(int(groups[2]), pid,
+                             "Process ID mismatch: '%s' should be '%s'" %
+                             (groups[2], pid))
+                self.assertEqual(int(groups[3]), int(previous_groups[3]) + 1,
+                             "Non-sequential counter: '%s' before '%s'" %
+                             (previous_groups[3], groups[3]))
+                self.assertEqual(groups[4], hostname,
+                             "Host name mismatch: '%s' should be '%s'" %
+                             (groups[4], hostname))
+            previous_groups = groups
+            tmp_file.write(_bytes_sample_message)
+            tmp_file.seek(0)
+            self.assertEqual(tmp_file.read(), _bytes_sample_message)
+            tmp_file.close()
+        file_count = len(os.listdir(os.path.join(self._path, "tmp")))
+        self.assertEqual(file_count, repetitions,
+                     "Wrong file count: '%s' should be '%s'" %
+                     (file_count, repetitions))
+
+    def test_refresh(self):
+        # Update the table of contents
+        self.assertEqual(self._box._toc, {})
+        key0 = self._box.add(self._template % 0)
+        key1 = self._box.add(self._template % 1)
+        self.assertEqual(self._box._toc, {})
+        self._box._refresh()
+        self.assertEqual(self._box._toc, {key0: os.path.join('new', key0),
+                                          key1: os.path.join('new', key1)})
+        key2 = self._box.add(self._template % 2)
+        self.assertEqual(self._box._toc, {key0: os.path.join('new', key0),
+                                          key1: os.path.join('new', key1)})
+        self._box._refresh()
+        self.assertEqual(self._box._toc, {key0: os.path.join('new', key0),
+                                          key1: os.path.join('new', key1),
+                                          key2: os.path.join('new', key2)})
+
+    def test_refresh_after_safety_period(self):
+        # Issue #13254: Call _refresh after the "file system safety
+        # period" of 2 seconds has passed; _toc should still be
+        # updated because this is the first call to _refresh.
+        key0 = self._box.add(self._template % 0)
+        key1 = self._box.add(self._template % 1)
+
+        self._box = self._factory(self._path)
+        self.assertEqual(self._box._toc, {})
+
+        # Emulate sleeping. Instead of sleeping for 2 seconds, use the
+        # skew factor to make _refresh think that the filesystem
+        # safety period has passed and re-reading the _toc is only
+        # required if mtimes differ.
+        self._box._skewfactor = -3
+
+        self._box._refresh()
+        self.assertEqual(sorted(self._box._toc.keys()), sorted([key0, key1]))
+
+    def test_lookup(self):
+        # Look up message subpaths in the TOC
+        self.assertRaises(KeyError, lambda: self._box._lookup('foo'))
+        key0 = self._box.add(self._template % 0)
+        self.assertEqual(self._box._lookup(key0), os.path.join('new', key0))
+        os.remove(os.path.join(self._path, 'new', key0))
+        self.assertEqual(self._box._toc, {key0: os.path.join('new', key0)})
+        # Be sure that the TOC is read back from disk (see issue #6896
+        # about bad mtime behaviour on some systems).
+        self._box.flush()
+        self.assertRaises(KeyError, lambda: self._box._lookup(key0))
+        self.assertEqual(self._box._toc, {})
+
+    def test_lock_unlock(self):
+        # Lock and unlock the mailbox. For Maildir, this does nothing.
+        self._box.lock()
+        self._box.unlock()
+
+    def test_get_info(self):
+        # Test getting message info from Maildir, not the message.
+        msg = mailbox.MaildirMessage(self._template % 0)
+        key = self._box.add(msg)
+        self.assertEqual(self._box.get_info(key), '')
+        msg.set_info('OurTestInfo')
+        self._box[key] = msg
+        self.assertEqual(self._box.get_info(key), 'OurTestInfo')
+
+    def test_set_info(self):
+        # Test setting message info from Maildir, not the message.
+        # This should immediately rename the message file.
+        msg = mailbox.MaildirMessage(self._template % 0)
+        key = self._box.add(msg)
+        def check_info(oldinfo, newinfo):
+            oldfilename = os.path.join(self._box._path, self._box._lookup(key))
+            newsubpath = self._box._lookup(key).split(self._box.colon)[0]
+            if newinfo:
+                newsubpath += self._box.colon + newinfo
+            newfilename = os.path.join(self._box._path, newsubpath)
+            # assert initial conditions
+            self.assertEqual(self._box.get_info(key), oldinfo)
+            if not oldinfo:
+                self.assertNotIn(self._box._lookup(key), self._box.colon)
+            self.assertTrue(os.path.exists(oldfilename))
+            if oldinfo != newinfo:
+                self.assertFalse(os.path.exists(newfilename))
+            # do the rename
+            self._box.set_info(key, newinfo)
+            # assert post conditions
+            if not newinfo:
+                self.assertNotIn(self._box._lookup(key), self._box.colon)
+            if oldinfo != newinfo:
+                self.assertFalse(os.path.exists(oldfilename))
+            self.assertTrue(os.path.exists(newfilename))
+            self.assertEqual(self._box.get_info(key), newinfo)
+        # none -> has info
+        check_info('', 'info1')
+        # has info -> same info
+        check_info('info1', 'info1')
+        # has info -> different info
+        check_info('info1', 'info2')
+        # has info -> none
+        check_info('info2', '')
+        # none -> none
+        check_info('', '')
+
+    def test_get_flags(self):
+        # Test getting message flags from Maildir, not the message.
+        msg = mailbox.MaildirMessage(self._template % 0)
+        key = self._box.add(msg)
+        self.assertEqual(self._box.get_flags(key), '')
+        msg.set_flags('T')
+        self._box[key] = msg
+        self.assertEqual(self._box.get_flags(key), 'T')
+
+    def test_set_flags(self):
+        msg = mailbox.MaildirMessage(self._template % 0)
+        key = self._box.add(msg)
+        self.assertEqual(self._box.get_flags(key), '')
+        self._box.set_flags(key, 'S')
+        self.assertEqual(self._box.get_flags(key), 'S')
+
+    def test_add_flag(self):
+        msg = mailbox.MaildirMessage(self._template % 0)
+        key = self._box.add(msg)
+        self.assertEqual(self._box.get_flags(key), '')
+        self._box.add_flag(key, 'B')
+        self.assertEqual(self._box.get_flags(key), 'B')
+        self._box.add_flag(key, 'B')
+        self.assertEqual(self._box.get_flags(key), 'B')
+        self._box.add_flag(key, 'AC')
+        self.assertEqual(self._box.get_flags(key), 'ABC')
+
+    def test_remove_flag(self):
+        msg = mailbox.MaildirMessage(self._template % 0)
+        key = self._box.add(msg)
+        self._box.set_flags(key, 'abc')
+        self.assertEqual(self._box.get_flags(key), 'abc')
+        self._box.remove_flag(key, 'b')
+        self.assertEqual(self._box.get_flags(key), 'ac')
+        self._box.remove_flag(key, 'b')
+        self.assertEqual(self._box.get_flags(key), 'ac')
+        self._box.remove_flag(key, 'ac')
+        self.assertEqual(self._box.get_flags(key), '')
+
+    def test_folder (self):
+        # Test for bug #1569790: verify that folders returned by .get_folder()
+        # use the same factory function.
+        def dummy_factory (s):
+            return None
+        box = self._factory(self._path, factory=dummy_factory)
+        folder = box.add_folder('folder1')
+        self.assertIs(folder._factory, dummy_factory)
+
+        folder1_alias = box.get_folder('folder1')
+        self.assertIs(folder1_alias._factory, dummy_factory)
+
+    def test_directory_in_folder (self):
+        # Test that mailboxes still work if there's a stray extra directory
+        # in a folder.
+        for i in range(10):
+            self._box.add(mailbox.Message(_sample_message))
+
+        # Create a stray directory
+        os.mkdir(os.path.join(self._path, 'cur', 'stray-dir'))
+
+        # Check that looping still works with the directory present.
+        for msg in self._box:
+            pass
+
+    @unittest.skipUnless(hasattr(os, 'umask'), 'test needs os.umask()')
+    def test_file_permissions(self):
+        # Verify that message files are created without execute permissions
+        msg = mailbox.MaildirMessage(self._template % 0)
+        orig_umask = os.umask(0)
+        try:
+            key = self._box.add(msg)
+        finally:
+            os.umask(orig_umask)
+        path = os.path.join(self._path, self._box._lookup(key))
+        mode = os.stat(path).st_mode
+        self.assertFalse(mode & 0o111)
+
+    @unittest.skipUnless(hasattr(os, 'umask'), 'test needs os.umask()')
+    def test_folder_file_perms(self):
+        # From bug #3228, we want to verify that the file created inside a Maildir
+        # subfolder isn't marked as executable.
+        orig_umask = os.umask(0)
+        try:
+            subfolder = self._box.add_folder('subfolder')
+        finally:
+            os.umask(orig_umask)
+
+        path = os.path.join(subfolder._path, 'maildirfolder')
+        st = os.stat(path)
+        perms = st.st_mode
+        self.assertFalse((perms & 0o111)) # Execute bits should all be off.
+
+    def test_reread(self):
+        # Do an initial unconditional refresh
+        self._box._refresh()
+
+        # Put the last modified times more than two seconds into the past
+        # (because mtime may have a two second granularity)
+        for subdir in ('cur', 'new'):
+            os.utime(os.path.join(self._box._path, subdir),
+                     (time.time()-5,)*2)
+
+        # Because mtime has a two second granularity in worst case (FAT), a
+        # refresh is done unconditionally if called for within
+        # two-second-plus-a-bit of the last one, just in case the mbox has
+        # changed; so now we have to wait for that interval to expire.
+        #
+        # Because this is a test, emulate sleeping. Instead of
+        # sleeping for 2 seconds, use the skew factor to make _refresh
+        # think that 2 seconds have passed and re-reading the _toc is
+        # only required if mtimes differ.
+        self._box._skewfactor = -3
+
+        # Re-reading causes the ._toc attribute to be assigned a new dictionary
+        # object, so we'll check that the ._toc attribute isn't a different
+        # object.
+        orig_toc = self._box._toc
+        def refreshed():
+            return self._box._toc is not orig_toc
+
+        self._box._refresh()
+        self.assertFalse(refreshed())
+
+        # Now, write something into cur and remove it.  This changes
+        # the mtime and should cause a re-read. Note that "sleep
+        # emulation" is still in effect, as skewfactor is -3.
+        filename = os.path.join(self._path, 'cur', 'stray-file')
+        os_helper.create_empty_file(filename)
+        os.unlink(filename)
+        self._box._refresh()
+        self.assertTrue(refreshed())
+
+
+class _TestSingleFile(TestMailbox):
+    '''Common tests for single-file mailboxes'''
+
+    def test_add_doesnt_rewrite(self):
+        # When only adding messages, flush() should not rewrite the
+        # mailbox file. See issue #9559.
+
+        # Inode number changes if the contents are written to another
+        # file which is then renamed over the original file. So we
+        # must check that the inode number doesn't change.
+        inode_before = os.stat(self._path).st_ino
+
+        self._box.add(self._template % 0)
+        self._box.flush()
+
+        inode_after = os.stat(self._path).st_ino
+        self.assertEqual(inode_before, inode_after)
+
+        # Make sure the message was really added
+        self._box.close()
+        self._box = self._factory(self._path)
+        self.assertEqual(len(self._box), 1)
+
+    def test_permissions_after_flush(self):
+        # See issue #5346
+
+        # Make the mailbox world writable. It's unlikely that the new
+        # mailbox file would have these permissions after flush(),
+        # because umask usually prevents it.
+        mode = os.stat(self._path).st_mode | 0o666
+        os.chmod(self._path, mode)
+
+        self._box.add(self._template % 0)
+        i = self._box.add(self._template % 1)
+        # Need to remove one message to make flush() create a new file
+        self._box.remove(i)
+        self._box.flush()
+
+        self.assertEqual(os.stat(self._path).st_mode, mode)
+
+    @unittest.skipUnless(hasattr(os, 'chown'), 'requires os.chown')
+    def test_ownership_after_flush(self):
+        # See issue gh-117467
+
+        pwd = import_helper.import_module('pwd')
+        grp = import_helper.import_module('grp')
+        st = os.stat(self._path)
+
+        for e in pwd.getpwall():
+            if e.pw_uid != st.st_uid:
+                other_uid = e.pw_uid
+                break
+        else:
+            self.skipTest("test needs more than one user")
+
+        for e in grp.getgrall():
+            if e.gr_gid != st.st_gid:
+                other_gid = e.gr_gid
+                break
+        else:
+            self.skipTest("test needs more than one group")
+
+        try:
+            os.chown(self._path, other_uid, other_gid)
+        except OSError:
+            self.skipTest('test needs root privilege')
+        # Change permissions as in test_permissions_after_flush.
+        mode = st.st_mode | 0o666
+        os.chmod(self._path, mode)
+
+        self._box.add(self._template % 0)
+        i = self._box.add(self._template % 1)
+        # Need to remove one message to make flush() create a new file
+        self._box.remove(i)
+        self._box.flush()
+
+        st = os.stat(self._path)
+        self.assertEqual(st.st_uid, other_uid)
+        self.assertEqual(st.st_gid, other_gid)
+        self.assertEqual(st.st_mode, mode)
+
+
+class _TestMboxMMDF(_TestSingleFile):
+
+    def tearDown(self):
+        super().tearDown()
+        self._box.close()
+        self._delete_recursively(self._path)
+        for lock_remnant in glob.glob(glob.escape(self._path) + '.*'):
+            os_helper.unlink(lock_remnant)
+
+    def assertMailboxEmpty(self):
+        with open(self._path, 'rb') as f:
+            self.assertEqual(f.readlines(), [])
+
+    def test_get_bytes_from(self):
+        # Get bytes representations of messages with _unixfrom.
+        unixfrom = 'From foo@bar blah\n'
+        key0 = self._box.add(unixfrom + self._template % 0)
+        key1 = self._box.add(unixfrom + _sample_message)
+        self.assertEqual(self._box.get_bytes(key0, from_=False),
+            (self._template % 0).encode('ascii'))
+        self.assertEqual(self._box.get_bytes(key1, from_=False),
+            _bytes_sample_message)
+        self.assertEqual(self._box.get_bytes(key0, from_=True),
+            (unixfrom + self._template % 0).encode('ascii'))
+        self.assertEqual(self._box.get_bytes(key1, from_=True),
+            unixfrom.encode('ascii') + _bytes_sample_message)
+
+    def test_get_string_from(self):
+        # Get string representations of messages with _unixfrom.
+        unixfrom = 'From foo@bar blah\n'
+        key0 = self._box.add(unixfrom + self._template % 0)
+        key1 = self._box.add(unixfrom + _sample_message)
+        self.assertEqual(self._box.get_string(key0, from_=False),
+                         self._template % 0)
+        self.assertEqual(self._box.get_string(key1, from_=False).split('\n'),
+                         _sample_message.split('\n'))
+        self.assertEqual(self._box.get_string(key0, from_=True),
+                         unixfrom + self._template % 0)
+        self.assertEqual(self._box.get_string(key1, from_=True).split('\n'),
+                         (unixfrom + _sample_message).split('\n'))
+
+    def test_add_from_string(self):
+        # Add a string starting with 'From ' to the mailbox
+        key = self._box.add('From foo@bar blah\nFrom: foo\n\n0\n')
+        self.assertEqual(self._box[key].get_from(), 'foo@bar blah')
+        self.assertEqual(self._box[key].get_unixfrom(), 'From foo@bar blah')
+        self.assertEqual(self._box[key].get_payload(), '0\n')
+
+    def test_add_from_bytes(self):
+        # Add a byte string starting with 'From ' to the mailbox
+        key = self._box.add(b'From foo@bar blah\nFrom: foo\n\n0\n')
+        self.assertEqual(self._box[key].get_from(), 'foo@bar blah')
+        self.assertEqual(self._box[key].get_unixfrom(), 'From foo@bar blah')
+        self.assertEqual(self._box[key].get_payload(), '0\n')
+
+    def test_add_mbox_or_mmdf_message(self):
+        # Add an mboxMessage or MMDFMessage
+        for class_ in (mailbox.mboxMessage, mailbox.MMDFMessage):
+            msg = class_('From foo@bar blah\nFrom: foo\n\n0\n')
+            key = self._box.add(msg)
+
+    def test_open_close_open(self):
+        # Open and inspect previously-created mailbox
+        values = [self._template % i for i in range(3)]
+        for value in values:
+            self._box.add(value)
+        self._box.close()
+        mtime = os.path.getmtime(self._path)
+        self._box = self._factory(self._path)
+        self.assertEqual(len(self._box), 3)
+        for key in self._box.iterkeys():
+            self.assertIn(self._box.get_string(key), values)
+        self._box.close()
+        self.assertEqual(mtime, os.path.getmtime(self._path))
+
+    def test_add_and_close(self):
+        # Verifying that closing a mailbox doesn't change added items
+        self._box.add(_sample_message)
+        for i in range(3):
+            self._box.add(self._template % i)
+        self._box.add(_sample_message)
+        self._box._file.flush()
+        self._box._file.seek(0)
+        contents = self._box._file.read()
+        self._box.close()
+        with open(self._path, 'rb') as f:
+            self.assertEqual(contents, f.read())
+        self._box = self._factory(self._path)
+
+    @support.requires_fork()
+    @unittest.skipUnless(hasattr(socket, 'socketpair'), "Test needs socketpair().")
+    def test_lock_conflict(self):
+        # Fork off a child process that will lock the mailbox temporarily,
+        # unlock it and exit.
+        c, p = socket.socketpair()
+        self.addCleanup(c.close)
+        self.addCleanup(p.close)
+
+        pid = os.fork()
+        if pid == 0:
+            # child
+            try:
+                # lock the mailbox, and signal the parent it can proceed
+                self._box.lock()
+                c.send(b'c')
+
+                # wait until the parent is done, and unlock the mailbox
+                c.recv(1)
+                self._box.unlock()
+            finally:
+                os._exit(0)
+
+        # In the parent, wait until the child signals it locked the mailbox.
+        p.recv(1)
+        try:
+            self.assertRaises(mailbox.ExternalClashError,
+                              self._box.lock)
+        finally:
+            # Signal the child it can now release the lock and exit.
+            p.send(b'p')
+            # Wait for child to exit.  Locking should now succeed.
+            support.wait_process(pid, exitcode=0)
+
+        self._box.lock()
+        self._box.unlock()
+
+    def test_relock(self):
+        # Test case for bug #1575506: the mailbox class was locking the
+        # wrong file object in its flush() method.
+        msg = "Subject: sub\n\nbody\n"
+        key1 = self._box.add(msg)
+        self._box.flush()
+        self._box.close()
+
+        self._box = self._factory(self._path)
+        self._box.lock()
+        key2 = self._box.add(msg)
+        self._box.flush()
+        self.assertTrue(self._box._locked)
+        self._box.close()
+
+
+class TestMbox(_TestMboxMMDF, unittest.TestCase, ExtraAssertions):
+
+    _factory = lambda self, path, factory=None: mailbox.mbox(path, factory)
+
+    @unittest.skipUnless(hasattr(os, 'umask'), 'test needs os.umask()')
+    def test_file_perms(self):
+        # From bug #3228, we want to verify that the mailbox file isn't executable,
+        # even if the umask is set to something that would leave executable bits set.
+        # We only run this test on platforms that support umask.
+        try:
+            old_umask = os.umask(0o077)
+            self._box.close()
+            os.unlink(self._path)
+            self._box = mailbox.mbox(self._path, create=True)
+            self._box.add('')
+            self._box.close()
+        finally:
+            os.umask(old_umask)
+
+        st = os.stat(self._path)
+        perms = st.st_mode
+        self.assertFalse((perms & 0o111)) # Execute bits should all be off.
+
+    def test_terminating_newline(self):
+        message = email.message.Message()
+        message['From'] = 'john@example.com'
+        message.set_payload('No newline at the end')
+        i = self._box.add(message)
+
+        # A newline should have been appended to the payload
+        message = self._box.get(i)
+        self.assertEqual(message.get_payload(), 'No newline at the end\n')
+
+    def test_message_separator(self):
+        # Check there's always a single blank line after each message
+        self._box.add('From: foo\n\n0')  # No newline at the end
+        with open(self._path, encoding='utf-8') as f:
+            data = f.read()
+            self.assertEndsWith(data, '0\n\n')
+
+        self._box.add('From: foo\n\n0\n')  # Newline at the end
+        with open(self._path, encoding='utf-8') as f:
+            data = f.read()
+            self.assertEndsWith(data, '0\n\n')
+
+
+class TestMMDF(_TestMboxMMDF, unittest.TestCase):
+
+    _factory = lambda self, path, factory=None: mailbox.MMDF(path, factory)
+
+
+class TestMH(TestMailbox, unittest.TestCase):
+
+    _factory = lambda self, path, factory=None: mailbox.MH(path, factory)
+
+    def assertMailboxEmpty(self):
+        self.assertEqual(os.listdir(self._path), ['.mh_sequences'])
+
+    def test_list_folders(self):
+        # List folders
+        self._box.add_folder('one')
+        self._box.add_folder('two')
+        self._box.add_folder('three')
+        self.assertEqual(len(self._box.list_folders()), 3)
+        self.assertEqual(set(self._box.list_folders()),
+                     set(('one', 'two', 'three')))
+
+    def test_get_folder(self):
+        # Open folders
+        def dummy_factory (s):
+            return None
+        self._box = self._factory(self._path, dummy_factory)
+
+        new_folder = self._box.add_folder('foo.bar')
+        folder0 = self._box.get_folder('foo.bar')
+        folder0.add(self._template % 'bar')
+        self.assertTrue(os.path.isdir(os.path.join(self._path, 'foo.bar')))
+        folder1 = self._box.get_folder('foo.bar')
+        self.assertEqual(folder1.get_string(folder1.keys()[0]),
+                         self._template % 'bar')
+
+        # Test for bug #1569790: verify that folders returned by .get_folder()
+        # use the same factory function.
+        self.assertIs(new_folder._factory, self._box._factory)
+        self.assertIs(folder0._factory, self._box._factory)
+
+    def test_add_and_remove_folders(self):
+        # Delete folders
+        self._box.add_folder('one')
+        self._box.add_folder('two')
+        self.assertEqual(len(self._box.list_folders()), 2)
+        self.assertEqual(set(self._box.list_folders()), set(('one', 'two')))
+        self._box.remove_folder('one')
+        self.assertEqual(len(self._box.list_folders()), 1)
+        self.assertEqual(set(self._box.list_folders()), set(('two',)))
+        self._box.add_folder('three')
+        self.assertEqual(len(self._box.list_folders()), 2)
+        self.assertEqual(set(self._box.list_folders()), set(('two', 'three')))
+        self._box.remove_folder('three')
+        self.assertEqual(len(self._box.list_folders()), 1)
+        self.assertEqual(set(self._box.list_folders()), set(('two',)))
+        self._box.remove_folder('two')
+        self.assertEqual(len(self._box.list_folders()), 0)
+        self.assertEqual(self._box.list_folders(), [])
+
+    def test_sequences(self):
+        # Get and set sequences
+        self.assertEqual(self._box.get_sequences(), {})
+        msg0 = mailbox.MHMessage(self._template % 0)
+        msg0.add_sequence('foo')
+        key0 = self._box.add(msg0)
+        self.assertEqual(self._box.get_sequences(), {'foo':[key0]})
+        msg1 = mailbox.MHMessage(self._template % 1)
+        msg1.set_sequences(['bar', 'replied', 'foo'])
+        key1 = self._box.add(msg1)
+        self.assertEqual(self._box.get_sequences(),
+                     {'foo':[key0, key1], 'bar':[key1], 'replied':[key1]})
+        msg0.set_sequences(['flagged'])
+        self._box[key0] = msg0
+        self.assertEqual(self._box.get_sequences(),
+                     {'foo':[key1], 'bar':[key1], 'replied':[key1],
+                      'flagged':[key0]})
+        self._box.remove(key1)
+        self.assertEqual(self._box.get_sequences(), {'flagged':[key0]})
+
+        self._box.set_sequences({'foo':[key0]})
+        self.assertEqual(self._box.get_sequences(), {'foo':[key0]})
+
+    def test_no_dot_mh_sequences_file(self):
+        path = os.path.join(self._path, 'foo.bar')
+        os.mkdir(path)
+        box = self._factory(path)
+        self.assertEqual(os.listdir(path), [])
+        self.assertEqual(box.get_sequences(), {})
+        self.assertEqual(os.listdir(path), [])
+        box.set_sequences({})
+        self.assertEqual(os.listdir(path), ['.mh_sequences'])
+
+    def test_lock_unlock_no_dot_mh_sequences_file(self):
+        path = os.path.join(self._path, 'foo.bar')
+        os.mkdir(path)
+        box = self._factory(path)
+        self.assertEqual(os.listdir(path), [])
+        box.lock()
+        box.unlock()
+        self.assertEqual(os.listdir(path), ['.mh_sequences'])
+
+    def test_issue2625(self):
+        msg0 = mailbox.MHMessage(self._template % 0)
+        msg0.add_sequence('foo')
+        key0 = self._box.add(msg0)
+        refmsg0 = self._box.get_message(key0)
+
+    def test_issue7627(self):
+        msg0 = mailbox.MHMessage(self._template % 0)
+        key0 = self._box.add(msg0)
+        self._box.lock()
+        self._box.remove(key0)
+        self._box.unlock()
+
+    def test_pack(self):
+        # Pack the contents of the mailbox
+        msg0 = mailbox.MHMessage(self._template % 0)
+        msg1 = mailbox.MHMessage(self._template % 1)
+        msg2 = mailbox.MHMessage(self._template % 2)
+        msg3 = mailbox.MHMessage(self._template % 3)
+        msg0.set_sequences(['foo', 'unseen'])
+        msg1.set_sequences(['foo'])
+        msg2.set_sequences(['foo', 'flagged'])
+        msg3.set_sequences(['foo', 'bar', 'replied'])
+        key0 = self._box.add(msg0)
+        key1 = self._box.add(msg1)
+        key2 = self._box.add(msg2)
+        key3 = self._box.add(msg3)
+        self.assertEqual(self._box.get_sequences(),
+                     {'foo':[key0,key1,key2,key3], 'unseen':[key0],
+                      'flagged':[key2], 'bar':[key3], 'replied':[key3]})
+        self._box.remove(key2)
+        self.assertEqual(self._box.get_sequences(),
+                     {'foo':[key0,key1,key3], 'unseen':[key0], 'bar':[key3],
+                      'replied':[key3]})
+        self._box.pack()
+        self.assertEqual(self._box.keys(), [1, 2, 3])
+        key0 = key0
+        key1 = key0 + 1
+        key2 = key1 + 1
+        self.assertEqual(self._box.get_sequences(),
+                     {'foo':[1, 2, 3], 'unseen':[1], 'bar':[3], 'replied':[3]})
+
+        # Test case for packing while holding the mailbox locked.
+        key0 = self._box.add(msg1)
+        key1 = self._box.add(msg1)
+        key2 = self._box.add(msg1)
+        key3 = self._box.add(msg1)
+
+        self._box.remove(key0)
+        self._box.remove(key2)
+        self._box.lock()
+        self._box.pack()
+        self._box.unlock()
+        self.assertEqual(self._box.get_sequences(),
+                     {'foo':[1, 2, 3, 4, 5],
+                      'unseen':[1], 'bar':[3], 'replied':[3]})
+
+    def _get_lock_path(self):
+        return os.path.join(self._path, '.mh_sequences.lock')
+
+
+class TestBabyl(_TestSingleFile, unittest.TestCase):
+
+    _factory = lambda self, path, factory=None: mailbox.Babyl(path, factory)
+
+    def assertMailboxEmpty(self):
+        with open(self._path, 'rb') as f:
+            self.assertEqual(f.readlines(), [])
+
+    def tearDown(self):
+        super().tearDown()
+        self._box.close()
+        self._delete_recursively(self._path)
+        for lock_remnant in glob.glob(glob.escape(self._path) + '.*'):
+            os_helper.unlink(lock_remnant)
+
+    def test_labels(self):
+        # Get labels from the mailbox
+        self.assertEqual(self._box.get_labels(), [])
+        msg0 = mailbox.BabylMessage(self._template % 0)
+        msg0.add_label('foo')
+        key0 = self._box.add(msg0)
+        self.assertEqual(self._box.get_labels(), ['foo'])
+        msg1 = mailbox.BabylMessage(self._template % 1)
+        msg1.set_labels(['bar', 'answered', 'foo'])
+        key1 = self._box.add(msg1)
+        self.assertEqual(set(self._box.get_labels()), set(['foo', 'bar']))
+        msg0.set_labels(['blah', 'filed'])
+        self._box[key0] = msg0
+        self.assertEqual(set(self._box.get_labels()),
+                     set(['foo', 'bar', 'blah']))
+        self._box.remove(key1)
+        self.assertEqual(set(self._box.get_labels()), set(['blah']))
+
+
+class FakeFileLikeObject:
+
+    def __init__(self):
+        self.closed = False
+
+    def close(self):
+        self.closed = True
+
+
+class FakeMailBox(mailbox.Mailbox):
+
+    def __init__(self):
+        mailbox.Mailbox.__init__(self, '', lambda file: None)
+        self.files = [FakeFileLikeObject() for i in range(10)]
+
+    def get_file(self, key):
+        return self.files[key]
+
+
+class TestFakeMailBox(unittest.TestCase):
+
+    def test_closing_fd(self):
+        box = FakeMailBox()
+        for i in range(10):
+            self.assertFalse(box.files[i].closed)
+        for i in range(10):
+            box[i]
+        for i in range(10):
+            self.assertTrue(box.files[i].closed)
+
+
+class TestMessage(TestBase, unittest.TestCase):
+
+    _factory = mailbox.Message      # Overridden by subclasses to reuse tests
+
+    def setUp(self):
+        self._path = os_helper.TESTFN
+
+    def tearDown(self):
+        self._delete_recursively(self._path)
+
+    def test_initialize_with_eMM(self):
+        # Initialize based on email.message.Message instance
+        eMM = email.message_from_string(_sample_message)
+        msg = self._factory(eMM)
+        self._post_initialize_hook(msg)
+        self._check_sample(msg)
+
+    def test_initialize_with_string(self):
+        # Initialize based on string
+        msg = self._factory(_sample_message)
+        self._post_initialize_hook(msg)
+        self._check_sample(msg)
+
+    def test_initialize_with_file(self):
+        # Initialize based on contents of file
+        with open(self._path, 'w+', encoding='utf-8') as f:
+            f.write(_sample_message)
+            f.seek(0)
+            msg = self._factory(f)
+            self._post_initialize_hook(msg)
+            self._check_sample(msg)
+
+    def test_initialize_with_binary_file(self):
+        # Initialize based on contents of binary file
+        with open(self._path, 'wb+') as f:
+            f.write(_bytes_sample_message)
+            f.seek(0)
+            msg = self._factory(f)
+            self._post_initialize_hook(msg)
+            self._check_sample(msg)
+
+    def test_initialize_with_nothing(self):
+        # Initialize without arguments
+        msg = self._factory()
+        self._post_initialize_hook(msg)
+        self.assertIsInstance(msg, email.message.Message)
+        self.assertIsInstance(msg, mailbox.Message)
+        self.assertIsInstance(msg, self._factory)
+        self.assertEqual(msg.keys(), [])
+        self.assertFalse(msg.is_multipart())
+        self.assertIsNone(msg.get_payload())
+
+    def test_initialize_incorrectly(self):
+        # Initialize with invalid argument
+        self.assertRaises(TypeError, lambda: self._factory(object()))
+
+    def test_all_eMM_attributes_exist(self):
+        # Issue 12537
+        eMM = email.message_from_string(_sample_message)
+        msg = self._factory(_sample_message)
+        for attr in eMM.__dict__:
+            self.assertIn(attr, msg.__dict__,
+                '{} attribute does not exist'.format(attr))
+
+    def test_become_message(self):
+        # Take on the state of another message
+        eMM = email.message_from_string(_sample_message)
+        msg = self._factory()
+        msg._become_message(eMM)
+        self._check_sample(msg)
+
+    def test_explain_to(self):
+        # Copy self's format-specific data to other message formats.
+        # This test is superficial; better ones are in TestMessageConversion.
+        msg = self._factory()
+        for class_ in self.all_mailbox_types:
+            other_msg = class_()
+            msg._explain_to(other_msg)
+        other_msg = email.message.Message()
+        self.assertRaises(TypeError, lambda: msg._explain_to(other_msg))
+
+    def _post_initialize_hook(self, msg):
+        # Overridden by subclasses to check extra things after initialization
+        pass
+
+
+class TestMaildirMessage(TestMessage, unittest.TestCase):
+
+    _factory = mailbox.MaildirMessage
+
+    def _post_initialize_hook(self, msg):
+        self.assertEqual(msg._subdir, 'new')
+        self.assertEqual(msg._info, '')
+
+    def test_subdir(self):
+        # Use get_subdir() and set_subdir()
+        msg = mailbox.MaildirMessage(_sample_message)
+        self.assertEqual(msg.get_subdir(), 'new')
+        msg.set_subdir('cur')
+        self.assertEqual(msg.get_subdir(), 'cur')
+        msg.set_subdir('new')
+        self.assertEqual(msg.get_subdir(), 'new')
+        self.assertRaises(ValueError, lambda: msg.set_subdir('tmp'))
+        self.assertEqual(msg.get_subdir(), 'new')
+        msg.set_subdir('new')
+        self.assertEqual(msg.get_subdir(), 'new')
+        self._check_sample(msg)
+
+    def test_flags(self):
+        # Use get_flags(), set_flags(), add_flag(), remove_flag()
+        msg = mailbox.MaildirMessage(_sample_message)
+        self.assertEqual(msg.get_flags(), '')
+        self.assertEqual(msg.get_subdir(), 'new')
+        msg.set_flags('F')
+        self.assertEqual(msg.get_subdir(), 'new')
+        self.assertEqual(msg.get_flags(), 'F')
+        msg.set_flags('SDTP')
+        self.assertEqual(msg.get_flags(), 'DPST')
+        msg.add_flag('FT')
+        self.assertEqual(msg.get_flags(), 'DFPST')
+        msg.remove_flag('TDRP')
+        self.assertEqual(msg.get_flags(), 'FS')
+        self.assertEqual(msg.get_subdir(), 'new')
+        self._check_sample(msg)
+
+    def test_date(self):
+        # Use get_date() and set_date()
+        msg = mailbox.MaildirMessage(_sample_message)
+        self.assertLess(abs(msg.get_date() - time.time()), 60)
+        msg.set_date(0.0)
+        self.assertEqual(msg.get_date(), 0.0)
+
+    def test_info(self):
+        # Use get_info() and set_info()
+        msg = mailbox.MaildirMessage(_sample_message)
+        self.assertEqual(msg.get_info(), '')
+        msg.set_info('1,foo=bar')
+        self.assertEqual(msg.get_info(), '1,foo=bar')
+        self.assertRaises(TypeError, lambda: msg.set_info(None))
+        self._check_sample(msg)
+
+    def test_info_and_flags(self):
+        # Test interaction of info and flag methods
+        msg = mailbox.MaildirMessage(_sample_message)
+        self.assertEqual(msg.get_info(), '')
+        msg.set_flags('SF')
+        self.assertEqual(msg.get_flags(), 'FS')
+        self.assertEqual(msg.get_info(), '2,FS')
+        msg.set_info('1,')
+        self.assertEqual(msg.get_flags(), '')
+        self.assertEqual(msg.get_info(), '1,')
+        msg.remove_flag('RPT')
+        self.assertEqual(msg.get_flags(), '')
+        self.assertEqual(msg.get_info(), '1,')
+        msg.add_flag('D')
+        self.assertEqual(msg.get_flags(), 'D')
+        self.assertEqual(msg.get_info(), '2,D')
+        self._check_sample(msg)
+
+
+class _TestMboxMMDFMessage:
+
+    _factory = mailbox._mboxMMDFMessage
+
+    def _post_initialize_hook(self, msg):
+        self._check_from(msg)
+
+    def test_initialize_with_unixfrom(self):
+        # Initialize with a message that already has a _unixfrom attribute
+        msg = mailbox.Message(_sample_message)
+        msg.set_unixfrom('From foo@bar blah')
+        msg = mailbox.mboxMessage(msg)
+        self.assertEqual(msg.get_from(), 'foo@bar blah')
+        self.assertEqual(msg.get_unixfrom(), 'From foo@bar blah')
+
+    def test_from(self):
+        # Get and set "From " line
+        msg = mailbox.mboxMessage(_sample_message)
+        self._check_from(msg)
+        self.assertIsNone(msg.get_unixfrom())
+        msg.set_from('foo bar')
+        self.assertEqual(msg.get_from(), 'foo bar')
+        self.assertIsNone(msg.get_unixfrom())
+        msg.set_from('foo@bar', True)
+        self._check_from(msg, 'foo@bar')
+        self.assertIsNone(msg.get_unixfrom())
+        msg.set_from('blah@temp', time.localtime())
+        self._check_from(msg, 'blah@temp')
+        self.assertIsNone(msg.get_unixfrom())
+
+    def test_flags(self):
+        # Use get_flags(), set_flags(), add_flag(), remove_flag()
+        msg = mailbox.mboxMessage(_sample_message)
+        self.assertEqual(msg.get_flags(), '')
+        msg.set_flags('F')
+        self.assertEqual(msg.get_flags(), 'F')
+        msg.set_flags('XODR')
+        self.assertEqual(msg.get_flags(), 'RODX')
+        msg.add_flag('FA')
+        self.assertEqual(msg.get_flags(), 'RODFAX')
+        msg.remove_flag('FDXA')
+        self.assertEqual(msg.get_flags(), 'RO')
+        self._check_sample(msg)
+
+    def _check_from(self, msg, sender=None):
+        # Check contents of "From " line
+        if sender is None:
+            sender = "MAILER-DAEMON"
+        self.assertIsNotNone(re.match(
+                sender + r" \w{3} \w{3} [\d ]\d [\d ]\d:\d{2}:\d{2} \d{4}",
+                msg.get_from()))
+
+
+class TestMboxMessage(_TestMboxMMDFMessage, TestMessage):
+
+    _factory = mailbox.mboxMessage
+
+
+class TestMHMessage(TestMessage, unittest.TestCase):
+
+    _factory = mailbox.MHMessage
+
+    def _post_initialize_hook(self, msg):
+        self.assertEqual(msg._sequences, [])
+
+    def test_sequences(self):
+        # Get, set, join, and leave sequences
+        msg = mailbox.MHMessage(_sample_message)
+        self.assertEqual(msg.get_sequences(), [])
+        msg.set_sequences(['foobar'])
+        self.assertEqual(msg.get_sequences(), ['foobar'])
+        msg.set_sequences([])
+        self.assertEqual(msg.get_sequences(), [])
+        msg.add_sequence('unseen')
+        self.assertEqual(msg.get_sequences(), ['unseen'])
+        msg.add_sequence('flagged')
+        self.assertEqual(msg.get_sequences(), ['unseen', 'flagged'])
+        msg.add_sequence('flagged')
+        self.assertEqual(msg.get_sequences(), ['unseen', 'flagged'])
+        msg.remove_sequence('unseen')
+        self.assertEqual(msg.get_sequences(), ['flagged'])
+        msg.add_sequence('foobar')
+        self.assertEqual(msg.get_sequences(), ['flagged', 'foobar'])
+        msg.remove_sequence('replied')
+        self.assertEqual(msg.get_sequences(), ['flagged', 'foobar'])
+        msg.set_sequences(['foobar', 'replied'])
+        self.assertEqual(msg.get_sequences(), ['foobar', 'replied'])
+
+
+class TestBabylMessage(TestMessage, unittest.TestCase):
+
+    _factory = mailbox.BabylMessage
+
+    def _post_initialize_hook(self, msg):
+        self.assertEqual(msg._labels, [])
+
+    def test_labels(self):
+        # Get, set, join, and leave labels
+        msg = mailbox.BabylMessage(_sample_message)
+        self.assertEqual(msg.get_labels(), [])
+        msg.set_labels(['foobar'])
+        self.assertEqual(msg.get_labels(), ['foobar'])
+        msg.set_labels([])
+        self.assertEqual(msg.get_labels(), [])
+        msg.add_label('filed')
+        self.assertEqual(msg.get_labels(), ['filed'])
+        msg.add_label('resent')
+        self.assertEqual(msg.get_labels(), ['filed', 'resent'])
+        msg.add_label('resent')
+        self.assertEqual(msg.get_labels(), ['filed', 'resent'])
+        msg.remove_label('filed')
+        self.assertEqual(msg.get_labels(), ['resent'])
+        msg.add_label('foobar')
+        self.assertEqual(msg.get_labels(), ['resent', 'foobar'])
+        msg.remove_label('unseen')
+        self.assertEqual(msg.get_labels(), ['resent', 'foobar'])
+        msg.set_labels(['foobar', 'answered'])
+        self.assertEqual(msg.get_labels(), ['foobar', 'answered'])
+
+    def test_visible(self):
+        # Get, set, and update visible headers
+        msg = mailbox.BabylMessage(_sample_message)
+        visible = msg.get_visible()
+        self.assertEqual(visible.keys(), [])
+        self.assertIsNone(visible.get_payload())
+        visible['User-Agent'] = 'FooBar 1.0'
+        visible['X-Whatever'] = 'Blah'
+        self.assertEqual(msg.get_visible().keys(), [])
+        msg.set_visible(visible)
+        visible = msg.get_visible()
+        self.assertEqual(visible.keys(), ['User-Agent', 'X-Whatever'])
+        self.assertEqual(visible['User-Agent'], 'FooBar 1.0')
+        self.assertEqual(visible['X-Whatever'], 'Blah')
+        self.assertIsNone(visible.get_payload())
+        msg.update_visible()
+        self.assertEqual(visible.keys(), ['User-Agent', 'X-Whatever'])
+        self.assertIsNone(visible.get_payload())
+        visible = msg.get_visible()
+        self.assertEqual(visible.keys(), ['User-Agent', 'Date', 'From', 'To',
+                                          'Subject'])
+        for header in ('User-Agent', 'Date', 'From', 'To', 'Subject'):
+            self.assertEqual(visible[header], msg[header])
+
+
+class TestMMDFMessage(_TestMboxMMDFMessage, TestMessage):
+
+    _factory = mailbox.MMDFMessage
+
+
+class TestMessageConversion(TestBase, unittest.TestCase):
+
+    def test_plain_to_x(self):
+        # Convert Message to all formats
+        for class_ in self.all_mailbox_types:
+            msg_plain = mailbox.Message(_sample_message)
+            msg = class_(msg_plain)
+            self._check_sample(msg)
+
+    def test_x_to_plain(self):
+        # Convert all formats to Message
+        for class_ in self.all_mailbox_types:
+            msg = class_(_sample_message)
+            msg_plain = mailbox.Message(msg)
+            self._check_sample(msg_plain)
+
+    def test_x_from_bytes(self):
+        # Convert all formats to Message
+        for class_ in self.all_mailbox_types:
+            msg = class_(_bytes_sample_message)
+            self._check_sample(msg)
+
+    def test_x_to_invalid(self):
+        # Convert all formats to an invalid format
+        for class_ in self.all_mailbox_types:
+            self.assertRaises(TypeError, lambda: class_(False))
+
+    def test_type_specific_attributes_removed_on_conversion(self):
+        reference = {class_: class_(_sample_message).__dict__
+                        for class_ in self.all_mailbox_types}
+        for class1 in self.all_mailbox_types:
+            for class2 in self.all_mailbox_types:
+                if class1 is class2:
+                    continue
+                source = class1(_sample_message)
+                target = class2(source)
+                type_specific = [a for a in reference[class1]
+                                   if a not in reference[class2]]
+                for attr in type_specific:
+                    self.assertNotIn(attr, target.__dict__,
+                        "while converting {} to {}".format(class1, class2))
+
+    def test_maildir_to_maildir(self):
+        # Convert MaildirMessage to MaildirMessage
+        msg_maildir = mailbox.MaildirMessage(_sample_message)
+        msg_maildir.set_flags('DFPRST')
+        msg_maildir.set_subdir('cur')
+        date = msg_maildir.get_date()
+        msg = mailbox.MaildirMessage(msg_maildir)
+        self._check_sample(msg)
+        self.assertEqual(msg.get_flags(), 'DFPRST')
+        self.assertEqual(msg.get_subdir(), 'cur')
+        self.assertEqual(msg.get_date(), date)
+
+    def test_maildir_to_mboxmmdf(self):
+        # Convert MaildirMessage to mboxmessage and MMDFMessage
+        pairs = (('D', ''), ('F', 'F'), ('P', ''), ('R', 'A'), ('S', 'R'),
+                 ('T', 'D'), ('DFPRST', 'RDFA'))
+        for class_ in (mailbox.mboxMessage, mailbox.MMDFMessage):
+            msg_maildir = mailbox.MaildirMessage(_sample_message)
+            msg_maildir.set_date(0.0)
+            for setting, result in pairs:
+                msg_maildir.set_flags(setting)
+                msg = class_(msg_maildir)
+                self.assertEqual(msg.get_flags(), result)
+                self.assertEqual(msg.get_from(), 'MAILER-DAEMON %s' %
+                             time.asctime(time.gmtime(0.0)))
+                self.assertIsNone(msg.get_unixfrom())
+            msg_maildir.set_subdir('cur')
+            self.assertEqual(class_(msg_maildir).get_flags(), 'RODFA')
+
+    def test_maildir_to_mh(self):
+        # Convert MaildirMessage to MHMessage
+        msg_maildir = mailbox.MaildirMessage(_sample_message)
+        pairs = (('D', ['unseen']), ('F', ['unseen', 'flagged']),
+                 ('P', ['unseen']), ('R', ['unseen', 'replied']), ('S', []),
+                 ('T', ['unseen']), ('DFPRST', ['replied', 'flagged']))
+        for setting, result in pairs:
+            msg_maildir.set_flags(setting)
+            self.assertEqual(mailbox.MHMessage(msg_maildir).get_sequences(),
+                             result)
+
+    def test_maildir_to_babyl(self):
+        # Convert MaildirMessage to Babyl
+        msg_maildir = mailbox.MaildirMessage(_sample_message)
+        pairs = (('D', ['unseen']), ('F', ['unseen']),
+                 ('P', ['unseen', 'forwarded']), ('R', ['unseen', 'answered']),
+                 ('S', []), ('T', ['unseen', 'deleted']),
+                 ('DFPRST', ['deleted', 'answered', 'forwarded']))
+        for setting, result in pairs:
+            msg_maildir.set_flags(setting)
+            self.assertEqual(mailbox.BabylMessage(msg_maildir).get_labels(),
+                             result)
+
+    def test_mboxmmdf_to_maildir(self):
+        # Convert mboxMessage and MMDFMessage to MaildirMessage
+        for class_ in (mailbox.mboxMessage, mailbox.MMDFMessage):
+            msg_mboxMMDF = class_(_sample_message)
+            msg_mboxMMDF.set_from('foo@bar', time.gmtime(0.0))
+            pairs = (('R', 'S'), ('O', ''), ('D', 'T'), ('F', 'F'), ('A', 'R'),
+                     ('RODFA', 'FRST'))
+            for setting, result in pairs:
+                msg_mboxMMDF.set_flags(setting)
+                msg = mailbox.MaildirMessage(msg_mboxMMDF)
+                self.assertEqual(msg.get_flags(), result)
+                self.assertEqual(msg.get_date(), 0.0)
+            msg_mboxMMDF.set_flags('O')
+            self.assertEqual(mailbox.MaildirMessage(msg_mboxMMDF).get_subdir(),
+                             'cur')
+
+    def test_mboxmmdf_to_mboxmmdf(self):
+        # Convert mboxMessage and MMDFMessage to mboxMessage and MMDFMessage
+        for class_ in (mailbox.mboxMessage, mailbox.MMDFMessage):
+            msg_mboxMMDF = class_(_sample_message)
+            msg_mboxMMDF.set_flags('RODFA')
+            msg_mboxMMDF.set_from('foo@bar')
+            self.assertIsNone(msg_mboxMMDF.get_unixfrom())
+            for class2_ in (mailbox.mboxMessage, mailbox.MMDFMessage):
+                msg2 = class2_(msg_mboxMMDF)
+                self.assertEqual(msg2.get_flags(), 'RODFA')
+                self.assertEqual(msg2.get_from(), 'foo@bar')
+                self.assertIsNone(msg2.get_unixfrom())
+
+    def test_mboxmmdf_to_mh(self):
+        # Convert mboxMessage and MMDFMessage to MHMessage
+        for class_ in (mailbox.mboxMessage, mailbox.MMDFMessage):
+            msg_mboxMMDF = class_(_sample_message)
+            pairs = (('R', []), ('O', ['unseen']), ('D', ['unseen']),
+                     ('F', ['unseen', 'flagged']),
+                     ('A', ['unseen', 'replied']),
+                     ('RODFA', ['replied', 'flagged']))
+            for setting, result in pairs:
+                msg_mboxMMDF.set_flags(setting)
+                self.assertEqual(mailbox.MHMessage(msg_mboxMMDF).get_sequences(),
+                                 result)
+
+    def test_mboxmmdf_to_babyl(self):
+        # Convert mboxMessage and MMDFMessage to BabylMessage
+        for class_ in (mailbox.mboxMessage, mailbox.MMDFMessage):
+            msg = class_(_sample_message)
+            pairs = (('R', []), ('O', ['unseen']),
+                     ('D', ['unseen', 'deleted']), ('F', ['unseen']),
+                     ('A', ['unseen', 'answered']),
+                     ('RODFA', ['deleted', 'answered']))
+            for setting, result in pairs:
+                msg.set_flags(setting)
+                self.assertEqual(mailbox.BabylMessage(msg).get_labels(), result)
+
+    def test_mh_to_maildir(self):
+        # Convert MHMessage to MaildirMessage
+        pairs = (('unseen', ''), ('replied', 'RS'), ('flagged', 'FS'))
+        for setting, result in pairs:
+            msg = mailbox.MHMessage(_sample_message)
+            msg.add_sequence(setting)
+            self.assertEqual(mailbox.MaildirMessage(msg).get_flags(), result)
+            self.assertEqual(mailbox.MaildirMessage(msg).get_subdir(), 'cur')
+        msg = mailbox.MHMessage(_sample_message)
+        msg.add_sequence('unseen')
+        msg.add_sequence('replied')
+        msg.add_sequence('flagged')
+        self.assertEqual(mailbox.MaildirMessage(msg).get_flags(), 'FR')
+        self.assertEqual(mailbox.MaildirMessage(msg).get_subdir(), 'cur')
+
+    def test_mh_to_mboxmmdf(self):
+        # Convert MHMessage to mboxMessage and MMDFMessage
+        pairs = (('unseen', 'O'), ('replied', 'ROA'), ('flagged', 'ROF'))
+        for setting, result in pairs:
+            msg = mailbox.MHMessage(_sample_message)
+            msg.add_sequence(setting)
+            for class_ in (mailbox.mboxMessage, mailbox.MMDFMessage):
+                self.assertEqual(class_(msg).get_flags(), result)
+        msg = mailbox.MHMessage(_sample_message)
+        msg.add_sequence('unseen')
+        msg.add_sequence('replied')
+        msg.add_sequence('flagged')
+        for class_ in (mailbox.mboxMessage, mailbox.MMDFMessage):
+            self.assertEqual(class_(msg).get_flags(), 'OFA')
+
+    def test_mh_to_mh(self):
+        # Convert MHMessage to MHMessage
+        msg = mailbox.MHMessage(_sample_message)
+        msg.add_sequence('unseen')
+        msg.add_sequence('replied')
+        msg.add_sequence('flagged')
+        self.assertEqual(mailbox.MHMessage(msg).get_sequences(),
+                         ['unseen', 'replied', 'flagged'])
+
+    def test_mh_to_babyl(self):
+        # Convert MHMessage to BabylMessage
+        pairs = (('unseen', ['unseen']), ('replied', ['answered']),
+                 ('flagged', []))
+        for setting, result in pairs:
+            msg = mailbox.MHMessage(_sample_message)
+            msg.add_sequence(setting)
+            self.assertEqual(mailbox.BabylMessage(msg).get_labels(), result)
+        msg = mailbox.MHMessage(_sample_message)
+        msg.add_sequence('unseen')
+        msg.add_sequence('replied')
+        msg.add_sequence('flagged')
+        self.assertEqual(mailbox.BabylMessage(msg).get_labels(),
+                         ['unseen', 'answered'])
+
+    def test_babyl_to_maildir(self):
+        # Convert BabylMessage to MaildirMessage
+        pairs = (('unseen', ''), ('deleted', 'ST'), ('filed', 'S'),
+                 ('answered', 'RS'), ('forwarded', 'PS'), ('edited', 'S'),
+                 ('resent', 'PS'))
+        for setting, result in pairs:
+            msg = mailbox.BabylMessage(_sample_message)
+            msg.add_label(setting)
+            self.assertEqual(mailbox.MaildirMessage(msg).get_flags(), result)
+            self.assertEqual(mailbox.MaildirMessage(msg).get_subdir(), 'cur')
+        msg = mailbox.BabylMessage(_sample_message)
+        for label in ('unseen', 'deleted', 'filed', 'answered', 'forwarded',
+                      'edited', 'resent'):
+            msg.add_label(label)
+        self.assertEqual(mailbox.MaildirMessage(msg).get_flags(), 'PRT')
+        self.assertEqual(mailbox.MaildirMessage(msg).get_subdir(), 'cur')
+
+    def test_babyl_to_mboxmmdf(self):
+        # Convert BabylMessage to mboxMessage and MMDFMessage
+        pairs = (('unseen', 'O'), ('deleted', 'ROD'), ('filed', 'RO'),
+                 ('answered', 'ROA'), ('forwarded', 'RO'), ('edited', 'RO'),
+                 ('resent', 'RO'))
+        for setting, result in pairs:
+            for class_ in (mailbox.mboxMessage, mailbox.MMDFMessage):
+                msg = mailbox.BabylMessage(_sample_message)
+                msg.add_label(setting)
+                self.assertEqual(class_(msg).get_flags(), result)
+        msg = mailbox.BabylMessage(_sample_message)
+        for label in ('unseen', 'deleted', 'filed', 'answered', 'forwarded',
+                      'edited', 'resent'):
+            msg.add_label(label)
+        for class_ in (mailbox.mboxMessage, mailbox.MMDFMessage):
+            self.assertEqual(class_(msg).get_flags(), 'ODA')
+
+    def test_babyl_to_mh(self):
+        # Convert BabylMessage to MHMessage
+        pairs = (('unseen', ['unseen']), ('deleted', []), ('filed', []),
+                 ('answered', ['replied']), ('forwarded', []), ('edited', []),
+                 ('resent', []))
+        for setting, result in pairs:
+            msg = mailbox.BabylMessage(_sample_message)
+            msg.add_label(setting)
+            self.assertEqual(mailbox.MHMessage(msg).get_sequences(), result)
+        msg = mailbox.BabylMessage(_sample_message)
+        for label in ('unseen', 'deleted', 'filed', 'answered', 'forwarded',
+                      'edited', 'resent'):
+            msg.add_label(label)
+        self.assertEqual(mailbox.MHMessage(msg).get_sequences(),
+                         ['unseen', 'replied'])
+
+    def test_babyl_to_babyl(self):
+        # Convert BabylMessage to BabylMessage
+        msg = mailbox.BabylMessage(_sample_message)
+        msg.update_visible()
+        for label in ('unseen', 'deleted', 'filed', 'answered', 'forwarded',
+                      'edited', 'resent'):
+            msg.add_label(label)
+        msg2 = mailbox.BabylMessage(msg)
+        self.assertEqual(msg2.get_labels(), ['unseen', 'deleted', 'filed',
+                                             'answered', 'forwarded', 'edited',
+                                             'resent'])
+        self.assertEqual(msg.get_visible().keys(), msg2.get_visible().keys())
+        for key in msg.get_visible().keys():
+            self.assertEqual(msg.get_visible()[key], msg2.get_visible()[key])
+
+
+class TestProxyFileBase(TestBase):
+
+    def _test_read(self, proxy):
+        # Read by byte
+        proxy.seek(0)
+        self.assertEqual(proxy.read(), b'bar')
+        proxy.seek(1)
+        self.assertEqual(proxy.read(), b'ar')
+        proxy.seek(0)
+        self.assertEqual(proxy.read(2), b'ba')
+        proxy.seek(1)
+        self.assertEqual(proxy.read(-1), b'ar')
+        proxy.seek(2)
+        self.assertEqual(proxy.read(1000), b'r')
+
+    def _test_readline(self, proxy):
+        # Read by line
+        linesep = os.linesep.encode()
+        proxy.seek(0)
+        self.assertEqual(proxy.readline(), b'foo' + linesep)
+        self.assertEqual(proxy.readline(), b'bar' + linesep)
+        self.assertEqual(proxy.readline(), b'fred' + linesep)
+        self.assertEqual(proxy.readline(), b'bob')
+        proxy.seek(2)
+        self.assertEqual(proxy.readline(), b'o' + linesep)
+        proxy.seek(6 + 2 * len(os.linesep))
+        self.assertEqual(proxy.readline(), b'fred' + linesep)
+        proxy.seek(6 + 2 * len(os.linesep))
+        self.assertEqual(proxy.readline(2), b'fr')
+        self.assertEqual(proxy.readline(-10), b'ed' + linesep)
+
+    def _test_readlines(self, proxy):
+        # Read multiple lines
+        linesep = os.linesep.encode()
+        proxy.seek(0)
+        self.assertEqual(proxy.readlines(), [b'foo' + linesep,
+                                           b'bar' + linesep,
+                                           b'fred' + linesep, b'bob'])
+        proxy.seek(0)
+        self.assertEqual(proxy.readlines(2), [b'foo' + linesep])
+        proxy.seek(3 + len(linesep))
+        self.assertEqual(proxy.readlines(4 + len(linesep)),
+                     [b'bar' + linesep, b'fred' + linesep])
+        proxy.seek(3)
+        self.assertEqual(proxy.readlines(1000), [linesep, b'bar' + linesep,
+                                               b'fred' + linesep, b'bob'])
+
+    def _test_iteration(self, proxy):
+        # Iterate by line
+        linesep = os.linesep.encode()
+        proxy.seek(0)
+        iterator = iter(proxy)
+        self.assertEqual(next(iterator), b'foo' + linesep)
+        self.assertEqual(next(iterator), b'bar' + linesep)
+        self.assertEqual(next(iterator), b'fred' + linesep)
+        self.assertEqual(next(iterator), b'bob')
+        self.assertRaises(StopIteration, next, iterator)
+
+    def _test_seek_and_tell(self, proxy):
+        # Seek and use tell to check position
+        linesep = os.linesep.encode()
+        proxy.seek(3)
+        self.assertEqual(proxy.tell(), 3)
+        self.assertEqual(proxy.read(len(linesep)), linesep)
+        proxy.seek(2, 1)
+        self.assertEqual(proxy.read(1 + len(linesep)), b'r' + linesep)
+        proxy.seek(-3 - len(linesep), 2)
+        self.assertEqual(proxy.read(3), b'bar')
+        proxy.seek(2, 0)
+        self.assertEqual(proxy.read(), b'o' + linesep + b'bar' + linesep)
+        proxy.seek(100)
+        self.assertFalse(proxy.read())
+
+    def _test_close(self, proxy):
+        # Close a file
+        self.assertFalse(proxy.closed)
+        proxy.close()
+        self.assertTrue(proxy.closed)
+        # Issue 11700 subsequent closes should be a no-op.
+        proxy.close()
+        self.assertTrue(proxy.closed)
+
+
+class TestProxyFile(TestProxyFileBase, unittest.TestCase):
+
+    def setUp(self):
+        self._path = os_helper.TESTFN
+        self._file = open(self._path, 'wb+')
+
+    def tearDown(self):
+        self._file.close()
+        self._delete_recursively(self._path)
+
+    def test_initialize(self):
+        # Initialize and check position
+        self._file.write(b'foo')
+        pos = self._file.tell()
+        proxy0 = mailbox._ProxyFile(self._file)
+        self.assertEqual(proxy0.tell(), pos)
+        self.assertEqual(self._file.tell(), pos)
+        proxy1 = mailbox._ProxyFile(self._file, 0)
+        self.assertEqual(proxy1.tell(), 0)
+        self.assertEqual(self._file.tell(), pos)
+
+    def test_read(self):
+        self._file.write(b'bar')
+        self._test_read(mailbox._ProxyFile(self._file))
+
+    def test_readline(self):
+        self._file.write(bytes('foo%sbar%sfred%sbob' % (os.linesep, os.linesep,
+                                                  os.linesep), 'ascii'))
+        self._test_readline(mailbox._ProxyFile(self._file))
+
+    def test_readlines(self):
+        self._file.write(bytes('foo%sbar%sfred%sbob' % (os.linesep, os.linesep,
+                                                  os.linesep), 'ascii'))
+        self._test_readlines(mailbox._ProxyFile(self._file))
+
+    def test_iteration(self):
+        self._file.write(bytes('foo%sbar%sfred%sbob' % (os.linesep, os.linesep,
+                                                  os.linesep), 'ascii'))
+        self._test_iteration(mailbox._ProxyFile(self._file))
+
+    def test_seek_and_tell(self):
+        self._file.write(bytes('foo%sbar%s' % (os.linesep, os.linesep), 'ascii'))
+        self._test_seek_and_tell(mailbox._ProxyFile(self._file))
+
+    def test_close(self):
+        self._file.write(bytes('foo%sbar%s' % (os.linesep, os.linesep), 'ascii'))
+        self._test_close(mailbox._ProxyFile(self._file))
+
+
+class TestPartialFile(TestProxyFileBase, unittest.TestCase):
+
+    def setUp(self):
+        self._path = os_helper.TESTFN
+        self._file = open(self._path, 'wb+')
+
+    def tearDown(self):
+        self._file.close()
+        self._delete_recursively(self._path)
+
+    def test_initialize(self):
+        # Initialize and check position
+        self._file.write(bytes('foo' + os.linesep + 'bar', 'ascii'))
+        pos = self._file.tell()
+        proxy = mailbox._PartialFile(self._file, 2, 5)
+        self.assertEqual(proxy.tell(), 0)
+        self.assertEqual(self._file.tell(), pos)
+
+    def test_read(self):
+        self._file.write(bytes('***bar***', 'ascii'))
+        self._test_read(mailbox._PartialFile(self._file, 3, 6))
+
+    def test_readline(self):
+        self._file.write(bytes('!!!!!foo%sbar%sfred%sbob!!!!!' %
+                         (os.linesep, os.linesep, os.linesep), 'ascii'))
+        self._test_readline(mailbox._PartialFile(self._file, 5,
+                                                 18 + 3 * len(os.linesep)))
+
+    def test_readlines(self):
+        self._file.write(bytes('foo%sbar%sfred%sbob?????' %
+                         (os.linesep, os.linesep, os.linesep), 'ascii'))
+        self._test_readlines(mailbox._PartialFile(self._file, 0,
+                                                  13 + 3 * len(os.linesep)))
+
+    def test_iteration(self):
+        self._file.write(bytes('____foo%sbar%sfred%sbob####' %
+                         (os.linesep, os.linesep, os.linesep), 'ascii'))
+        self._test_iteration(mailbox._PartialFile(self._file, 4,
+                                                  17 + 3 * len(os.linesep)))
+
+    def test_seek_and_tell(self):
+        self._file.write(bytes('(((foo%sbar%s$$$' % (os.linesep, os.linesep), 'ascii'))
+        self._test_seek_and_tell(mailbox._PartialFile(self._file, 3,
+                                                      9 + 2 * len(os.linesep)))
+
+    def test_close(self):
+        self._file.write(bytes('&foo%sbar%s^' % (os.linesep, os.linesep), 'ascii'))
+        self._test_close(mailbox._PartialFile(self._file, 1,
+                                              6 + 3 * len(os.linesep)))
+
+
+## Start: tests from the original module (for backward compatibility).
+
+FROM_ = "From some.body@dummy.domain  Sat Jul 24 13:43:35 2004\n"
+DUMMY_MESSAGE = """\
+From: some.body@dummy.domain
+To: me@my.domain
+Subject: Simple Test
+
+This is a dummy message.
+"""
+
+class MaildirTestCase(unittest.TestCase):
+
+    def setUp(self):
+        # create a new maildir mailbox to work with:
+        self._dir = os_helper.TESTFN
+        if os.path.isdir(self._dir):
+            os_helper.rmtree(self._dir)
+        elif os.path.isfile(self._dir):
+            os_helper.unlink(self._dir)
+        os.mkdir(self._dir)
+        os.mkdir(os.path.join(self._dir, "cur"))
+        os.mkdir(os.path.join(self._dir, "tmp"))
+        os.mkdir(os.path.join(self._dir, "new"))
+        self._counter = 1
+        self._msgfiles = []
+
+    def tearDown(self):
+        list(map(os.unlink, self._msgfiles))
+        os_helper.rmdir(os.path.join(self._dir, "cur"))
+        os_helper.rmdir(os.path.join(self._dir, "tmp"))
+        os_helper.rmdir(os.path.join(self._dir, "new"))
+        os_helper.rmdir(self._dir)
+
+    def createMessage(self, dir, mbox=False):
+        t = int(time.time() % 1000000)
+        pid = self._counter
+        self._counter += 1
+        filename = ".".join((str(t), str(pid), "myhostname", "mydomain"))
+        tmpname = os.path.join(self._dir, "tmp", filename)
+        newname = os.path.join(self._dir, dir, filename)
+        with open(tmpname, "w", encoding="utf-8") as fp:
+            self._msgfiles.append(tmpname)
+            if mbox:
+                fp.write(FROM_)
+            fp.write(DUMMY_MESSAGE)
+        try:
+            os.link(tmpname, newname)
+        except (AttributeError, PermissionError):
+            with open(newname, "w") as fp:
+                fp.write(DUMMY_MESSAGE)
+        self._msgfiles.append(newname)
+        return tmpname
+
+    def test_empty_maildir(self):
+        """Test an empty maildir mailbox"""
+        # Test for regression on bug #117490:
+        # Make sure the boxes attribute actually gets set.
+        self.mbox = mailbox.Maildir(os_helper.TESTFN)
+        #self.assertHasAttr(self.mbox, "boxes")
+        #self.assertEqual(len(self.mbox.boxes), 0)
+        self.assertIsNone(self.mbox.next())
+        self.assertIsNone(self.mbox.next())
+
+    def test_nonempty_maildir_cur(self):
+        self.createMessage("cur")
+        self.mbox = mailbox.Maildir(os_helper.TESTFN)
+        #self.assertEqual(len(self.mbox.boxes), 1)
+        self.assertIsNotNone(self.mbox.next())
+        self.assertIsNone(self.mbox.next())
+        self.assertIsNone(self.mbox.next())
+
+    def test_nonempty_maildir_new(self):
+        self.createMessage("new")
+        self.mbox = mailbox.Maildir(os_helper.TESTFN)
+        #self.assertEqual(len(self.mbox.boxes), 1)
+        self.assertIsNotNone(self.mbox.next())
+        self.assertIsNone(self.mbox.next())
+        self.assertIsNone(self.mbox.next())
+
+    def test_nonempty_maildir_both(self):
+        self.createMessage("cur")
+        self.createMessage("new")
+        self.mbox = mailbox.Maildir(os_helper.TESTFN)
+        #self.assertEqual(len(self.mbox.boxes), 2)
+        self.assertIsNotNone(self.mbox.next())
+        self.assertIsNotNone(self.mbox.next())
+        self.assertIsNone(self.mbox.next())
+        self.assertIsNone(self.mbox.next())
+
+## End: tests from the original module (for backward compatibility).
+
+
+_sample_message = """\
+Return-Path: <gkj@gregorykjohnson.com>
+X-Original-To: gkj+person@localhost
+Delivered-To: gkj+person@localhost
+Received: from localhost (localhost [127.0.0.1])
+        by andy.gregorykjohnson.com (Postfix) with ESMTP id 356ED9DD17
+        for <gkj+person@localhost>; Wed, 13 Jul 2005 17:23:16 -0400 (EDT)
+Delivered-To: gkj@sundance.gregorykjohnson.com
+Received: from localhost [127.0.0.1]
+        by localhost with POP3 (fetchmail-6.2.5)
+        for gkj+person@localhost (single-drop); Wed, 13 Jul 2005 17:23:16 -0400 (EDT)
+Received: from andy.gregorykjohnson.com (andy.gregorykjohnson.com [64.32.235.228])
+        by sundance.gregorykjohnson.com (Postfix) with ESMTP id 5B056316746
+        for <gkj@gregorykjohnson.com>; Wed, 13 Jul 2005 17:23:11 -0400 (EDT)
+Received: by andy.gregorykjohnson.com (Postfix, from userid 1000)
+        id 490CD9DD17; Wed, 13 Jul 2005 17:23:11 -0400 (EDT)
+Date: Wed, 13 Jul 2005 17:23:11 -0400
+From: "Gregory K. Johnson" <gkj@gregorykjohnson.com>
+To: gkj@gregorykjohnson.com
+Subject: Sample message
+Message-ID: <20050713212311.GC4701@andy.gregorykjohnson.com>
+Mime-Version: 1.0
+Content-Type: multipart/mixed; boundary="NMuMz9nt05w80d4+"
+Content-Disposition: inline
+User-Agent: Mutt/1.5.9i
+
+
+--NMuMz9nt05w80d4+
+Content-Type: text/plain; charset=us-ascii
+Content-Disposition: inline
+
+This is a sample message.
+
+--
+Gregory K. Johnson
+
+--NMuMz9nt05w80d4+
+Content-Type: application/octet-stream
+Content-Disposition: attachment; filename="text.gz"
+Content-Transfer-Encoding: base64
+
+H4sICM2D1UIAA3RleHQAC8nILFYAokSFktSKEoW0zJxUPa7wzJIMhZLyfIWczLzUYj0uAHTs
+3FYlAAAA
+
+--NMuMz9nt05w80d4+--
+"""
+
+_bytes_sample_message = _sample_message.encode('ascii')
+
+_sample_headers = [
+    ("Return-Path", "<gkj@gregorykjohnson.com>"),
+    ("X-Original-To", "gkj+person@localhost"),
+    ("Delivered-To", "gkj+person@localhost"),
+    ("Received", """from localhost (localhost [127.0.0.1])
+        by andy.gregorykjohnson.com (Postfix) with ESMTP id 356ED9DD17
+        for <gkj+person@localhost>; Wed, 13 Jul 2005 17:23:16 -0400 (EDT)"""),
+    ("Delivered-To", "gkj@sundance.gregorykjohnson.com"),
+    ("Received", """from localhost [127.0.0.1]
+        by localhost with POP3 (fetchmail-6.2.5)
+        for gkj+person@localhost (single-drop); Wed, 13 Jul 2005 17:23:16 -0400 (EDT)"""),
+    ("Received", """from andy.gregorykjohnson.com (andy.gregorykjohnson.com [64.32.235.228])
+        by sundance.gregorykjohnson.com (Postfix) with ESMTP id 5B056316746
+        for <gkj@gregorykjohnson.com>; Wed, 13 Jul 2005 17:23:11 -0400 (EDT)"""),
+    ("Received", """by andy.gregorykjohnson.com (Postfix, from userid 1000)
+        id 490CD9DD17; Wed, 13 Jul 2005 17:23:11 -0400 (EDT)"""),
+    ("Date", "Wed, 13 Jul 2005 17:23:11 -0400"),
+    ("From", """"Gregory K. Johnson" <gkj@gregorykjohnson.com>"""),
+    ("To", "gkj@gregorykjohnson.com"),
+    ("Subject", "Sample message"),
+    ("Mime-Version", "1.0"),
+    ("Content-Type", """multipart/mixed; boundary="NMuMz9nt05w80d4+\""""),
+    ("Content-Disposition", "inline"),
+    ("User-Agent", "Mutt/1.5.9i"),
+]
+
+_sample_payloads = ("""This is a sample message.
+
+--
+Gregory K. Johnson
+""",
+"""H4sICM2D1UIAA3RleHQAC8nILFYAokSFktSKEoW0zJxUPa7wzJIMhZLyfIWczLzUYj0uAHTs
+3FYlAAAA
+""")
+
+
+class MiscTestCase(unittest.TestCase):
+    def test__all__(self):
+        support.check__all__(self, mailbox,
+                             not_exported={"linesep", "fcntl"})
+
+
+def tearDownModule():
+    support.reap_children()
+    # reap_children may have re-populated caches:
+    if refleak_helper.hunting_for_refleaks():
+        sys._clear_internal_caches()
+
+
+if __name__ == '__main__':
+    unittest.main()