diff --git a/Lib/mailbox.py b/Lib/mailbox.py
index b00d9e863..364af6bb0 100644
--- a/Lib/mailbox.py
+++ b/Lib/mailbox.py
@@ -2183,11 +2183,7 @@ def _unlock_file(f):
 
 def _create_carefully(path):
     """Create a file if it doesn't exist and open for reading and writing."""
-    fd = os.open(path, os.O_CREAT | os.O_EXCL | os.O_RDWR, 0o666)
-    try:
-        return open(path, 'rb+')
-    finally:
-        os.close(fd)
+    return open(path, 'xb+')
 
 def _create_temporary(path):
     """Create a temp file based on path and open for reading and writing."""
diff --git a/Lib/test/test_mailbox.py b/Lib/test/test_mailbox.py
index 940baf394..0169948e4 100644
--- a/Lib/test/test_mailbox.py
+++ b/Lib/test/test_mailbox.py
@@ -1,7 +1,6 @@
 import os
 import sys
 import time
-import stat
 import socket
 import email
 import email.message
@@ -13,7 +12,6 @@ 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
@@ -1267,7 +1265,7 @@ class _TestMboxMMDF(_TestSingleFile):
         self._box.close()
 
 
-class TestMbox(_TestMboxMMDF, unittest.TestCase, ExtraAssertions):
+class TestMbox(_TestMboxMMDF, unittest.TestCase):
 
     _factory = lambda self, path, factory=None: mailbox.mbox(path, factory)