Index: Lib/test/test_mimetypes.py
===================================================================
--- Lib/test/test_mimetypes.py	(revision 82055)
+++ Lib/test/test_mimetypes.py	(working copy)
@@ -62,7 +62,17 @@
         all = self.db.guess_all_extensions('image/jpg', strict=True)
         eq(all, [])
 
+    def test_init_reinitializes(self):
+        # Issue 4936: make sure an init starts clean
+        # First, put some poison into the types table
+        self.db.add_type('foo/bar', '.foobar')
+        self.assertEqual(self.db.guess_extension('foo/bar'), '.foobar')
+        # Reinitialize
+        mimetypes.init()
+        # Poison should be gone.
+        self.assertEqual(self.db.guess_extension('foo/bar'), None)
 
+
 @unittest.skipUnless(sys.platform.startswith("win"), "Windows only")
 class Win32MimeTypesTestCase(unittest.TestCase):
     def setUp(self):