diff -r 20bb8babc505 Lib/dbm/dumb.py
--- a/Lib/dbm/dumb.py	Wed Nov 30 12:10:54 2016 +0100
+++ b/Lib/dbm/dumb.py	Thu Dec 01 14:41:10 2016 +0200
@@ -68,7 +68,7 @@ class _Database(collections.MutableMappi
 
         # Handle the creation
         self._create(flag)
-        self._update()
+        self._update(flag)
 
     def _create(self, flag):
         if flag == 'n':
@@ -92,13 +92,19 @@ class _Database(collections.MutableMappi
             f.close()
 
     # Read directory file into the in-memory index dict.
-    def _update(self):
+    def _update(self, flag):
         self._index = {}
         try:
             f = _io.open(self._dirfile, 'r', encoding="Latin-1")
         except OSError:
-            pass
+            self._modified = not self._readonly
+            if flag not in ('c', 'n'):
+                import warnings
+                warnings.warn("The index file is missing, the "
+                              "semantics of the 'c' flag will be used.",
+                              DeprecationWarning, stacklevel=4)
         else:
+            self._modified = False
             with f:
                 for line in f:
                     line = line.rstrip()
@@ -113,7 +119,7 @@ class _Database(collections.MutableMappi
         # CAUTION:  It's vital that _commit() succeed, and _commit() can
         # be called from __del__().  Therefore we must never reference a
         # global in this routine.
-        if self._index is None:
+        if self._index is None or not self._modified:
             return  # nothing to do
 
         try:
@@ -197,6 +203,7 @@ class _Database(collections.MutableMappi
         elif not isinstance(val, (bytes, bytearray)):
             raise TypeError("values must be bytes or strings")
         self._verify_open()
+        self._modified = True
         if key not in self._index:
             self._addkey(key, self._addval(val))
         else:
@@ -229,6 +236,7 @@ class _Database(collections.MutableMappi
         if isinstance(key, str):
             key = key.encode('utf-8')
         self._verify_open()
+        self._modified = True
         # The blocks used by the associated value are lost.
         del self._index[key]
         # XXX It's unclear why we do a _commit() here (the code always
diff -r 20bb8babc505 Lib/test/test_dbm_dumb.py
--- a/Lib/test/test_dbm_dumb.py	Wed Nov 30 12:10:54 2016 +0100
+++ b/Lib/test/test_dbm_dumb.py	Thu Dec 01 14:41:10 2016 +0200
@@ -5,6 +5,7 @@
 import io
 import operator
 import os
+import stat
 import unittest
 import warnings
 import dbm.dumb as dumbdbm
@@ -251,6 +252,20 @@ class DumbDBMTestCase(unittest.TestCase)
                 f = dumbdbm.open(_fname, value)
             f.close()
 
+    def test_missing_index(self):
+        with dumbdbm.open(_fname, 'n') as f:
+            pass
+        os.unlink(_fname + '.dir')
+        for value in ('r', 'w'):
+            with self.assertWarnsRegex(DeprecationWarning,
+                                       "The index file is missing, the "
+                                       "semantics of the 'c' flag will "
+                                       "be used."):
+                f = dumbdbm.open(_fname, value)
+            f.close()
+            self.assertEqual(os.path.exists(_fname + '.dir'), value == 'w')
+            self.assertFalse(os.path.exists(_fname + '.bak'))
+
     def test_invalid_flag(self):
         for flag in ('x', 'rf', None):
             with self.assertWarnsRegex(DeprecationWarning,
@@ -259,6 +274,21 @@ class DumbDBMTestCase(unittest.TestCase)
                 f = dumbdbm.open(_fname, flag)
             f.close()
 
+    @unittest.skipUnless(hasattr(os, 'chmod'), 'test needs os.chmod()')
+    def test_readonly_files(self):
+        with support.temp_dir() as dir:
+            fname = os.path.join(dir, 'db')
+            with dumbdbm.open(fname, 'n') as f:
+                self.assertEqual(list(f.keys()), [])
+                for key in self._dict:
+                    f[key] = self._dict[key]
+            os.chmod(fname + ".dir", stat.S_IRUSR)
+            os.chmod(fname + ".dat", stat.S_IRUSR)
+            os.chmod(dir, stat.S_IRUSR|stat.S_IXUSR)
+            with dumbdbm.open(fname, 'r') as f:
+                self.assertEqual(sorted(f.keys()), sorted(self._dict))
+                f.close()  # don't write
+
     def tearDown(self):
         _delete_files()