diff -r 7b9ad68db14e Doc/library/dbm.rst
--- a/Doc/library/dbm.rst	Mon Jun 20 00:05:40 2016 +0300
+++ b/Doc/library/dbm.rst	Mon Jun 20 01:03:11 2016 +0300
@@ -341,6 +341,10 @@ The module defines the following:
       :func:`.open` always creates a new database when the flag has the value
       ``'n'``.
 
+   .. deprecated-removed:: 3.6 3.8
+      Creating database in ``'r'`` and ``'w'`` modes.  Modifying database in
+      ``'r'`` mode.
+
    In addition to the methods provided by the
    :class:`collections.abc.MutableMapping` class, :class:`dumbdbm` objects
    provide the following methods:
diff -r 7b9ad68db14e Doc/whatsnew/3.6.rst
--- a/Doc/whatsnew/3.6.rst	Mon Jun 20 00:05:40 2016 +0300
+++ b/Doc/whatsnew/3.6.rst	Mon Jun 20 01:03:11 2016 +0300
@@ -575,6 +575,11 @@ Deprecated features
   ``__package__`` are not defined now raises an :exc:`ImportWarning`.
   (Contributed by Rose Ames in :issue:`25791`.)
 
+* Unlike to other :mod:`dbm` implementations, the :mod:`dbm.dumb` module
+  creates database in ``'r'`` and ``'w'`` modes if it doesn't exist and
+  allows modifying database in ``'r'`` mode.  This behavior is now deprecated
+  and will be removed in 3.8.
+  (Contributed by Serhiy Storchaka in :issue:`21708`.)
 
 Deprecated Python behavior
 --------------------------
diff -r 7b9ad68db14e Lib/dbm/dumb.py
--- a/Lib/dbm/dumb.py	Mon Jun 20 00:05:40 2016 +0300
+++ b/Lib/dbm/dumb.py	Mon Jun 20 01:03:11 2016 +0300
@@ -47,6 +47,7 @@ class _Database(collections.MutableMappi
 
     def __init__(self, filebasename, mode, flag='c'):
         self._mode = mode
+        self._readonly = (flag == 'r')
 
         # The directory file is a text file.  Each line looks like
         #    "%r, (%d, %d)\n" % (key, pos, siz)
@@ -80,6 +81,11 @@ class _Database(collections.MutableMappi
         try:
             f = _io.open(self._datfile, 'r', encoding="Latin-1")
         except OSError:
+            if flag not in ('c', 'n'):
+                import warnings
+                warnings.warn("The database file is missing, the "
+                              "semantics of the 'c' flag will be used.",
+                              DeprecationWarning, stacklevel=4)
             with _io.open(self._datfile, 'w', encoding="Latin-1") as f:
                 self._chmod(self._datfile)
         else:
@@ -178,6 +184,10 @@ class _Database(collections.MutableMappi
             f.write("%r, %r\n" % (key.decode("Latin-1"), pos_and_siz_pair))
 
     def __setitem__(self, key, val):
+        if self._readonly:
+            import warnings
+            warnings.warn('The database is opened for reading only',
+                          DeprecationWarning, stacklevel=2)
         if isinstance(key, str):
             key = key.encode('utf-8')
         elif not isinstance(key, (bytes, bytearray)):
@@ -212,6 +222,10 @@ class _Database(collections.MutableMappi
             # (so that _commit() never gets called).
 
     def __delitem__(self, key):
+        if self._readonly:
+            import warnings
+            warnings.warn('The database is opened for reading only',
+                          DeprecationWarning, stacklevel=2)
         if isinstance(key, str):
             key = key.encode('utf-8')
         self._verify_open()
@@ -300,4 +314,8 @@ def open(file, flag='c', mode=0o666):
     else:
         # Turn off any bits that are set in the umask
         mode = mode & (~um)
+    if flag not in ('r', 'w', 'c', 'n'):
+        import warnings
+        warnings.warn("Flag must be one of 'r', 'w', 'c', or 'n'",
+                      DeprecationWarning, stacklevel=2)
     return _Database(file, mode, flag=flag)
diff -r 7b9ad68db14e Lib/test/test_dbm_dumb.py
--- a/Lib/test/test_dbm_dumb.py	Mon Jun 20 00:05:40 2016 +0300
+++ b/Lib/test/test_dbm_dumb.py	Mon Jun 20 01:03:11 2016 +0300
@@ -6,6 +6,7 @@ import io
 import operator
 import os
 import unittest
+import warnings
 import dbm.dumb as dumbdbm
 from test import support
 from functools import partial
@@ -78,6 +79,12 @@ class DumbDBMTestCase(unittest.TestCase)
         self.init_db()
         f = dumbdbm.open(_fname, 'r')
         self.read_helper(f)
+        with self.assertWarnsRegex(DeprecationWarning,
+                                   'The database is opened for reading only'):
+            f[b'g'] = b'x'
+        with self.assertWarnsRegex(DeprecationWarning,
+                                   'The database is opened for reading only'):
+            del f[b'a']
         f.close()
 
     def test_dumbdbm_keys(self):
@@ -148,7 +155,7 @@ class DumbDBMTestCase(unittest.TestCase)
             self.assertEqual(self._dict[key], f[key])
 
     def init_db(self):
-        f = dumbdbm.open(_fname, 'w')
+        f = dumbdbm.open(_fname, 'n')
         for k in self._dict:
             f[k] = self._dict[k]
         f.close()
@@ -234,6 +241,24 @@ class DumbDBMTestCase(unittest.TestCase)
                     pass
             self.assertEqual(stdout.getvalue(), '')
 
+    def test_warn_on_ignored_flags(self):
+        for value in ('r', 'w'):
+            _delete_files()
+            with self.assertWarnsRegex(DeprecationWarning,
+                                       "The database file is missing, the "
+                                       "semantics of the 'c' flag will "
+                                       "be used."):
+                f = dumbdbm.open(_fname, value)
+            f.close()
+
+    def test_invalid_flag(self):
+        for flag in ('x', 'rf', None):
+            with self.assertWarnsRegex(DeprecationWarning,
+                                       "Flag must be one of "
+                                       "'r', 'w', 'c', or 'n'"):
+                f = dumbdbm.open(_fname, flag)
+            f.close()
+
     def tearDown(self):
         _delete_files()