bpo-31026: Fix test_dbm if dbm.ndbm is build with Berkeley DB. (GH-6632)

(cherry picked from commit 70af06cd)
Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Lib/test/test_dbm.py

@@ -38,8 +38,7 @@ def delete_files():
 
 
 class AnyDBMTestCase:
-    _dict = {'0': b'',
-             'a': b'Python:',
+    _dict = {'a': b'Python:',
              'b': b'Programming',
              'c': b'the',
              'd': b'way',
@@ -109,6 +108,20 @@ class AnyDBMTestCase:
         keys = self.keys_helper(f)
         f.close()
 
+    def test_empty_value(self):
+        if getattr(dbm._defaultmod, 'library', None) == 'Berkeley DB':
+            self.skipTest("Berkeley DB doesn't distinguish the empty value "
+                          "from the absent one")
+        f = dbm.open(_fname, 'c')
+        self.assertEqual(f.keys(), [])
+        f[b'empty'] = b''
+        self.assertEqual(f.keys(), [b'empty'])
+        self.assertIn(b'empty', f)
+        self.assertEqual(f[b'empty'], b'')
+        self.assertEqual(f.get(b'empty'), b'')
+        self.assertEqual(f.setdefault(b'empty'), b'')
+        f.close()
+
     def test_anydbm_access(self):
         self.init_db()
         f = dbm.open(_fname, 'r')

Lib/test/test_dbm_ndbm.py

@@ -36,6 +36,20 @@ class DbmTestCase(unittest.TestCase):
         self.assertEqual(self.d[b'xxx'], b'foo')
         self.d.close()
 
+    def test_empty_value(self):
+        if dbm.ndbm.library == 'Berkeley DB':
+            self.skipTest("Berkeley DB doesn't distinguish the empty value "
+                          "from the absent one")
+        self.d = dbm.ndbm.open(self.filename, 'c')
+        self.assertEqual(self.d.keys(), [])
+        self.d['empty'] = ''
+        self.assertEqual(self.d.keys(), [b'empty'])
+        self.assertIn(b'empty', self.d)
+        self.assertEqual(self.d[b'empty'], b'')
+        self.assertEqual(self.d.get(b'empty'), b'')
+        self.assertEqual(self.d.setdefault(b'empty'), b'')
+        self.d.close()
+
     def test_modes(self):
         for mode in ['r', 'rw', 'w', 'n']:
             try: