Enable custom mappings support

Author: Yury Selivanov

src/future/types/newbytes.py

@@ -177,17 +177,21 @@ def __rmul__(self, other):
     def __mod__(self, vals):
         if isinstance(vals, newbytes):
             vals = _builtin_bytes.__str__(vals)
-        elif isinstance(vals, dict):
-            for k, v in vals.items():
-                if isinstance(v, newbytes):
-                    vals[k] = _builtin_bytes.__str__(v)
+
         elif isinstance(vals, tuple):
             newvals = []
             for v in vals:
                 if isinstance(v, newbytes):
                     v = _builtin_bytes.__str__(v)
                 newvals.append(v)
             vals = tuple(newvals)
+
+        elif (hasattr(vals.__class__, '__getitem__') and
+                hasattr(vals.__class__, 'iteritems')):
+            for k, v in vals.iteritems():
+                if isinstance(v, newbytes):
+                    vals[k] = _builtin_bytes.__str__(v)
+
         return _builtin_bytes.__mod__(self, vals)
 
     def join(self, iterable_of_bytes):

tests/test_future/test_bytes.py

@@ -552,6 +552,20 @@ def test_maketrans(self):
         self.assertRaises(ValueError, bytes.maketrans, b'abc', b'xyzq')
         self.assertRaises(TypeError, bytes.maketrans, 'abc', 'def')
 
+    @unittest.skipUnless(utils.PY2, 'test requires Python 2')
+    def test_mod_custom_dict(self):
+        import UserDict
+
+        class MyDict(UserDict.UserDict):
+            pass
+
+        d = MyDict()
+        d['foo'] = bytes(b'bar')
+        self.assertFalse(isinstance(d, dict))
+        self.assertTrue(isinstance(d, UserDict.UserDict))
+
+        self.assertEqual(bytes(b'%(foo)s') % d, b'bar')
+
     @unittest.skipUnless(utils.PY35 or utils.PY2,
                          'test requires Python 2 or 3.5+')
     def test_mod_more(self):