Teach newbytes to call __bytes__ if available

Currently, the newbytes implementation ignores __bytes__ on classes
and acts like python2's str.

Author: vodik

src/future/types/newbytes.py

@@ -100,6 +100,8 @@ def __new__(cls, *args, **kwargs):
                 newargs.append(errors)
             value = args[0].encode(*newargs)
             ###
+        elif hasattr(args[0], '__bytes__'):
+            value = args[0].__bytes__()
         elif isinstance(args[0], Iterable):
             if len(args[0]) == 0:
                 # This could be an empty list or tuple. Return b'' as on Py3.

tests/test_future/test_bytes.py

@@ -707,6 +707,31 @@ def test_issue_171_part_b(self):
         b = nativebytes(bytes(b'asdf'))
         self.assertEqual(b, b'asdf')
 
+    def test_cast_to_bytes(self):
+        """
+        Tests whether __bytes__ method is called
+        """
+
+        class TestObject:
+            def __bytes__(self):
+                return b'asdf'
+
+        self.assertEqual(bytes(TestObject()), b'asdf')
+
+    def test_cast_to_bytes_iter_precedence(self):
+        """
+        Tests that call to __bytes__ is preferred to iteration
+        """
+
+        class TestObject:
+            def __bytes__(self):
+                return b'asdf'
+
+            def __iter__(self):
+                return iter(b'hjkl')
+
+        self.assertEqual(bytes(TestObject()), b'asdf')
+
 
 if __name__ == '__main__':
     unittest.main()