I think Ezio's suggestion of a sentinel value would be better, allowing None to be using as a legitimate 'message' [1].  That is, somewhere at global scope, define '_subtest_msg_sentinel = object()', change the msg default at Lib/unittest/case.py:500 to be 'msg=_subtest_msg_sentinel', and change the check at Lib/unittest/case.py:1400 to check 'if message is not _subtest_msg_sentinel'.


[1] For example:

   class TruthTest(unittest.TestCase):
       def test_truth(self):
           for o in None, 1, 0, [], (4,):
               with self.subTest(o):
                   self.assertTrue(o)

Should print failure results including '[None]', '[0]', and '[[]]'.