When a single positional argument is passed to subTest(), if the argument is false, its value won't be displayed in the output -- (<subtest>) will appear instead:

>>> import unittest
>>> class NumbersTest(unittest.TestCase):
...   def test_odd(self):
...     for i in range(4):
...       with self.subTest(i):  # single positional arg
...         self.assertNotEqual(i%2, 0)
... 
>>> unittest.main(exit=False)

======================================================================
FAIL: test_odd (main.NumbersTest) (<subtest>)
----------------------------------------------------------------------

Traceback (most recent call last):
  File "<stdin>", line 5, in test_odd
AssertionError: 0 == 0

======================================================================
FAIL: test_odd (main.NumbersTest) [2]
----------------------------------------------------------------------

Traceback (most recent call last):
  File "<stdin>", line 5, in test_odd
AssertionError: 0 == 0

Ran 1 test in 0.001s
FAILED (failures=2)

This is because subTest() accepts a positional "msg" arg, passes it to _SubTest (Lib/unittest/case.py:515), and then _SubTest checks using "if self._message:" (Lib/unittest/case.py:1400).

I think it would be better to check the message against a sentinel value instead.

Made it check against None explicitly. My concern is if [] is passed in, if will show [[]]. But this case should be rare.

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 '[[]]'.

New changeset ba743894e793 by Berker Peksag in branch '3.5':
Issue bpo-25651: Allow falsy values to be used for msg parameter of subTest()
https://hg.python.org/cpython/rev/ba743894e793

New changeset ddbf92168a44 by Berker Peksag in branch '3.6':
Issue bpo-25651: Merge from 3.5
https://hg.python.org/cpython/rev/ddbf92168a44

New changeset e5888f5b9cf8 by Berker Peksag in branch 'default':
Issue bpo-25651: Merge from 3.6
https://hg.python.org/cpython/rev/e5888f5b9cf8

Fixed. I lost some time because of this today :)