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If the exception argument is None or repr(None), it is omitted from the report when a thread raises an unhandled exception:
>>> def raise_exception(e):
... raise e
... 
>>> t = Thread(target=raise_exception, args=(Exception(None),)); t.start(); t.join()
Exception in thread Thread-1:
Traceback (most recent call last):
 File "/home/proj/python/cpython/Lib/threading.py", line 916, in _bootstrap_inner
 self.run()
 File "/home/proj/python/cpython/Lib/threading.py", line 864, in run
 self._target(*self._args, **self._kwargs)
 File "<stdin>", line 2, in raise_exception
Exception
>>> t = Thread(target=raise_exception, args=(Exception("None"),)); t.start(); t.join()
Exception in thread Thread-2:
Traceback (most recent call last):
 File "/home/proj/python/cpython/Lib/threading.py", line 916, in _bootstrap_inner
 self.run()
 File "/home/proj/python/cpython/Lib/threading.py", line 864, in run
 self._target(*self._args, **self._kwargs)
 File "<stdin>", line 2, in raise_exception
Exception
Compare the result with other exception messages, the normal sys.excepthook() report, and Python 2:
>>> t = Thread(target=raise_exception, args=(Exception("NONE"),)); t.start(); t.join()
Exception in thread Thread-3:
Traceback (most recent call last):
 File "/home/proj/python/cpython/Lib/threading.py", line 916, in _bootstrap_inner
 self.run()
 File "/home/proj/python/cpython/Lib/threading.py", line 864, in run
 self._target(*self._args, **self._kwargs)
 File "<stdin>", line 2, in raise_exception
Exception: NONE
>>> raise Exception(None)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
Exception: None

I traced this back to revision 73afda5a4e4c (Issue 17911), which includes this change to traceback._format_final_exc_line():
-if value is None or not valuestr:
+if value == 'None' or value is None or not valuestr:

If I remove the value == 'None' check, there are two failures in the test suite:
1. test_decimal fails, but only because the test was changed in revision 5f3dd0a2b1ab to accommodate the very bug I am complaining about. IMO this was a case of "fixing" the test instead of fixing the bug, so it is okay to restore the test.
2. The following test, added in revision ecaafc32c500:
def test_without_exception(self):
 err = traceback.format_exception_only(None, None)
 self.assertEqual(err, ['None\n'])
It was apparently added so that print_exc() would output "None" when called with no exception set. This is the case for Python 2. However in the Python 3 versions I have tried, print_exc() either raises AttributeError or prints "NoneType". In any case, the test does not seem to be testing anything valid according to the documentation, so I propose to remove it.

hmm, can you give me a change to page this in? I'm pretty sure I saw breakage in external libraries prompting me to add the test and fix. I'd rather not recause that.

I agree that ignoring the exception message if str(exc) == 'None' is wrong.

Yeah sure I can give you a chance to consider this (I assume you meant "chance" :). But neither of the tests I mentioned were added by you as far as I know. Maybe you are thinking of some other test.

Have you had any luck reviewing this Robert?

I plan to commit this soon, in time for the next release.

"Issue #27348: In the traceback module, restore the formatting of exception messages like "Exception: None". This fixes a regression introduced in 3.5a2."
Humn, if it is described as a regression, it means that it's a bug no? You plan to push the change into Python 3.5, 3.6 and 3.7, right?
none-message.patch: LGTM.

Yes, a bug fix for 3.5+.

New changeset 5859a9e8b214 by Martin Panter in branch '3.5':
Issue #27348: Restore "Exception: None" formatting in traceback module
https://hg.python.org/cpython/rev/5859a9e8b214
New changeset d1455d14accd by Martin Panter in branch '3.6':
Issue #27348: Merge exception formatting fix from 3.5 into 3.6
https://hg.python.org/cpython/rev/d1455d14accd
New changeset 4261ae29d3e2 by Martin Panter in branch 'default':
Issue #27348: Merge exception formatting fix from 3.6
https://hg.python.org/cpython/rev/4261ae29d3e2