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doctest.py, line 2274 (in 2.7):
 def shortDescription(self):
 return "Skipping tests from %s" % module.__name__
"module" is not defined here. Assigning to David since he last changed this line. Test script attached.

It is possible to retrieve the current module using _normalize_module(None), or instead use the test name (dt_test.name) just like in DocTestCase.shortDescription.
Since there is no doc about it, IMHO we should use unittest's standard as guideline, which is: 
shortDescription()
 Returns a description of the test, or None if no description has been 
 provided. The default implementation of this method returns the first 
 line of the test method’s docstring, if available, or None.
(i.e., DocTestCase._dt_test.name)
What's your opinion?

The patch was actually by Brian Curtin, before he got commit privs :).
Attached is a fix. 
I don't know what t.py has to do with the issue, though. I'm experimenting to see if I can trigger the bug before I apply the fix.

OK, ./python -OO -m test test_json will trigger it.
Updated fix attached. I'm not completely happy with it, though, since it prints the 'skipping test from module XXX' twice, and I don't know why (some unittest thing I'm not getting right, doubtless).

Oh, in case anyone is wondering, the fix for -OO test runs was originally made because the unladen swallow folks ran the test suite that way. (I think someone else wanted it too, but I don't remember who). It's broken again, so I doubt anyone is actually doing this now, and if we want to keep supporting this use case (maybe we don't), then we should have a buildbot running the test suite with -OO. I might be able to set one up at some point.

New changeset 64f1b8ad9214 by R David Murray in branch '3.2':
#12757: Make doctest skipping in -OO mode work with unittest/regrtest -v
http://hg.python.org/cpython/rev/64f1b8ad9214
New changeset ff7957aa01a1 by R David Murray in branch 'default':
Merge #12757: Make doctest skipping in -OO mode work with unittest/regrtest -v
http://hg.python.org/cpython/rev/ff7957aa01a1
New changeset c50db3d06116 by R David Murray in branch '2.7':
#12757: Make doctest skipping in -OO mode work with unittest/regrtest -v
http://hg.python.org/cpython/rev/c50db3d06116 

Since nobody really cares about this issue :), I went ahead and applied the patch that at least avoids the tracebacks. Someone can open a new bug about the duplicated message if they really care.